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Figure 2-14. Formation of proteins, lipids, and cellular vesicles by the endoplasmic reticulum and Golgi apparatus.
图 2-14. 内质网和高尔基体形成蛋白质、脂质和细胞囊泡。
Processing of Endoplasmic Secretions by the Golgi Apparatus-Formation of Vesicles. Figure 2-14 summarizes the major functions of the endoplasmic reticulum and Golgi apparatus. As substances are formed in the endoplasmic reticulum, especially proteins, they are transported through the tubules toward portions of the smooth endoplasmic reticulum that lie nearest to the Golgi apparatus. At this point, transport vesicles composed of small envelopes of smooth endoplasmic reticulum continually break away and diffuse to the deepest layer of the Golgi apparatus. Inside these vesicles are synthesized proteins and other products from the endoplasmic reticulum.
高尔基体对内质网分泌物的处理 - 胞泡的形成。图 2-14 总结了内质网和高尔基体的主要功能。当物质在内质网中形成，特别是蛋白质时，它们通过小管运输到靠近高尔基体的光滑内质网部分。在这一点上，由光滑内质网的小囊泡组成的运输囊泡不断脱落并扩散到高尔基体的最深层。这些囊泡内含有合成的蛋白质和来自内质网的其他产品。
The transport vesicles instantly fuse with the Golgi apparatus and empty their contained substances into the vesicular spaces of the Golgi apparatus. Here, additional carbohydrate moieties are added to the secretions. Also, an important function of the Golgi apparatus is to compact the endoplasmic reticular secretions into highly concentrated packets. As the secretions pass toward the outermost layers of the Golgi apparatus, the compaction and processing proceed. Finally, both small and large vesicles continually break away from the Golgi apparatus, carrying with them the compacted secretory substances and diffusing throughout the cell.
运输囊泡瞬间与高尔基体融合，将其所含物质排入高尔基体的囊泡空间。在这里，额外的碳水化合物部分被添加到分泌物中。此外，高尔基体的一个重要功能是将内质网的分泌物压缩成高度浓缩的包裹。当分泌物向高尔基体的最外层移动时，压缩和处理过程继续进行。最后，小囊泡和大囊泡不断从高尔基体断裂，携带着压缩的分泌物并扩散到细胞内。
The following example provides an idea of the timing of these processes. When a glandular cell is bathed in amino acids, newly formed protein molecules can be detected in the granular endoplasmic reticulum within 3 to 5 minutes. Within 20 minutes, newly formed proteins are already present in the Golgi apparatus and, within 1 to 2 hours, the proteins are secreted from the surface of the cell.
以下示例提供了这些过程的时间概念。当腺体细胞浸泡在氨基酸中时，新的蛋白质分子可以在 3 到 5 分钟内在颗粒状内质网中被检测到。在 20 分钟内，新的蛋白质已经出现在高尔基体中，而在 1 到 2 小时内，蛋白质从细胞表面分泌出来。
Types of Vesicles Formed by the Golgi ApparatusSecretory Vesicles and Lysosomes. In a highly secretory cell, the vesicles formed by the Golgi apparatus are mainly secretory vesicles containing proteins that are secreted through the surface of the cell membrane. These secretory vesicles first diffuse to the cell membrane and then fuse with it and empty their substances to the exterior by the mechanism called exocytosis. Exocytosis, in most cases, is stimulated by entry of calcium ions into the cell. Calcium ions interact with the vesicular membrane and cause its fusion with the cell membrane, followed by exocytosis-opening of the membrane’s outer surface and extrusion of its contents outside the cell. Some vesicles, however, are destined for intracellular use.
高尔基体形成的囊泡类型：分泌囊泡和溶酶体。在高度分泌的细胞中，高尔基体形成的囊泡主要是含有通过细胞膜表面分泌的蛋白质的分泌囊泡。这些分泌囊泡首先扩散到细胞膜，然后与其融合，通过称为胞吐作用的机制将其物质排放到细胞外。在大多数情况下，胞吐作用是由钙离子进入细胞刺激的。钙离子与囊泡膜相互作用，导致其与细胞膜融合，随后发生胞吐作用——膜的外表面打开，内容物被挤出细胞外。然而，一些囊泡则是用于细胞内的。

Use of Intracellular Vesicles to Replenish Cellular
使用细胞内囊泡补充细胞

Membranes. Some intracellular vesicles formed by the Golgi apparatus fuse with the cell membrane or with the membranes of intracellular structures such as the mitochondria and even the endoplasmic reticulum. This fusion increases the expanse of these membranes and replenishes the membranes as they are used up. For example, the cell membrane loses much of its substance every time it forms a phagocytic or pinocytotic vesicle, and the vesicular membranes of the Golgi apparatus continually replenish the cell membrane.
In summary, the membranous system of the endoplasmic reticulum and Golgi apparatus are highly metabolic and capable of forming new intracellular structures and secretory substances to be extruded from the cell.
总之，内质网和高尔基体的膜系统具有高度的代谢能力，能够形成新的细胞内结构和分泌物，以便从细胞中排出。

THE MITOCHONDRIA EXTRACT ENERGY FROM NUTRIENTS
线粒体从营养物质中提取能量

The principal substances from which cells extract energy are foods that react chemically with oxygen-carbohydrates, fats, and proteins. In the human body, essentially all carbohydrates are converted into glucose by the digestive tract and liver before they reach the other cells of the body. Similarly, proteins are converted into amino acids, and fats are converted into fatty acids. Figure 2-15 shows oxygen and the foodstuffs-glucose, fatty acids, and amino acids-all entering the cell. Inside the cell, they react chemically with oxygen under the influence of enzymes that control the reactions and channel the energy released in the proper direction. The details of all these digestive and metabolic functions are provided in Chapters 63 through 73.
细胞提取能量的主要物质是与氧发生化学反应的食物——碳水化合物、脂肪和蛋白质。在人体内，几乎所有的碳水化合物在到达身体其他细胞之前，都会被消化道和肝脏转化为葡萄糖。同样，蛋白质被转化为氨基酸，脂肪被转化为脂肪酸。图 2-15 显示了氧气和食物成分——葡萄糖、脂肪酸和氨基酸——都进入细胞。在细胞内，它们在酶的影响下与氧气发生化学反应，酶控制反应并将释放的能量引导到正确的方向。所有这些消化和代谢功能的详细信息在第 63 到 73 章中提供。
Briefly, almost all these oxidative reactions occur inside the mitochondria, and the energy that is released is used to form the high-energy compound ATP. Then, ATP, not the original food, is used throughout the cell to energize almost all the subsequent intracellular metabolic reactions.
简而言之，几乎所有这些氧化反应都发生在线粒体内，释放的能量用于形成高能化合物 ATP。然后，ATP 而不是原始食物被细胞内的几乎所有后续代谢反应所利用。
Figure 2-15. Formation of adenosine triphosphate (ATP) in the cell showing that most of the ATP is formed in the mitochondria. (ADP, Adenosine diphosphate; CoA, coenzyme A.)
图 2-15. 细胞中三磷酸腺苷（ATP）的形成，显示大部分 ATP 是在线粒体中形成的。（ADP，腺苷二磷酸；CoA，辅酶 A。）

Functional Characteristics of Adenosine Triphosphate
腺苷三磷酸的功能特性

Nc1ncnc2c1ncn2C1OC2COP(=O)([O-])OP(=O)([O-])OP(=O)([O-])OCC2C1ONH2NNNNOOPOO-OPOO-OPOO-OOH
Adenosine triphosphate 三磷酸腺苷
ATP is a nucleotide composed of the following: (1) the nitrogenous base adenine; (2) the pentose sugar ribose; and (3) three phosphate radicals. The last two phosphate radicals are connected with the remainder of the molecule by high-energy phosphate bonds, which are represented in the formula shown by the symbol . Under the physical and chemical conditions of the body, each of these high-energy bonds contains about 12,000 calories of energy per mole of ATP, which is many times greater than the energy stored in the average chemical bond, thus giving rise to the term high-energy bond. Furthermore, the high-energy phosphate bond is very labile, so that it can be split instantly on demand whenever energy is required to promote other intracellular reactions.
ATP 是一种核苷酸，由以下成分组成：（1）氮碱基腺嘌呤；（2）五碳糖核糖；以及（3）三个磷酸基团。最后两个磷酸基团通过高能磷酸键与分子的其余部分相连，这在公式中用符号表示。在身体的物理和化学条件下，每个高能键每摩尔 ATP 大约含有 12,000 卡路里的能量，这远远超过平均化学键中储存的能量，因此产生了高能键这一术语。此外，高能磷酸键非常不稳定，因此可以在需要能量以促进其他细胞内反应时立即断裂。
When ATP releases its energy, a phosphoric acid radical is split away, and adenosine diphosphate (ADP) is formed. This released energy is used to energize many of the cell’s other functions, such as syntheses of substances and muscular contraction.
当 ATP 释放其能量时，一个磷酸根被分离，形成腺苷二磷酸（ADP）。释放的能量用于为细胞的许多其他功能提供能量，例如物质合成和肌肉收缩。
To reconstitute the cellular ATP as it is used up, energy derived from the cellular nutrients causes ADP and phosphoric acid to recombine to form new ATP, and the entire process is repeated over and over. For these reasons, ATP has been called the energy currency of the cell because it can be spent and reformed continually, having a turnover time of only a few minutes.
为了在细胞中重新合成 ATP，当 ATP 被消耗时，来自细胞营养物质的能量使 ADP 和磷酸重新结合形成新的 ATP，这个过程不断重复。因此，ATP 被称为细胞的能量货币，因为它可以不断被消耗和再生，周转时间仅为几分钟。
Chemical Processes in the Formation of ATP—Role of the Mitochondria. On entry into the cells, glucose is converted by enzymes in the cytoplasm into pyruvic acid (a process called glycolysis). A small amount of ADP is changed into ATP by the energy released during this conversion, but this amount accounts for less than 5 % 5 % 5%5 \% of the overall energy metabolism of the cell.
ATP 形成中的化学过程——线粒体的作用。当葡萄糖进入细胞时，细胞质中的酶将其转化为丙酮酸（这一过程称为糖酵解）。在这一转化过程中释放的能量使少量 ADP 转变为 ATP，但这一数量占细胞整体能量代谢的比例不足 5 % 5 % 5%5 \% 。
About 95 % 95 % 95%95 \% of the cell’s ATP formation occurs in the mitochondria. The pyruvic acid derived from carbohydrates, fatty acids from lipids, and amino acids from proteins is eventually converted into the compound acetyl-coenzyme A ( CoA ) A ( CoA ) A(CoA)A(\mathrm{CoA}) in the matrix of mitochondria. This substance, in turn, is further dissolved (for the purpose of extracting its energy) by another series of enzymes in the mitochondrion matrix, undergoing dissolution in a sequence of chemical reactions called the citric acid cycle, or Krebs cycle. These chemical reactions are so important that they are explained in detail in Chapter 68.
细胞的 ATP 形成约 95 % 95 % 95%95 \% 发生在线粒体中。来自碳水化合物的丙酮酸、来自脂质的脂肪酸以及来自蛋白质的氨基酸最终在线粒体基质中转化为化合物乙酰辅酶 A ( CoA ) A ( CoA ) A(CoA)A(\mathrm{CoA}) 。这种物质随后通过线粒体基质中的一系列酶进一步溶解（以提取其能量），经历一系列称为柠檬酸循环或克雷布斯循环的化学反应。这些化学反应非常重要，以至于在第 68 章中进行了详细解释。
In this citric acid cycle, acetyl-CoA is split into its component parts, hydrogen atoms and carbon dioxide. The carbon dioxide diffuses out of the mitochondria and eventually out of the cell; finally, it is excreted from the body through the lungs.
在这个柠檬酸循环中，乙酰辅酶 A 被分解成其组成部分，氢原子和二氧化碳。二氧化碳扩散出线粒体，最终离开细胞；最后，它通过肺部排出体外。
The hydrogen atoms, conversely, are highly reactive; they combine with oxygen that has also diffused into the mitochondria. This combination releases a tremendous amount of energy, which is used by mitochondria to convert large amounts of ADP to ATP. The processes of these reactions are complex, requiring the participation of many protein enzymes that are integral parts of mitochondrial membranous shelves that protrude into the mitochondrial matrix. The initial event is the removal of an electron from the hydrogen atom, thus converting it to a hydrogen ion. The terminal event is the combination of hydrogen ions with oxygen to form water and the release of large amounts of energy to globular proteins that protrude like knobs from the membranes of the mitochondrial shelves; these proteins are called ATP synthetase. Finally, the enzyme ATP synthetase uses the energy from the hydrogen ions to convert ADP to ATP. The newly formed ATP is transported out of the mitochondria into all parts of the cell cytoplasm and nucleoplasm, where it energizes multiple cell functions.
氢原子则高度活泼；它们与也扩散到线粒体中的氧结合。这种结合释放出大量能量，线粒体利用这些能量将大量 ADP 转化为 ATP。这些反应的过程复杂，涉及许多蛋白质酶的参与，这些酶是突入线粒体基质的线粒体膜架的组成部分。初始事件是从氢原子中去除一个电子，从而将其转化为氢离子。最终事件是氢离子与氧结合形成水，并释放大量能量给从线粒体膜架突出的球状蛋白质；这些蛋白质被称为 ATP 合成酶。最后，酶 ATP 合成酶利用氢离子的能量将 ADP 转化为 ATP。新形成的 ATP 被运输出线粒体，进入细胞质和细胞核质的各个部分，为多种细胞功能提供能量。
This overall process for formation of ATP is called the chemiosmotic mechanism of ATP formation. The chemical and physical details of this mechanism are presented
这种 ATP 形成的整体过程称为 ATP 形成的化学渗透机制。该机制的化学和物理细节已被呈现。
Figure 2-16. Use of adenosine triphosphate (ATP; formed in the mitochondrion) to provide energy for three major cellular functionsmembrane transport, protein synthesis, and muscle contraction. (ADP, Adenosine diphosphate.)
图 2-16. 使用三磷酸腺苷（ATP；在线粒体中形成）为三大主要细胞功能提供能量：膜运输、蛋白质合成和肌肉收缩。（ADP，二磷酸腺苷。）
in Chapter 68, and many of the detailed metabolic functions of ATP in the body are discussed in Chapters 68 through 72 .
在第 68 章中，ATP 在体内的许多详细代谢功能在第 68 章到第 72 章中进行了讨论。
Uses of ATP for Cellular Function. Energy from ATP is used to promote three major categories of cellular functions: (1) transport of substances through multiple cell membranes; (2) synthesis of chemical compounds throughout the cell; and (3) mechanical work. These uses of ATP are illustrated by the examples in Figure 2-16: (1) to supply energy for the transport of sodium through the cell membrane; (2) to promote protein synthesis by the ribosomes; and (3) to supply the energy needed during muscle contraction.
ATP 在细胞功能中的用途。ATP 提供的能量用于促进三大类细胞功能：(1) 通过多个细胞膜运输物质；(2) 在细胞内合成化合物；(3) 机械工作。这些 ATP 的用途通过图 2-16 中的例子进行说明：(1) 为钠通过细胞膜的运输提供能量；(2) 促进核糖体的蛋白质合成；(3) 在肌肉收缩过程中提供所需的能量。
In addition to the membrane transport of sodium, energy from ATP is required for the membrane transport of potassium, calcium, magnesium, phosphate, chloride, urate, and hydrogen ions and many other ions, as well as various organic substances. Membrane transport is so important to cell function that some cells-the renal tubular cells, for example-use as much as 80 % 80 % 80%80 \% of the ATP that they form for this purpose alone.
除了钠的膜运输外，钾、钙、镁、磷酸盐、氯化物、尿酸和氢离子以及许多其他离子的膜运输还需要 ATP 提供能量，此外还包括各种有机物质。膜运输对细胞功能至关重要，以至于某些细胞——例如肾小管细胞——为此目的使用了它们所形成的 ATP 的 80 % 80 % 80%80 \% 。
In addition to synthesizing proteins, cells make phospholipids, cholesterol, purines, pyrimidines, and many other substances. Synthesis of almost any chemical compound requires energy. For example, a single protein molecule might be composed of as many as several thousand amino acids attached to one another by peptide linkages. The formation of each of these linkages requires energy derived from the breakdown of four high-energy bonds; thus, many thousand ATP molecules must release their energy as each protein molecule is formed. Indeed, some cells use as much as 75 % 75 % 75%75 \% of all the ATP formed in the cell
除了合成蛋白质，细胞还制造磷脂、胆固醇、嘌呤、嘧啶和许多其他物质。几乎任何化学化合物的合成都需要能量。例如，一个单一的蛋白质分子可能由多达几千个氨基酸通过肽键连接在一起。形成每一个肽键都需要从四个高能键的分解中获得能量；因此，在每个蛋白质分子形成时，必须释放出许多千个 ATP 分子的能量。实际上，一些细胞使用的 ATP 量可能高达 75 % 75 % 75%75 \% 。
Figure 2-17. Ameboid motion by a cell.
图 2-17. 细胞的变形虫运动。
simply to synthesize new chemical compounds, especially protein molecules; this is particularly true during the growth phase of cells.
简单地合成新的化合物，特别是蛋白质分子；在细胞的生长阶段，这一点尤其明显。
Another use of ATP is to supply energy for special cells to perform mechanical work. We discuss in Chapter 6 that each contraction of a muscle fiber requires the expenditure of large quantities of ATP energy. Other cells perform mechanical work in other ways, especially by ciliary and ameboid motion, described later in this chapter. The source of energy for all these types of mechanical work is ATP.
ATP 的另一个用途是为特殊细胞提供能量以进行机械工作。我们在第六章中讨论到，每次肌肉纤维的收缩都需要消耗大量的 ATP 能量。其他细胞以其他方式进行机械工作，特别是通过纤毛和变形虫运动，后面将在本章中描述。所有这些机械工作的能量来源都是 ATP。
In summary, ATP is readily available to release its energy rapidly wherever it is needed in the cell. To replace ATP used by the cell, much slower chemical reactions break down carbohydrates, fats, and proteins and use the energy derived from these processes to form new ATP. More than 95 % 95 % 95%95 \% of this ATP is formed in the mitochondria, which is why the mitochondria are called the powerhouses of the cell.
总之，ATP 可以迅速释放能量，随时在细胞中所需的地方使用。为了补充细胞消耗的 ATP，较慢的化学反应分解碳水化合物、脂肪和蛋白质，并利用这些过程产生的能量形成新的 ATP。超过 95 % 95 % 95%95 \% 的 ATP 是在线粒体中形成的，这就是为什么线粒体被称为细胞的动力源。

LOCOMOTION OF CELLS 细胞的运动

The most obvious type of movement in the body is that which occurs in skeletal, cardiac, and smooth muscle cells, which constitute almost 50 % 50 % 50%50 \% of the entire body mass. The specialized functions of these cells are discussed in Chapters 6 through 9. Two other types of movement-ameboid locomotion and ciliary movement-occur in other cells.
身体中最明显的运动类型是发生在骨骼、心脏和平滑肌细胞中的运动，这些细胞几乎占据了整个身体质量的 50 % 50 % 50%50 \% 。这些细胞的专门功能将在第 6 到第 9 章中讨论。还有两种其他类型的运动——变形虫运动和纤毛运动——发生在其他细胞中。

AMEBOID MOVEMENT 变形运动

Ameboid movement is a crawling-like movement of an entire cell in relation to its surroundings, such as movement of white blood cells through tissues. This type of movement gets its name from the fact that amebae move in this manner, and amebae have provided an excellent tool for studying the phenomenon.
变形虫运动是细胞相对于其周围环境的爬行式运动，例如白血细胞在组织中的移动。这种运动得名于变形虫以这种方式移动的事实，而变形虫为研究这一现象提供了极好的工具。
Typically, ameboid locomotion begins with the protrusion of a pseudopodium from one end of the cell. The pseudopodium projects away from the cell body and partially secures itself in a new tissue area; then the remainder of the cell is pulled toward the pseudopodium. Figure 2-17
通常，变形虫运动始于细胞一端伪足的突出。伪足从细胞体向外延伸，并部分固定在新的组织区域；然后细胞的其余部分被拉向伪足。图 2-17
demonstrates this process, showing an elongated cell, the right-hand end of which is a protruding pseudopodium. The membrane of this end of the cell is continually moving forward, and the membrane at the left-hand end of the cell is continually following along as the cell moves.
展示了这个过程，显示了一个细长的细胞，右端是一个突出的伪足。细胞这一端的膜不断向前移动，而细胞左端的膜则在细胞移动时不断跟随。
Mechanism of Ameboid Locomotion. Figure 2-17 shows the general principle of ameboid motion. Basically, this results from the continual formation of new cell membrane at the leading edge of the pseudopodium and continual absorption of the membrane in the mid and rear portions of the cell. Two other effects are also essential for forward movement of the cell. The first is attachment of the pseudopodium to surrounding tissues so that it becomes fixed in its leading position while the remainder of the cell body is being pulled forward toward the point of attachment. This attachment is caused by receptor proteins that line the insides of exocytotic vesicles. When the vesicles become part of the pseudopodial membrane, they open so that their insides evert to the outside, and the receptors now protrude to the outside and attach to ligands in the surrounding tissues.
变形虫运动机制。图 2-17 显示了变形运动的一般原理。基本上，这种运动是由于伪足前缘不断形成新的细胞膜，以及细胞中部和后部不断吸收膜。细胞向前移动还需要另外两个重要因素。第一个是伪足与周围组织的附着，使其在前进位置固定，而细胞体的其余部分则向附着点拉动。这种附着是由排列在外排泡内侧的受体蛋白引起的。当泡体成为伪足膜的一部分时，它们会打开，使内部翻转到外部，受体现在突出到外部并附着在周围组织的配体上。
At the opposite end of the cell, the receptors pull away from their ligands and form new endocytotic vesicles. Then, inside the cell, these vesicles stream toward the pseudopodial end of the cell, where they are used to form new membrane for the pseudopodium.
在细胞的另一端，受体与其配体分离并形成新的内吞囊泡。然后，在细胞内部，这些囊泡朝向伪足端流动，在那里它们被用来形成伪足的新膜。
The second essential effect for locomotion is to provide the energy required to pull the cell body in the direction of the pseudopodium. A moderate to large amount of the protein actin is in the cytoplasm of all cells. Much of the actin is in the form of single molecules that do not provide any motive power; however, these molecules polymerize to form a filamentous network, and the network contracts when it binds with an actin-binding protein such as myosin. The entire process is energized by the high-energy compound ATP. This is what occurs in the pseudopodium of a moving cell, where such a network of actin filaments forms anew inside the enlarging pseudopodium. Contraction also occurs in the ectoplasm of the cell body, where a preexisting actin network is already present beneath the cell membrane.
运动的第二个基本效果是提供所需的能量，以便将细胞体拉向伪足。所有细胞的细胞质中都有适量到大量的蛋白质肌动蛋白。大部分肌动蛋白以单个分子的形式存在，这些分子并不提供任何动力；然而，这些分子聚合形成丝状网络，当它与肌动蛋白结合蛋白（如肌球蛋白）结合时，网络会收缩。整个过程由高能化合物 ATP 提供能量。这就是在移动细胞的伪足中发生的情况，在那里，肌动蛋白丝状网络在不断扩大的伪足内部重新形成。收缩也发生在细胞体的外质中，那里细胞膜下已经存在一个预先存在的肌动蛋白网络。

Types of Cells That Exhibit Ameboid Locomotion.
表现变形虫运动的细胞类型。

The most common cells to exhibit ameboid locomotion in the human body are the white blood cells when they move out of the blood into the tissues to form tissue macrophages. Other types of cells can also move by ameboid locomotion under certain circumstances. For example, fibroblasts move into a damaged area to help repair the damage, and even the germinal cells of the skin, although ordinarily completely sessile cells, move toward a cut area to repair the opening. Cell locomotion is also especially important in the development of the embryo and fetus after fertilization of an ovum. For example, embryonic cells often must migrate long distances from their sites of origin to new areas during the development of special structures.
在人体中，最常见的表现出变形虫运动的细胞是白血球，当它们从血液中移动到组织中形成组织巨噬细胞时。其他类型的细胞在某些情况下也可以通过变形虫运动移动。例如，成纤维细胞会移动到受损区域以帮助修复损伤，甚至皮肤的生殖细胞，尽管通常是完全固定的细胞，也会向切口区域移动以修复开口。细胞运动在受精卵受精后胚胎和胎儿的发展中也特别重要。例如，胚胎细胞在特殊结构发育过程中，通常必须从其起源地迁移到新的区域。
Some types of cancer cells, such as sarcomas, which arise from connective tissue cells, are especially proficient at ameboid movement. This partially accounts for their relatively rapid spreading from one part of the body to another, known as metastasis.
某些类型的癌细胞，如起源于结缔组织细胞的肉瘤，特别擅长变形虫运动。这在一定程度上解释了它们从身体一部分迅速扩散到另一部分的相对快速传播，称为转移。
Control of Ameboid Locomotion-Chemotaxis. An important initiator of ameboid locomotion is the process called chemotaxis, which results from the appearance of certain chemical substances in the tissues. Any chemical substance that causes chemotaxis to occur is called a chemotactic substance. Most cells that exhibit ameboid locomotion move toward the source of a chemotactic substance-that is, from an area of lower concentration toward an area of higher concentration. This is called positive chemotaxis. Some cells move away from the source, which is called negative chemotaxis.
变形虫运动的控制-趋化性。变形虫运动的重要启动因素是称为趋化性的过程，这一过程是由于某些化学物质在组织中的出现所引起的。任何导致趋化性发生的化学物质称为趋化物质。大多数表现出变形虫运动的细胞朝向趋化物质的来源移动，即从低浓度区域向高浓度区域移动。这被称为正趋化性。有些细胞则远离来源，这被称为负趋化性。
How does chemotaxis control the direction of ameboid locomotion? Although the answer is not certain, it is known that the side of the cell most exposed to the chemotactic substance develops membrane changes that cause pseudopodial protrusion.
化学趋向是如何控制变形虫运动方向的？虽然答案尚不确定，但已知细胞最暴露于化学趋向物质的一侧会发生膜变化，从而导致伪足突出。

CILIA AND CILIARY MOVEMENTS
纤毛和纤毛运动

There are two types of cilia, motile and nonmotile, or primary, cilia. Motile cilia can undergo a whiplike movement on the surfaces of cells. This movement occurs mainly in two places in the human body, on the surfaces of the respiratory airways and on the inside surfaces of the uterine tubes (fallopian tubes) of the reproductive tract. In the nasal cavity and lower respiratory airways, the whiplike motion of motile cilia causes a layer of mucus to move at a rate of about 1 cm / min 1 cm / min 1cm//min1 \mathrm{~cm} / \mathrm{min} toward the pharynx, in this way continually clearing these passageways of mucus and particles that have become trapped in the mucus. In the uterine tubes, cilia cause slow movement of fluid from the ostium of the uterine tube toward the uterus cavity; this movement of fluid transports the ovum from the ovary to the uterus.
有两种类型的纤毛，运动性纤毛和非运动性纤毛或原纤毛。运动性纤毛可以在细胞表面进行鞭状运动。这种运动主要发生在人体的两个地方：呼吸道的表面和生殖道的输卵管内表面。在鼻腔和下呼吸道，运动性纤毛的鞭状运动使一层粘液以约 1 cm / min 1 cm / min 1cm//min1 \mathrm{~cm} / \mathrm{min} 的速度向咽部移动，从而不断清除这些通道中的粘液和被粘液困住的颗粒。在输卵管中，纤毛使液体从输卵管的开口缓慢移动到子宫腔；这种液体的运动将卵子从卵巢运输到子宫。
As shown in Figure 2-18, a cilium has the appearance of a sharp-pointed straight or curved hair that projects 2 to 4 micrometers from the surface of the cell. Often, many motile cilia project from a single cell-for example, as many as 200 cilia on the surface of each epithelial cell inside the respiratory passageways. The cilium is covered by an outcropping of the cell membrane, and it is supported by 11 microtubules-nine double tubules located around the periphery of the cilium and two single tubules down the center, as demonstrated in the cross section shown in Figure 2-18. Each cilium is an outgrowth of a structure that lies immediately beneath the cell membrane, called the basal body of the cilium.
如图 2-18 所示，纤毛呈现出尖锐的直发或弯曲的外观，从细胞表面突出 2 到 4 微米。通常，许多运动纤毛从单个细胞中突出——例如，在呼吸道内，每个上皮细胞表面上可有多达 200 根纤毛。纤毛被细胞膜的突起覆盖，并由 11 根微管支撑——9 根双微管位于纤毛的周边，2 根单微管位于中心，如图 2-18 所示的横截面所示。每根纤毛是位于细胞膜正下方的结构的突起，称为纤毛的基体。
The flagellum of a sperm is similar to a motile cilium; in fact, it has much the same type of structure and same type of contractile mechanism. The flagellum, however, is much longer and moves in quasisinusoidal waves instead of whiplike movements.
精子的鞭毛类似于运动纤毛；实际上，它们的结构类型和收缩机制非常相似。然而，鞭毛要长得多，并且以准正弦波的方式运动，而不是像鞭子一样的运动。
Figure 2-18. Structure and function of the cilium. (Modified from Satir P: Cilia. Sci Am 204:108, 1961.)
图 2-18. 纤毛的结构和功能。（改编自 Satir P: Cilia. Sci Am 204:108, 1961。）
In the inset of Figure 2-18, movement of the motile cilium is shown. The cilium moves forward with a sudden, rapid whiplike stroke 10 to 20 times per second, bending sharply where it projects from the surface of the cell. Then it moves backward slowly to its initial position. The rapid, forward-thrusting, whiplike movement pushes the fluid lying adjacent to the cell in the direction that the cilium moves; the slow dragging movement in the backward direction has almost no effect on fluid movement. As a result, the fluid is continually propelled in the direction of the fast-forward stroke. Because most motile ciliated cells have large numbers of cilia on their surfaces, and because all the cilia are oriented in the same direction, this is an effective means for moving fluids from one part of the surface to another.
在图 2-18 的插图中，显示了运动纤毛的运动。纤毛以每秒 10 到 20 次的突然快速鞭状击打向前移动，在从细胞表面突出的位置急剧弯曲。然后它缓慢地向后移动到初始位置。快速的向前推进的鞭状运动将紧邻细胞的液体推向纤毛移动的方向；而向后的缓慢拖动几乎对液体运动没有影响。因此，液体不断被推动向快速向前击打的方向。由于大多数运动纤毛细胞的表面有大量纤毛，并且所有纤毛都朝同一方向排列，这是一种有效的方式，将液体从表面的一个部分移动到另一个部分。
Mechanism of Ciliary Movement. Although not all aspects of ciliary movement are known, we are aware of the following elements. First, the nine double tubules and two single tubules are all linked to one another by a complex of protein cross-linkages; this total complex of tubules and cross-linkages is called the axoneme. Second, even after removal of the membrane and destruction of other elements of the cilium in addition to the axoneme, the cilium can still beat under appropriate conditions. Third, two conditions are necessary for continued beating of the axoneme after removal of the other structures of the cilium: (1) the availability of ATP; and (2) appropriate ionic conditions, especially appropriate concentrations of magnesium and calcium. Fourth, during forward motion of the cilium, the double tubules on the front edge of the cilium slide outward toward the tip of the cilium, whereas those on the back edge remain in place. Fifth, multiple protein arms composed of the protein dynein, which has adenosine triphosphatase (ATPase) enzymatic activity, project from each double tubule toward an adjacent double tubule.
纤毛运动机制。尽管纤毛运动的所有方面尚不完全清楚，但我们知道以下几个要素。首先，九对双管和两根单管通过复杂的蛋白质交联相互连接；这个由管道和交联组成的整体被称为轴突。其次，即使在去除膜和破坏纤毛其他元素（除了轴突）后，纤毛在适当条件下仍然可以继续摆动。第三，去除纤毛其他结构后，轴突持续摆动需要两个条件：（1）ATP 的可用性；（2）适当的离子条件，特别是适当的镁和钙浓度。第四，在纤毛向前运动时，纤毛前缘的双管向外滑动至纤毛尖端，而后缘的双管则保持不动。第五，由蛋白质动力蛋白组成的多个蛋白臂从每个双管向相邻的双管伸出，动力蛋白具有腺苷三磷酸酶（ATPase）酶活性。
Given this basic information, it has been determined that the release of energy from ATP in contact with the ATPase dynein arms causes the heads of these arms to “crawl” rapidly along the surface of the adjacent double tubule. If the front tubules crawl outward while the back tubules remain stationary, bending occurs.
根据这些基本信息，已确定与 ATP 酶动力蛋白臂接触时，ATP 释放的能量使这些臂的头部迅速沿着相邻双管的表面“爬行”。如果前面的管向外爬行，而后面的管保持静止，就会发生弯曲。
The way in which cilia contraction is controlled is not well understood. The cilia of some genetically abnormal cells do not have the two central single tubules, and these cilia fail to beat. Therefore, it is presumed that some signal, perhaps an electrochemical signal, is transmitted along these two central tubules to activate the dynein arms.
纤毛收缩的控制方式尚不清楚。一些基因异常细胞的纤毛没有两个中央单管，这些纤毛无法摆动。因此，推测某种信号，可能是电化学信号，通过这两个中央单管传递，以激活动力蛋白臂。
Nonmotile Primary Cilia Serve as Cell Sensory “Antennae.” Primary cilia are nonmotile and generally occur only as a single cilium on each cell. Although the physiological functions of primary cilia are not fully understood, current evidence indicates that they function as cellular “sensory antennae,” which coordinate cellular signaling pathways involved in chemical and mechanical sensation, signal transduction, and cell growth. In the kidneys, for example, primary cilia are found in most epithelial cells of the tubules, projecting into the tubule lumen and acting as a flow sensor. In response to fluid flow over the tubular epithelial cells, the primary cilia bend and cause flow-induced changes in intracellular calcium signaling. These signals, in turn, initiate multiple effects on the cells. Defects in signaling by primary cilia in renal tubular epithelial cells are thought to contribute to various disorders, including the development of large fluid-filled cysts, a condition called polycystic kidney disease.
非运动性初级纤毛作为细胞感知“天线”。初级纤毛是非运动性的，通常每个细胞仅有一个。尽管初级纤毛的生理功能尚未完全理解，但目前的证据表明，它们作为细胞的“感知天线”，协调参与化学和机械感知、信号转导及细胞生长的细胞信号通路。例如，在肾脏中，初级纤毛存在于大多数肾小管上皮细胞中，突入肾小管腔，充当流动传感器。作为对流体流动的反应，初级纤毛弯曲并引起细胞内钙信号的流动诱导变化。这些信号反过来又对细胞产生多重影响。初级纤毛在肾小管上皮细胞中的信号传导缺陷被认为与多种疾病有关，包括大液体囊肿的形成，这种情况被称为多囊肾病。

Bibliography 参考书目

Alberts B, Johnson A, Lewis J, et al: Molecular Biology of the Cell, 6th ed. New York: Garland Science, 2014.
阿尔伯茨 B, 约翰逊 A, 刘易斯 J, 等: 细胞的分子生物学, 第 6 版. 纽约: 加兰科学, 2014.
Brandizzi F, Barlowe C: Organization of the ER-Golgi interface for membrane traffic control. Nat Rev Mol Cell Biol 14:382, 2013
Brandizzi F, Barlowe C: 膜交通控制的内质网-高尔基体界面的组织。自然评论分子细胞生物学 14:382, 2013
Dikic I, Elazar Z. Mechanism and medical implications of mammalian autophagy. Nat Rev Mol Cell Biol 19:349, 2018.
Dikic I, Elazar Z. 哺乳动物自噬的机制及其医学意义。自然评论分子细胞生物学 19:349, 2018.
Eisner V, Picard M, Hajnóczky G. Mitochondrial dynamics in adaptive and maladaptive cellular stress responses. Nat Cell Biol 20:755, 2018.
Eisner V, Picard M, Hajnóczky G. 线粒体动态在适应性和不适应性细胞压力反应中的作用。自然细胞生物学 20:755, 2018.
Galluzzi L, Yamazaki T, Kroemer G. Linking cellular stress responses to systemic homeostasis. Nat Rev Mol Cell Biol 19:731, 2018.
Galluzzi L, Yamazaki T, Kroemer G. 将细胞应激反应与系统稳态联系起来。Nat Rev Mol Cell Biol 19:731, 2018.
Guerriero CJ, Brodsky JL: The delicate balance between secreted protein folding and endoplasmic reticulum-associated degradation in human physiology. Physiol Rev 92:537, 2012.
Guerriero CJ, Brodsky JL: 人体生理中分泌蛋白折叠与内质网相关降解之间的微妙平衡。生理学评论 92:537, 2012.
Harayama T, Riezman H. Understanding the diversity of membrane lipid composition. Nat Rev Mol Cell Biol 19:281, 2018.
原文：Harayama T, Riezman H. 理解膜脂质组成的多样性。Nat Rev Mol Cell Biol 19:281, 2018.
Insall R: The interaction between pseudopods and extracellular signalling during chemotaxis and directed migration. Curr Opin Cell Biol 25:526, 2013.
Insall R: 假足与细胞外信号在趋化性和定向迁移中的相互作用。当前细胞生物学意见 25:526, 2013.
Kaksonen M, Roux A. Mechanisms of clathrin-mediated endocytosis. Nat Rev Mol Cell Biol 19:313, 2018.
Kaksonen M, Roux A. 胞吞作用中网格蛋白介导的机制。自然评论分子细胞生物学 19:313, 2018.
Lawrence RE, Zoncu R. The lysosome as a cellular centre for signalling, metabolism and quality control. Nat Cell Biol 21: 133, 2019.
劳伦斯 RE, 佐恩库 R. 溶酶体作为细胞信号传导、代谢和质量控制的中心. 自然细胞生物学 21: 133, 2019.
Nakamura N, Wei JH, Seemann J: Modular organization of the mammalian Golgi apparatus. Curr Opin Cell Biol 24:467, 2012.
中村 N, 魏 JH, 西曼 J: 哺乳动物高尔基体的模块化组织. 细胞生物学当前观点 24:467, 2012.
Palikaras K, Lionaki E, Tavernarakis N. Mechanisms of mitophagy in cellular homeostasis, physiology and pathology. Nat Cell Biol 20:1013, 2018.
Palikaras K, Lionaki E, Tavernarakis N. 细胞稳态、生理和病理中的线粒体自噬机制。自然细胞生物学 20:1013, 2018.
Sezgin E, Levental I, Mayor S, Eggeling C. The mystery of membrane organization: composition, regulation and roles of lipid rafts. Nat Rev Mol Cell Biol 18:361, 2017.
Sezgin E, Levental I, Mayor S, Eggeling C. 膜组织的奥秘：脂筏的组成、调控和作用。自然评论分子细胞生物学 18:361, 2017.
Spinelli JB, Haigis MC. The multifaceted contributions of mitochondria to cellular metabolism. Nat Cell Biol. 20:745, 2018.
斯皮内利 JB, 海吉斯 MC. 线粒体对细胞代谢的多方面贡献. 自然细胞生物学. 20:745, 2018.
Walker CL, Pomatto LCD, Tripathi DN, Davies KJA. Redox regulation of homeostasis and proteostasis in peroxisomes. Physiol Rev 98:89, 2018.
沃克 CL, 波马托 LCD, 特里帕提 DN, 戴维斯 KJA. 过氧化物酶体中的稳态和蛋白质稳态的氧化还原调节. 生理学评论 98:89, 2018.
Zhou K, Gaullier G, Luger K. Nucleosome structure and dynamics are coming of age. Nat Struct Mol Biol 26:3, 2019.
周凯, 高利耶 G, 卢格 K. 核小体结构与动态正逐渐成熟. 自然结构与分子生物学 26:3, 2019.

Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
蛋白质合成、细胞功能和细胞繁殖的遗传控制

Genes, which are located in the nuclei of all cells of the body, control heredity from parents to children, as well as the daily functioning of all the body’s cells. The genes control cell function by determining which structures, enzymes, and chemicals are synthesized within the cell.
基因位于身体所有细胞的细胞核中，控制着从父母到孩子的遗传以及身体所有细胞的日常功能。基因通过决定细胞内合成哪些结构、酶和化学物质来控制细胞功能。
Figure 3-1 shows the general schema of genetic control. Each gene, which is composed of deoxyribonucleic acid (DNA), controls the formation of another nucleic acid, ribonucleic acid (RNA); this RNA then spreads throughout the cell to control formation of a specific protein. The entire process, from transcription of the genetic code in the nucleus to translation of the RNA code and the formation of proteins in the cell cytoplasm, is often referred to as gene expression.
图 3-1 显示了遗传控制的一般模式。每个由脱氧核糖核酸（DNA）组成的基因控制另一种核酸，即核糖核酸（RNA）的形成；这种 RNA 随后在细胞内传播，以控制特定蛋白质的形成。整个过程，从细胞核中遗传密码的转录到 RNA 密码的翻译以及细胞质中蛋白质的形成，通常被称为基因表达。
Because the human body has approximately 20,000 to 25,000 different genes that code for proteins in each cell, it is possible to form a large number of different cellular proteins. In fact, RNA molecules transcribed from the same segment of DNA-the same gene-can be processed in more than one way by the cell, giving rise to alternate versions of the protein. The total number of different proteins produced by the various cell types in humans is estimated to be at least 100,000.
由于人体每个细胞大约有 20,000 到 25,000 个不同的基因编码蛋白质，因此可以形成大量不同的细胞蛋白质。事实上，从同一段 DNA（同一基因）转录的 RNA 分子可以通过细胞以多种方式进行处理，从而产生蛋白质的不同版本。估计人类各种细胞类型产生的不同蛋白质总数至少为 100,000 种。
Some of the cellular proteins are structural proteins, which, in association with various lipids and carbohydrates, form structures of the various intracellular organelles discussed in Chapter 2. However, most of the proteins are enzymes that catalyze different chemical reactions in the cells. For example, enzymes promote all the oxidative reactions that supply energy to the cell, along with synthesis of all the cell chemicals, such as lipids, glycogen, and adenosine triphosphate (ATP).
一些细胞蛋白质是结构蛋白，它们与各种脂质和碳水化合物结合，形成第 2 章讨论的各种细胞内细胞器的结构。然而，大多数蛋白质是催化细胞内不同化学反应的酶。例如，酶促进所有提供能量给细胞的氧化反应，以及合成所有细胞化学物质，如脂质、糖原和三磷酸腺苷（ATP）。

CELL NUCLEUS GENES CONTROL PROTEIN SYNTHESIS
细胞核基因控制蛋白质合成

In the cell nucleus, large numbers of genes are attached end on end in extremely long, double-stranded helical molecules of DNA having molecular weights measured in the billions. A very short segment of such a molecule is shown in Figure 3 − 2 3 − 2 3-23-2. This molecule is composed of several simple chemical compounds bound together in a regular pattern, the details of which are explained in the next few paragraphs.
在细胞核中，大量基因以极长的双链螺旋 DNA 分子端对端连接，分子量以十亿计。图 3 − 2 3 − 2 3-23-2 中显示了这样一个分子的一小段。该分子由几个简单的化合物以规律的模式结合在一起，具体细节将在接下来的几段中解释。

Building Blocks of DNA DNA 的基本构件

Figure 3-3 shows the basic chemical compounds involved in the formation of DNA. These compounds include the following: (1) phosphoric acid; (2) a sugar called deoxyribose; and (3) four nitrogenous bases (two purines, adenine and guanine, and two pyrimidines, thymine and cytosine). The phosphoric acid and deoxyribose form the two helical strands that are the backbone of the DNA molecule, and the nitrogenous bases lie between the two strands and connect them, as illustrated in Figure 3-2.
图 3-3 显示了参与 DNA 形成的基本化学化合物。这些化合物包括以下几种：(1) 磷酸；(2) 一种叫做脱氧核糖的糖；以及(3) 四种氮碱基（两种嘌呤，腺嘌呤和鸟嘌呤，以及两种嘧啶，胸腺嘧啶和胞嘧啶）。磷酸和脱氧核糖形成 DNA 分子的两个螺旋链，构成其骨架，而氮碱基位于两个链之间并将它们连接起来，如图 3-2 所示。

Nucleotides 核苷酸

The first stage of DNA formation is to combine one molecule of phosphoric acid, one molecule of deoxyribose, and one of the four bases to form an acidic nucleotide. Four separate nucleotides are thus formed, one for each of the four bases: deoxyadenylic, deoxythymidylic, deoxyguanylic, and deoxycytidylic acids. Figure 3-4 shows the chemical
DNA 形成的第一阶段是将一个磷酸分子、一个脱氧核糖分子和四种碱基中的一种结合在一起，形成一个酸性核苷酸。因此形成了四种独立的核苷酸，每种对应一种碱基：脱氧腺苷酸、脱氧胸苷酸、脱氧鸟苷酸和脱氧胞苷酸。图 3-4 显示了化学结构。
Figure 3-1 The general schema whereby genes control cell function. m R N A m R N A mRNAm R N A, Messenger RNA.
图 3-1 基因控制细胞功能的一般模式。 m R N A m R N A mRNAm R N A ，信使 RNA。

structure of deoxyadenylic acid, and Figure 3-5 shows simple symbols for the four nucleotides that form DNA.
脱氧腺苷酸的结构，图 3-5 显示了形成 DNA 的四种核苷酸的简单符号。

Nucleotides Are Organized to Form Two Strands of DNA Loosely Bound to Each Other
核苷酸被组织成两条松散结合的 DNA 链

Figure 3-2 shows the manner in which multiple nucleotides are bound together to form two strands of DNA. The two strands are, in turn, loosely bonded with each other by weak cross-linkages, as illustrated in Figure 3-6 by the central dashed lines. Note that the backbone of each DNA strand is composed of alternating phosphoric acid and deoxyribose molecules. In turn, purine and pyrimidine bases are attached to the sides of the deoxyribose molecules. Then, by means of loose hydrogen bonds (dashed
图 3-2 显示了多个核苷酸结合在一起形成两条 DNA 链的方式。这两条链又通过弱的交联以松散的方式相互结合，如图 3-6 中的中心虚线所示。请注意，每条 DNA 链的骨架由交替的磷酸和脱氧核糖分子组成。接着，嘌呤和嘧啶碱基附着在脱氧核糖分子的两侧。然后，通过松散的氢键（虚线）连接。
Figure 3-4. Deoxyadenylic acid, one of the nucleotides that make up DNA.
图 3-4. 脱氧腺苷酸，构成 DNA 的核苷酸之一。
Deoxyadenylic acid 脱氧腺苷酸
Deoxyguanylic acid 脱氧鸟苷酸
Deoxythymidylic acid 脱氧胸苷酸
Deoxycytidylic acid 脱氧胞苷酸
Figure 3-5. Symbols for the four nucleotides that combine to form DNA. Each nucleotide contains phosphoric acid §, deoxyribose (D), and one of the four nucleotide bases: adenine (A); thymine (T); guanine (G); or cytosine ©. lines) between the purine and pyrimidine bases, the two respective DNA strands are held together. Note the following caveats, however:
图 3-5。组成 DNA 的四种核苷酸的符号。每个核苷酸包含磷酸§、脱氧核糖(D)和四种核苷酸碱基之一：腺嘌呤(A)；胸腺嘧啶(T)；鸟嘌呤(G)；或胞嘧啶(C)。在嘌呤和嘧啶碱基之间的（线）连接着两条相应的 DNA 链。然而，请注意以下警告：
  1. Each purine base adenine of one strand always bonds with a pyrimidine base thymine of the other strand.
    每条链上的嘌呤碱基腺嘌呤总是与另一条链上的嘧啶碱基胸腺嘧啶结合。
  2. Each purine base guanine always bonds with a pyrimidine base cytosine.
    每个嘌呤碱基鸟嘌呤总是与一个嘧啶碱基胞嘧啶结合。
    Thus, in Figure 3-6, the sequence of complementary pairs of bases is CG, CG, GC, TA, CG, TA, GC, AT, and AT. Because of the looseness of the hydrogen bonds, the two strands can pull apart with ease, and they do so many times during the course of their function in the cell.
    因此，在图 3-6 中，互补碱基对的顺序是 CG、CG、GC、TA、CG、TA、GC、AT 和 AT。由于氢键的松弛，这两条链可以轻松分开，并且在细胞功能的过程中，它们会多次这样做。
To put the DNA of Figure 3-6 into its proper physical perspective, one could merely pick up the two ends and twist them into a helix. Ten pairs of nucleotides are present in each full turn of the helix in the DNA molecule.
为了将图 3-6 中的 DNA 放入适当的物理视角，只需抓住两端并将其扭成螺旋。每个 DNA 分子的完整螺旋圈中有十对核苷酸。

GENETIC CODE 遗传密码

The importance of DNA lies in its ability to control the formation of proteins in the cell, which it achieves by means of a genetic code. That is, when the two strands of a DNA molecule are split apart, the purine and pyrimidine bases projecting to the side of each DNA strand are exposed, as shown by the top strand in Figure 3-7. It is these projecting bases that form the genetic code.
DNA 的重要性在于它能够控制细胞内蛋白质的形成，这通过遗传密码实现。也就是说，当 DNA 分子的两条链分开时，突出的嘌呤和嘧啶碱基暴露在每条 DNA 链的侧面，如图 3-7 的上方链所示。正是这些突出的碱基形成了遗传密码。
The genetic code consists of successive “triplets” of bases-that is, each three successive bases is a code word. The successive triplets eventually control the sequence of amino acids in a protein molecule that is to be synthesized in the cell. Note in Figure 3-6 that the top strand of DNA, reading from left to right, has the genetic code GGC, AGA, CTT, with the triplets being separated from one another by the arrows. As we follow this genetic code through Figure 3-7 and Figure 3-8, we see that these three respective triplets are responsible for successive placement of the three amino acids, proline, serine, and glutamic acid, in a newly formed molecule of protein.
遗传密码由连续的“三联体”碱基组成——也就是说，每三个连续的碱基是一个密码子。这些连续的三联体最终控制细胞中合成的蛋白质分子中氨基酸的顺序。请注意图 3-6，DNA 的上链从左到右读取，遗传密码为 GGC、AGA、CTT，三联体之间用箭头分隔。当我们通过图 3-7 和图 3-8 跟踪这个遗传密码时，我们看到这三个相应的三联体负责在新形成的蛋白质分子中依次放置三个氨基酸：脯氨酸、丝氨酸和谷氨酸。

TRANSCRIPTION-TRANSFER OF CELL NUCLEUS DNA CODE TO CYTOPLASM RNA CODE
转录-细胞核 DNA 代码转移到细胞质 RNA 代码

Because DNA is located in the cell nucleus, yet most of the cell functions are carried out in the cytoplasm, there must be some means for DNA genes of the nucleus to control chemical reactions of the cytoplasm. This control
因为 DNA 位于细胞核中，而大多数细胞功能是在细胞质中进行的，因此细胞核中的 DNA 基因必须有某种方式来控制细胞质的化学反应。这个控制
Figure 3-6. Arrangement of deoxyribose nucleotides in a double strand of DNA.
图 3-6. DNA 双链中脱氧核糖核苷酸的排列。
Figure 3-7. Combination of ribose nucleotides with a strand of DNA to form a molecule of RNA that carries the genetic code from the gene to the cytoplasm. The RNA polymerase enzyme moves along the DNA strand and builds the RNA molecule.
图 3-7。核糖核苷酸与 DNA 链结合，形成携带遗传密码的 RNA 分子，将其从基因传递到细胞质。RNA 聚合酶沿着 DNA 链移动并构建 RNA 分子。
Figure 3-8. A portion of an RNA molecule showing three RNA codons-CCG, UCU, and GAA-that control attachment of the three amino acids, proline, serine, and glutamic acid, respectively, to the growing RNA chain.
图 3-8. 一部分 RNA 分子显示三个 RNA 密码子-CCG、UCU 和 GAA-分别控制脯氨酸、丝氨酸和谷氨酸这三种氨基酸附着在不断增长的 RNA 链上。

is achieved through the intermediary of another type of nucleic acid, RNA, the formation of which is controlled by DNA of the nucleus. Thus, as shown in Figure 3-7, the code is transferred to RNA in a process called transcription. The RNA, in turn, diffuses from the nucleus through nuclear pores into the cytoplasmic compartment, where it controls protein synthesis.
通过另一种类型的核酸 RNA 的中介实现，RNA 的形成由细胞核中的 DNA 控制。因此，如图 3-7 所示，代码在一个称为转录的过程中转移到 RNA。RNA 随后通过核孔从细胞核扩散到细胞质区，在那里它控制蛋白质合成。

RNA IS SYNTHESIZED IN THE NUCLEUS FROM A DNA TEMPLATE
RNA 是在细胞核中从 DNA 模板合成的

During RNA synthesis, the two strands of DNA separate temporarily; one of these strands is used as a template for synthesis of an RNA molecule. The code triplets in the DNA result in the formation of complementary code triplets (called codons) in the RNA. These codons, in turn, will control the sequence of amino acids in a protein to be synthesized in the cell cytoplasm.
在 RNA 合成过程中，DNA 的两条链暂时分开；其中一条链被用作合成 RNA 分子的模板。DNA 中的密码子三联体导致 RNA 中形成互补的密码子三联体（称为密码子）。这些密码子反过来将控制在细胞质中合成的蛋白质的氨基酸序列。
Building Blocks of RNA. The basic building blocks of RNA are almost the same as those of DNA, except for two differences. First, the sugar deoxyribose is not used in RNA formation. In its place is another sugar of slightly different composition, ribose, which contains an extra hydroxyl ion appended to the ribose ring structure. Second, thymine is replaced by another pyrimidine, uracil.
RNA 的基本构建块几乎与 DNA 的相同，只有两个不同之处。首先，RNA 的形成中不使用脱氧核糖。取而代之的是另一种成分略有不同的糖，核糖，它的环结构上附加了一个额外的羟基离子。其次，胸腺嘧啶被另一种嘧啶，尿嘧啶所替代。
Formation of RNA Nucleotides. The basic building blocks of RNA form RNA nucleotides, exactly as described previously for DNA synthesis. Here again, four separate nucleotides are used to form RNA. These nucleotides contain the bases adenine, guanine, cytosine, and uracil. Note that these bases are the same as in DNA, except that uracil in RNA replaces thymine in DNA.
RNA 核苷酸的形成。RNA 的基本构建块形成 RNA 核苷酸，正如之前描述的 DNA 合成一样。在这里，再次使用四种独立的核苷酸来形成 RNA。这些核苷酸包含碱基腺嘌呤、鸟嘌呤、胞嘧啶和尿嘧啶。请注意，这些碱基与 DNA 中的相同，只是 RNA 中的尿嘧啶取代了 DNA 中的胸腺嘧啶。
“Activation” of RNA Nucleotides. The next step in the synthesis of RNA is “activation” of RNA nucleotides by an enzyme, RNA polymerase. This activation occurs by adding two extra phosphate radicals to each nucleotide to form triphosphates (shown in Figure 3-7 by the two RNA nucleotides to the far right during RNA chain formation). These last two phosphates are combined with the nucleotide by high-energy phosphate bonds derived from ATP in the cell.
RNA 核苷酸的“激活”。RNA 合成的下一步是通过酶 RNA 聚合酶对 RNA 核苷酸进行“激活”。这种激活是通过向每个核苷酸添加两个额外的磷酸基团来形成三磷酸盐（在 RNA 链形成过程中，图 3-7 中最右侧的两个 RNA 核苷酸所示）。这两个磷酸基团通过来自细胞中 ATP 的高能磷酸键与核苷酸结合。
The result of this activation process is that large quantities of ATP energy are made available to each of the nucleotides. This energy is used to promote chemical reactions that add each new RNA nucleotide at the end of the developing RNA chain.
这种激活过程的结果是大量的 ATP 能量被提供给每个核苷酸。这种能量用于促进化学反应，将每个新的 RNA 核苷酸添加到正在发展的 RNA 链的末端。

RNA CHAIN ASSEMBLY FROM ACTIVATED NUCLEOTIDES USING THE DNA STRAND AS A TEMPLATE
使用 DNA 链作为模板从活化核苷酸组装 RNA 链

As shown in Figure 3-7, assembly of RNA is accomplished under the influence of an enzyme, RNA polymerase. This large protein enzyme has many functional properties necessary for formation of RNA, as follows:
如图 3-7 所示，RNA 的组装是在酶 RNA 聚合酶的影响下完成的。这种大型蛋白质酶具有许多形成 RNA 所需的功能特性，如下所示：
  1. In the DNA strand immediately ahead of the gene to be transcribed is a sequence of nucleotides called the promoter. The RNA polymerase has an appropriate complementary structure that recognizes this promoter and becomes attached to it, which is the essential step for initiating the formation of RNA.
    在待转录基因前面的 DNA 链上有一段称为启动子的核苷酸序列。RNA 聚合酶具有适当的互补结构，可以识别这个启动子并与之结合，这是启动 RNA 形成的关键步骤。
  2. After the RNA polymerase attaches to the promoter, the polymerase causes unwinding of about two turns of the DNA helix and separation of the unwound portions of the two strands.
    在 RNA 聚合酶附着到启动子后，聚合酶导致 DNA 螺旋的约两个转的解旋，并使两条链的解旋部分分开。
  3. The polymerase then moves along the DNA strand, temporarily unwinding and separating the two DNA strands at each stage of its movement. As it moves along, at each stage it adds a new activated RNA nucleotide to the end of the newly forming RNA chain through the following steps:
    聚合酶随后沿着 DNA 链移动，暂时解开并分开在其移动的每个阶段的两条 DNA 链。在移动的过程中，在每个阶段，它通过以下步骤向新形成的 RNA 链的末端添加一个新的活化 RNA 核苷酸：
    a. First, it causes a hydrogen bond to form between the end base of the DNA strand and the base of an RNA nucleotide in the nucleoplasm.
    首先，它导致 DNA 链的末端碱基与核质中 RNA 核苷酸的碱基之间形成氢键。
    b. Then, one at a time, the RNA polymerase breaks two of the three phosphate radicals away from each of these RNA nucleotides, liberating large amounts of energy from the broken high-energy phosphate bonds. This energy is used to cause covalent linkage of the remaining phosphate on the nucleotide with the ribose on the end of the growing RNA chain.
    然后，RNA 聚合酶逐个地从每个 RNA 核苷酸中断裂三个磷酸基团中的两个，释放出大量能量，这些能量来自断裂的高能磷酸键。这些能量用于使核苷酸上剩余的磷酸与正在增长的 RNA 链末端的核糖形成共价连接。
    c. When the RNA polymerase reaches the end of the DNA gene, it encounters a new sequence of DNA nucleotides called the chain-terminating sequence, which causes the polymerase and the newly formed RNA chain to break away from the DNA strand. The polymerase then can be used again and again to form more new RNA chains.
    当 RNA 聚合酶到达 DNA 基因的末端时，它会遇到一段新的 DNA 核苷酸序列，称为链终止序列，这使得聚合酶和新形成的 RNA 链从 DNA 链上断开。然后，聚合酶可以一次又一次地被用来形成更多新的 RNA 链。
    d. As the new RNA strand is formed, its weak hydrogen bonds with the DNA template break away because the DNA has a high affinity for rebonding with its own complementary DNA strand. Thus, the RNA chain is forced away from the DNA and is released into the nucleoplasm.
    随着新的 RNA 链的形成，它与 DNA 模板之间的弱氢键断裂，因为 DNA 对与其自身互补的 DNA 链重新结合具有很强的亲和力。因此，RNA 链被迫远离 DNA，并释放到核质中。
    Therefore, the code that is present in the DNA strand is eventually transmitted in complementary form to the RNA chain. The ribose nucleotide bases always combine with the deoxyribose bases in the following combinations:
    因此，DNA 链中的代码最终以互补形式传递给 RNA 链。核糖核苷酸碱基总是以以下组合与脱氧核糖碱基结合：
DNA Base DNA 碱基 RNA Base RNA 碱基
guanine 鸟嘌呤 Cytosine 胞嘧啶
cytosine 胞嘧啶 Guanine 鸟嘌呤
adenine 腺苷 Uracil 尿嘧啶
thymine 胸腺嘧啶 adenine 腺苷
DNA Base RNA Base guanine Cytosine cytosine Guanine adenine Uracil thymine adenine| DNA Base | RNA Base | | :--- | :--- | | guanine | Cytosine | | cytosine | Guanine | | adenine | Uracil | | thymine | adenine |
There Are Several Different Types of RNA. As research on RNA has continued to advance, many different types of RNA have been discovered. Some types of RNA are involved in protein synthesis, whereas other types serve gene regulatory functions or are involved in posttranscriptional modification of RNA. The functions of some types of RNA, especially those that do not appear to code for proteins, are still mysterious. The following six types of RNA play independent and different roles in protein synthesis:
有几种不同类型的 RNA。随着对 RNA 研究的不断深入，发现了许多不同类型的 RNA。一些类型的 RNA 参与蛋白质合成，而其他类型则发挥基因调控功能或参与 RNA 的转录后修饰。一些类型的 RNA，特别是那些似乎不编码蛋白质的 RNA，其功能仍然是个谜。以下六种类型的 RNA 在蛋白质合成中发挥独立而不同的作用：
  1. Precursor messenger RNA (pre-mRNA) is a large, immature, single strand of RNA that is processed in the nucleus to form mature messenger RNA (mRNA). The pre-RNA includes two different types of segments, called introns, which are removed by a process called splicing, and exons, which are retained in the final mRNA.
    前体信使 RNA（pre-mRNA）是一种大型、未成熟的单链 RNA，在细胞核中经过处理形成成熟的信使 RNA（mRNA）。前体 RNA 包括两种不同类型的片段，称为内含子，这些片段通过一种称为剪接的过程被去除，而外显子则保留在最终的 mRNA 中。
  2. Small nuclear RNA (snRNA) directs the splicing of pre-mRNA to form mRNA.
    小核 RNA（snRNA）指导前 mRNA 的剪接以形成 mRNA。
  3. Messenger RNA (mRNA) carries the genetic code to the cytoplasm for controlling the type of protein formed.
    信使 RNA（mRNA）将遗传密码携带到细胞质中，以控制形成的蛋白质类型。
  4. Transfer RNA (tRNA) transports activated amino acids to the ribosomes to be used in assembling the protein molecule.
    转运 RNA（tRNA）将活化的氨基酸运输到核糖体，以用于组装蛋白质分子。
  5. Ribosomal RNA, along with about 75 different proteins, forms ribosomes, the physical and chemical structures on which protein molecules are actually assembled.
    核糖体 RNA 与大约 75 种不同的蛋白质一起形成核糖体，这是蛋白质分子实际组装的物理和化学结构。
  6. MicroRNAs (miRNAs) are single-stranded RNA molecules of 21 to 23 nucleotides that can regulate gene transcription and translation.
    微小 RNA（miRNA）是由 21 到 23 个核苷酸组成的单链 RNA 分子，能够调节基因转录和翻译。

MESSENGER RNA—THE CODONS 信使 RNA—密码子

Messenger RNA molecules are long single RNA strands that are suspended in the cytoplasm. These molecules are composed of several hundred to several thousand RNA nucleotides in unpaired strands, and they contain codons that are exactly complementary to the code triplets of the DNA genes. Figure 3-8 shows a small segment of mRNA. Its codons are CCG, UCU, and GAA, which are the codons for the amino acids proline, serine, and glutamic acid. The transcription of these codons from the DNA molecule to the RNA molecule is shown in Figure 3-7.
信使 RNA 分子是悬浮在细胞质中的长单链 RNA。这些分子由几百到几千个未配对的 RNA 核苷酸组成，包含与 DNA 基因的密码子三联体完全互补的密码子。图 3-8 显示了一小段 mRNA。它的密码子是 CCG、UCU 和 GAA，分别对应氨基酸脯氨酸、丝氨酸和谷氨酸。这些密码子从 DNA 分子转录到 RNA 分子的过程如图 3-7 所示。
RNA Codons for the Different Amino Acids. Table 3-1 lists the RNA codons for the 20 common amino acids found in protein molecules. Note that most of the amino acids are represented by more than one codon; also, one codon represents the signal “start manufacturing the protein molecule,” and three codons represent “stop manufacturing the protein molecule.” In Table 3-1, these two
RNA 密码子与不同氨基酸。表 3-1 列出了蛋白质分子中 20 种常见氨基酸的 RNA 密码子。请注意，大多数氨基酸由多个密码子表示；此外，一个密码子表示“开始合成蛋白质分子”，而三个密码子表示“停止合成蛋白质分子”。在表 3-1 中，这两个
Table 3-1 RNA Codons for Amino Acids and for Start and Stop
表 3-1 氨基酸的 RNA 密码子及起始和终止密码子
Amino Acid 氨基酸 RNA Codons RNA 密码子
Alanine 丙氨酸 GCU GCC GCA GCG
Arginine 精氨酸 CGU CGC CGA CGG AGA AGG
Asparagine 天冬氨酸 AAU AAC
Aspartic acid 天冬氨酸 GAU GAC
Cysteine 半胱氨酸 UGU UGC
Glutamic acid 谷氨酸 GAA GAG
Glutamine 谷氨酰胺 CAA CAG
Glycine 甘氨酸 GGU GGC GGA GGG
Histidine 组氨酸 CAU CAC
Isoleucine 异亮氨酸 AUU AUC AUA
Leucine 亮氨酸 CUU CUC CUA CUG UUA UUG
Lysine 赖氨酸 AAA AAG
Methionine 蛋氨酸 AUG
Phenylalanine 苯丙氨酸 UUU UUC
Proline 脯氨酸 CCU CCC CCA CCG
Serine 丝氨酸 UCU UCC UCA UCG AGC AGU
Threonine 苏氨酸 ACU ACC ACA ACG
Tryptophan 色氨酸 UGG
Tyrosine 酪氨酸 UAU UAC
Valine 缬氨酸 GUU GUC GUA GUG
Start (CI) 开始 (CI) AUG
Stop (CT) 停止 (CT) UAA UAG UGA
Amino Acid RNA Codons Alanine GCU GCC GCA GCG Arginine CGU CGC CGA CGG AGA AGG Asparagine AAU AAC Aspartic acid GAU GAC Cysteine UGU UGC Glutamic acid GAA GAG Glutamine CAA CAG Glycine GGU GGC GGA GGG Histidine CAU CAC Isoleucine AUU AUC AUA Leucine CUU CUC CUA CUG UUA UUG Lysine AAA AAG Methionine AUG Phenylalanine UUU UUC Proline CCU CCC CCA CCG Serine UCU UCC UCA UCG AGC AGU Threonine ACU ACC ACA ACG Tryptophan UGG Tyrosine UAU UAC Valine GUU GUC GUA GUG Start (CI) AUG Stop (CT) UAA UAG UGA | Amino Acid | RNA Codons | | | | | | | :--- | :--- | :--- | :--- | :--- | :--- | :--- | | Alanine | GCU | GCC | GCA | GCG | | | | Arginine | CGU | CGC | CGA | CGG | AGA | AGG | | Asparagine | AAU | AAC | | | | | | Aspartic acid | GAU | GAC | | | | | | Cysteine | UGU | UGC | | | | | | Glutamic acid | GAA | GAG | | | | | | Glutamine | CAA | CAG | | | | | | Glycine | GGU | GGC | GGA | GGG | | | | Histidine | CAU | CAC | | | | | | Isoleucine | AUU | AUC | AUA | | | | | Leucine | CUU | CUC | CUA | CUG | UUA | UUG | | Lysine | AAA | AAG | | | | | | Methionine | AUG | | | | | | | Phenylalanine | UUU | UUC | | | | | | Proline | CCU | CCC | CCA | CCG | | | | Serine | UCU | UCC | UCA | UCG | AGC | AGU | | Threonine | ACU | ACC | ACA | ACG | | | | Tryptophan | UGG | | | | | | | Tyrosine | UAU | UAC | | | | | | Valine | GUU | GUC | GUA | GUG | | | | Start (CI) | AUG | | | | | | | Stop (CT) | UAA | UAG | UGA | | | |
Cl, Chain-initiating; CT, chain-terminating.
Cl，链引发；CT，链终止。
types of codons are designated CI for “chain-initiating” or “start” codon and CT for “chain-terminating” or “stop” codon.
密码子的类型被指定为 CI，表示“链起始”或“起始”密码子，CT 表示“链终止”或“终止”密码子。

TRANSFER RNA—THE ANTICODONS
转移 RNA—反密码子

Another type of RNA that is essential for protein synthesis is called transfer RNA (tRNA) because it transfers amino acids to protein molecules as the protein is being synthesized. Each type of tRNA combines specifically with 1 of the 20 amino acids that are to be incorporated into proteins. The tRNA then acts as a carrier to transport its specific type of amino acid to the ribosomes, where protein molecules are forming. In the ribosomes, each specific type of tRNA recognizes a particular codon on the mRNA (described later) and thereby delivers the appropriate amino acid to the appropriate place in the chain of the newly forming protein molecule.
另一种对蛋白质合成至关重要的 RNA 被称为转运 RNA（tRNA），因为它在蛋白质合成过程中将氨基酸转移到蛋白质分子中。每种类型的 tRNA 与 20 种氨基酸中的一种特异性结合，这些氨基酸将被纳入蛋白质中。tRNA 随后充当载体，将其特定类型的氨基酸运输到核糖体，在那里蛋白质分子正在形成。在核糖体中，每种特定类型的 tRNA 识别 mRNA 上的特定密码子（稍后描述），从而将适当的氨基酸送到新形成的蛋白质分子链中的适当位置。
Transfer RNA, which contains only about 80 nucleotides, is a relatively small molecule in comparison with mRNA. It is a folded chain of nucleotides with a cloverleaf appearance similar to that shown in Figure 3-9. At one end of the molecule there is always an adenylic acid to which the transported amino acid attaches at a hydroxyl group of the ribose in the adenylic acid.
转运 RNA 仅包含约 80 个核苷酸，相较于信使 RNA，它是一种相对较小的分子。它是一个折叠的核苷酸链，呈现出类似于图 3-9 所示的三叶草形状。在分子的一个末端，总是有一个腺苷酸，运输的氨基酸附着在腺苷酸中核糖的羟基上。
Because the function of tRNA is to cause attachment of a specific amino acid to a forming protein chain, it is essential that each type of tRNA also have specificity for a particular codon in the mRNA. The specific code in the tRNA that allows it to recognize a specific codon is again a triplet of nucleotide bases and is called an anticodon. This anticodon is located approximately in the middle of the tRNA molecule (at the bottom of the cloverleaf configuration shown in Figure 3-9). During formation of the protein molecule, the anticodon bases combine loosely by hydrogen bonding with the codon bases of the mRNA. In this way, the respective amino acids are lined up one after another along the mRNA chain, thus establishing the
由于 tRNA 的功能是使特定氨基酸附着到形成的蛋白质链上，因此每种类型的 tRNA 也必须对 mRNA 中的特定密码子具有特异性。tRNA 中允许其识别特定密码子的特定代码再次是由三个核苷酸碱基组成，称为反密码子。这个反密码子大约位于 tRNA 分子的中间（在图 3-9 所示的三叶草结构的底部）。在蛋白质分子的形成过程中，反密码子碱基通过氢键与 mRNA 的密码子碱基松散结合。通过这种方式，各自的氨基酸沿着 mRNA 链一个接一个地排列，从而建立起
Figure 3-9. A messenger RNA strand is moving through two ribosomes. As each codon passes through, an amino acid is added to the growing protein chain, which is shown in the right-hand ribosome. The transfer RNA molecule transports each specific amino acid to the newly forming protein. appropriate sequence of amino acids in the newly forming protein molecule.
图 3-9。一条信使 RNA 链正在通过两个核糖体。当每个密码子通过时，一个氨基酸被添加到正在增长的蛋白质链中，右侧的核糖体显示了这一过程。转运 RNA 分子将每个特定的氨基酸运输到新形成的蛋白质中，确保氨基酸在新形成的蛋白质分子中按适当的顺序排列。

RIBOSOMAL RNA 核糖体 RNA

The third type of RNA in the cell is ribosomal RNA, which constitutes about 60 % 60 % 60%60 \% of the ribosome. The remainder of the ribosome is protein, including about 75 types of proteins that are both structural proteins and enzymes needed to manufacture proteins.
细胞中的第三种 RNA 是核糖体 RNA，它构成了约 60 % 60 % 60%60 \% 的核糖体。核糖体的其余部分是蛋白质，包括约 75 种既是结构蛋白又是制造蛋白质所需的酶的蛋白质。
The ribosome is the physical structure in the cytoplasm on which proteins are actually synthesized. However, it always functions in association with the other two types of RNA; t R N A t R N A tRNAt R N A transports amino acids to the ribosome for incorporation into the developing protein, whereas m R N A m R N A mRNAm R N A provides the information necessary for sequencing the amino acids in proper order for each specific type of protein to be manufactured. Thus, the ribosome acts as a manufacturing plant in which the protein molecules are formed.
核糖体是细胞质中实际合成蛋白质的物理结构。然而，它总是与另外两种类型的 RNA 一起工作； t R N A t R N A tRNAt R N A 将氨基酸运输到核糖体，以便将其纳入正在形成的蛋白质中，而 m R N A m R N A mRNAm R N A 提供了制造每种特定类型蛋白质所需的氨基酸按正确顺序排列的信息。因此，核糖体充当了一个制造工厂，在这里形成蛋白质分子。
Formation of Ribosomes in the Nucleolus. The DNA genes for the formation of ribosomal RNA are located in five pairs of chromosomes in the nucleus. Each of these chromosomes contains many duplicates of these particular genes because of the large amounts of ribosomal RNA required for cellular function.
在核仁中形成核糖体。形成核糖体 RNA 的 DNA 基因位于细胞核中的五对染色体上。这些染色体中的每一个都包含许多这些特定基因的重复，因为细胞功能所需的核糖体 RNA 量很大。
As the ribosomal RNA forms, it collects in the nucleolus, a specialized structure lying adjacent to the chromosomes. When large amounts of ribosomal RNA are being synthesized, as occurs in cells that manufacture large amounts of protein, the nucleolus is a large structure, whereas in cells that synthesize little protein, the nucleolus may not even be seen. Ribosomal RNA is specially processed in the nucleolus, where it binds with ribosomal proteins to form granular condensation products that are primordial subunits of ribosomes. These subunits are then released from the nucleolus and transported through the large pores of the nuclear envelope to almost all parts of the cytoplasm. After the subunits enter the cytoplasm, they are assembled to form mature functional ribosomes. Therefore, proteins are formed in the cytoplasm of the cell, but not in the cell nucleus, because the nucleus does not contain mature ribosomes.
随着核糖体 RNA 的形成，它在核仁中聚集，核仁是一个位于染色体旁边的特殊结构。当大量的核糖体 RNA 被合成时，例如在制造大量蛋白质的细胞中，核仁是一个较大的结构，而在合成少量蛋白质的细胞中，核仁甚至可能看不见。核糖体 RNA 在核仁中经过特殊处理，与核糖体蛋白结合形成颗粒状凝聚产物，这些产物是核糖体的原始亚单位。这些亚单位随后从核仁释放出来，通过核膜的大孔道运输到几乎所有细胞质的部分。在亚单位进入细胞质后，它们被组装成成熟的功能性核糖体。因此，蛋白质是在细胞的细胞质中形成的，而不是在细胞核中，因为细胞核不含成熟的核糖体。

miRNA AND SMALL INTERFERING RNA
miRNA 和小干扰 RNA

A fourth type of RNA in the cell is microRNA (miRNA); miRNA are short ( 21 to 23 nucleotides) single-stranded RNA fragments that regulate gene expression (Figure 3 − 10 ) 3 − 10 ) 3-10)3-10). The miRNAs are encoded from the transcribed DNA of genes, but they are not translated into proteins and are therefore often called noncoding RNA. The miRNAs are processed by the cell into molecules that are complementary to mRNA and act to decrease gene expression. The generation of miRNAs involves special processing of longer primary precursor RNAs called primiRNAs, which are the primary transcripts of the gene.
细胞中的第四种 RNA 是微小 RNA（miRNA）；miRNA 是短的（21 到 23 个核苷酸）单链 RNA 片段，调节基因表达（图 3 − 10 ) 3 − 10 ) 3-10)3-10) ）。miRNA 是从基因转录的 DNA 编码而来的，但它们不被翻译成蛋白质，因此通常被称为非编码 RNA。miRNA 被细胞处理成与 mRNA 互补的分子，并起到降低基因表达的作用。miRNA 的生成涉及对称为 primiRNA 的较长初级前体 RNA 的特殊处理，primiRNA 是基因的主要转录本。
Figure 3-10. Regulation of gene expression by microRNA (miRNA). Primary miRNA (pri-miRNA), the primary transcripts of a gene processed in the cell nucleus by the microprocessor complex, are converted to pre-miRNAs. These pre-miRNAs are then further processed in the cytoplasm by dicer, an enzyme that helps assemble an RNAinduced silencing complex (RISC) and generates miRNAs. The miRNAs regulate gene expression by binding to the complementary region of the RNA and repressing translation or promoting degradation of the messenger RNA (mRNA) before it can be translated by the ribosome.
图 3-10. 微小 RNA（miRNA）对基因表达的调控。初级 miRNA（pri-miRNA）是基因在细胞核中由微处理器复合体加工的初级转录本，转化为前体 miRNA（pre-miRNA）。这些前体 miRNA 随后在细胞质中被酶 Dicer 进一步加工，Dicer 帮助组装 RNA 诱导沉默复合体（RISC）并生成 miRNA。miRNA 通过与 RNA 的互补区域结合来调节基因表达，抑制翻译或促进信使 RNA（mRNA）的降解，从而在其被核糖体翻译之前进行调控。
The pri-miRNAs are then processed in the cell nucleus by the microprocessor complex to pre-miRNAs, which are 70-nucleotide, stem loop structures. These pre-miRNAs are then further processed in the cytoplasm by a specific dicer enzyme that helps assemble an RNA-induced silencing complex (RISC) and generates miRNAs.
初级 miRNA 在细胞核中被微处理器复合体加工成前体 miRNA，这些前体 miRNA 是 70 个核苷酸的茎环结构。随后，这些前体 miRNA 在细胞质中被特定的切割酶进一步加工，帮助组装 RNA 诱导沉默复合体（RISC），并生成 miRNA。
The miRNAs regulate gene expression by binding to the complementary region of the RNA and promoting repression of translation or degradation of the mRNA before it can be translated by the ribosome. miRNAs are believed to play an important role in normal regulation of cell function, and alterations in miRNA function have been associated with diseases such as cancer and heart disease.
miRNA 通过与 RNA 的互补区域结合来调节基因表达，促进翻译抑制或在 mRNA 被核糖体翻译之前降解 mRNA。miRNA 被认为在细胞功能的正常调节中发挥重要作用，miRNA 功能的改变与癌症和心脏病等疾病相关。
Another type of miRNA is small interfering RNA (siRNA), also called silencing RNA or short interfering R N A R N A RNAR N A. The siRNAs are short, double-stranded RNA molecules, comprised of 20 to 25 nucleotides, that interfere with expression of specific genes. siRNAs generally refer to synthetic miRNAs and can be administered to silence expression of specific genes. They are designed to avoid nuclear processing by the microprocessor complex and, after the siRNA enters the cytoplasm, it activates the RISC silencing complex, blocking the translation of mRNA. Because siRNAs can be tailored for any specific sequence in the gene, they can be used to block translation of any mRNA and therefore expression by any gene for which the nucleotide sequence is known. Researchers have proposed that siRNAs may become useful therapeutic tools to silence genes that contribute to the pathophysiology of diseases.
另一种类型的 miRNA 是小干扰 RNA（siRNA），也称为沉默 RNA 或短干扰 RNA。siRNA 是短的双链 RNA 分子，由 20 到 25 个核苷酸组成，干扰特定基因的表达。siRNA 通常指合成的 miRNA，可以被施用以沉默特定基因的表达。它们被设计为避免被微处理器复合体进行核处理，并且在 siRNA 进入细胞质后，激活 RISC 沉默复合体，阻止 mRNA 的翻译。由于 siRNA 可以针对基因中的任何特定序列进行定制，因此可以用来阻止任何 mRNA 的翻译，从而阻止任何已知核苷酸序列的基因的表达。研究人员提出，siRNA 可能成为有用的治疗工具，以沉默那些对疾病病理生理学有贡献的基因。

TRANSLATION-FORMATION OF PROTEINS ON THE RIBOSOMES
翻译-在核糖体上形成蛋白质

When a molecule of mRNA comes in contact with a ribosome, it travels through the ribosome, beginning at a predetermined end of the RNA molecule specified by an appropriate sequence of RNA bases called the chaininitiating codon. Then, as shown in Figure 3-9, while the mRNA travels through the ribosome, a protein molecule is formed, a process called translation. Thus, the ribosome reads the codons of the mRNA in much the same way that a tape is read as it passes through the playback head of a tape recorder. Then, when a “stop” (or “chainterminating”) codon slips past the ribosome, the end of a protein molecule is signaled, and the protein molecule is freed into the cytoplasm.
当一分子 mRNA 与核糖体接触时，它会通过核糖体移动，从 RNA 分子的一个预定端开始，该端由称为起始密码子的适当 RNA 碱基序列指定。然后，如图 3-9 所示，当 mRNA 在核糖体中移动时，形成一个蛋白质分子，这个过程称为翻译。因此，核糖体以类似于磁带通过磁带录音机的播放头时读取磁带的方式读取 mRNA 的密码子。然后，当一个“终止”（或“链终止”）密码子滑过核糖体时，蛋白质分子的结束被信号传递，蛋白质分子被释放到细胞质中。
Polyribosomes. A single mRNA molecule can form protein molecules in several ribosomes at the same time because the initial end of the RNA strand can pass to a successive ribosome as it leaves the first, as shown at the bottom left in Figure 3-9 and Figure 3-11. The protein molecules are in different stages of development in each ribosome. As a result, clusters of ribosomes frequently occur, with 3 to 10 ribosomes being attached to a single mRNA at the same time. These clusters are called polyribosomes.
多核糖体。单个 mRNA 分子可以在多个核糖体中同时形成蛋白质分子，因为 RNA 链的起始端可以在离开第一个核糖体时传递给下一个核糖体，如图 3-9 和图 3-11 左下角所示。每个核糖体中的蛋白质分子处于不同的发展阶段。因此，核糖体的聚集现象经常发生，通常有 3 到 10 个核糖体同时附着在单个 mRNA 上。这些聚集体被称为多核糖体。
An mRNA can cause formation of a protein molecule in any ribosome; there is no specificity of ribosomes for given types of protein. The ribosome is simply the physical manufacturing plant in which the chemical reactions take place.
mRNA 可以在任何核糖体中导致蛋白质分子的形成；核糖体对特定类型的蛋白质没有特异性。核糖体只是进行化学反应的物理制造工厂。
Many Ribosomes Attach to the Endoplasmic Reticulum. In Chapter 2, we noted that many ribosomes become attached to the endoplasmic reticulum. This attachment occurs because the initial ends of many forming protein molecules have amino acid sequences that immediately attach to specific receptor sites on the endoplasmic reticulum, causing these molecules to penetrate the
许多核糖体附着在内质网。 在第二章中，我们提到许多核糖体附着在内质网上。 这种附着发生是因为许多正在形成的蛋白质分子的初始末端具有氨基酸序列，这些序列立即附着在内质网上的特定受体位点上，导致这些分子渗透到
Figure 3-11. The physical structure of the ribosomes, as well as their functional relationship to messenger RNA, transfer RNA, and the endoplasmic reticulum during the formation of protein molecules.
图 3-11. 核糖体的物理结构，以及它们在蛋白质分子形成过程中与信使 RNA、转运 RNA 和内质网的功能关系。

reticulum wall and enter the endoplasmic reticulum matrix. This process gives a granular appearance to the portions of the reticulum where proteins are being formed and are entering the matrix of the reticulum.
网状结构壁并进入内质网基质。这个过程使得蛋白质形成并进入网状结构基质的部分呈现颗粒状外观。
Figure 3-11 shows the functional relationship of mRNA to the ribosomes and the manner in which the ribosomes attach to the membrane of the endoplasmic reticulum. Note the process of translation occurring in several ribosomes at the same time in response to the same strand of mRNA. Note also the newly forming polypeptide (protein) chains passing through the endoplasmic reticulum membrane into the endoplasmic matrix.
图 3-11 显示了 mRNA 与核糖体之间的功能关系，以及核糖体如何附着在内质网膜上。请注意，多个核糖体同时对同一条 mRNA 链进行翻译的过程。还要注意，新形成的多肽（蛋白质）链通过内质网膜进入内质基质。
It should be noted that except in glandular cells, in which large amounts of protein-containing secretory vesicles are formed, most proteins synthesized by the ribosomes are released directly into the cytosol instead of into the endoplasmic reticulum. These proteins are enzymes and internal structural proteins of the cell.
需要注意的是，除了腺体细胞外，在腺体细胞中形成大量含蛋白质的分泌囊泡，绝大多数由核糖体合成的蛋白质直接释放到细胞质中，而不是进入内质网。这些蛋白质是细胞的酶和内部结构蛋白。
Chemical Steps in Protein Synthesis. Some of the chemical events that occur in the synthesis of a protein molecule are shown in Figure 3-12. This Fig. shows representative reactions for three separate amino acids, AA 1 AA 1 AA_(1)\mathrm{AA}_{1}, AA 2 AA 2 AA_(2)\mathrm{AA}_{2}, and AA 20 AA 20 AA_(20)\mathrm{AA}_{20}. The stages of the reactions are as follows:
蛋白质合成中的化学步骤。图 3-12 显示了在合成蛋白质分子过程中发生的一些化学事件。该图展示了三个不同氨基酸 AA 1 AA 1 AA_(1)\mathrm{AA}_{1} 、 AA 2 AA 2 AA_(2)\mathrm{AA}_{2} 和 AA 20 AA 20 AA_(20)\mathrm{AA}_{20} 的代表性反应。反应的阶段如下：
  1. Each amino acid is activated by a chemical process in which ATP combines with the amino acid to form an adenosine monophosphate complex with the amino acid, giving up two high-energy phosphate bonds in the process.
    每种氨基酸通过一种化学过程被激活，在这个过程中，ATP 与氨基酸结合形成氨基酸的腺苷单磷酸复合物，同时释放出两个高能磷酸键。
  2. The activated amino acid, having an excess of energy, then combines with its specific tRNA to form an amino acid-tRNA complex and, at the same time, releases the adenosine monophosphate.
    活化的氨基酸因能量过剩而与其特定的 tRNA 结合，形成氨基酸-tRNA 复合物，同时释放腺苷单磷酸。
  3. The tRNA carrying the amino acid complex then comes in contact with the mRNA molecule in the ribosome, where the anticodon of the tRNA attaches temporarily to its specific codon of the mRNA, thus lining up the amino acid in the appropriate sequence to form a protein molecule.
    携带氨基酸复合物的 tRNA 随后与核糖体中的 mRNA 分子接触，tRNA 的反密码子暂时附着在 mRNA 的特定密码子上，从而将氨基酸按适当的顺序排列以形成蛋白质分子。
    Then, under the influence of the enzyme peptidyl transferase (one of the proteins in the ribosome), peptide bonds are formed between the successive amino acids, thus adding progressively to the protein chain. These chemical events require energy from two additional high-energy phosphate bonds, making a total of four high-energy bonds used for each amino acid added to the protein chain. Thus, the synthesis of proteins is one of the most energy-consuming processes of the cell.
    然后，在肽转移酶（核糖体中的一种蛋白质）的影响下，连续的氨基酸之间形成肽键，从而逐渐增加蛋白质链。这些化学反应需要来自两个额外高能磷酸键的能量，使得每添加一个氨基酸到蛋白质链上总共使用四个高能键。因此，蛋白质的合成是细胞中最耗能的过程之一。
Peptide Linkage-Combination of Amino Acids. The successive amino acids in the protein chain combine with one another according to the typical reaction.
肽键-氨基酸的结合。蛋白质链中的连续氨基酸根据典型反应相互结合。
In this chemical reaction, a hydroxyl radical ( OH − ) OH − (OH^(-))\left(\mathrm{OH}^{-}\right)is removed from the COOH portion of the first amino acid, and a hydrogen ( H + ) H + (H^(+))\left(\mathrm{H}^{+}\right)of the NH 2 NH 2 NH_(2)\mathrm{NH}_{2} portion of the other amino acid is removed. These combine to form water, and the two reactive sites left on the two successive amino acids bond with each other, resulting in a single molecule. This process is called peptide linkage. As each additional amino acid is added, an additional peptide linkage is formed.
在这个化学反应中，第一个氨基酸的 COOH 部分去掉了一个羟基自由基 ( OH − ) OH − (OH^(-))\left(\mathrm{OH}^{-}\right) ，而另一个氨基酸的 NH 2 NH 2 NH_(2)\mathrm{NH}_{2} 部分去掉了一个氢 ( H + ) H + (H^(+))\left(\mathrm{H}^{+}\right) 。这些结合形成水，两个连续氨基酸上留下的两个反应位点相互结合，形成一个单一分子。这个过程称为肽键。随着每个额外氨基酸的加入，形成一个额外的肽键。

SYNTHESIS OF OTHER SUBSTANCES IN THE CELL
细胞中其他物质的合成

Many thousand protein enzymes formed in the manner just described control essentially all the other chemical reactions that take place in cells. These enzymes promote synthesis of lipids, glycogen, purines, pyrimidines, and hundreds of other substances. We discuss many of these synthetic processes in relation to carbohydrate, lipid, and protein metabolism in Chapters 68 through 70. These substances each contribute to the various functions of the cells.
数以千计的蛋白质酶以刚才描述的方式形成，基本上控制着细胞内发生的所有其他化学反应。这些酶促进脂质、糖原、嘌呤、嘧啶及数百种其他物质的合成。我们在第 68 到 70 章中讨论了许多与碳水化合物、脂质和蛋白质代谢相关的合成过程。这些物质各自对细胞的各种功能做出贡献。

CONTROL OF GENE FUNCTION AND BIOCHEMICAL ACTIVITY IN CELLS
基因功能和细胞内生化活性的控制

From our discussion thus far, it is clear that the genes control both the physical and chemical functions of the cells. However, the degree of activation of respective genes must also be
从我们到目前为止的讨论来看，基因控制着细胞的物理和化学功能。然而，各个基因的激活程度也必须是
Figure 3-12. Chemical events in the formation of a protein molecule. AMP, Adenosine monophosphate; ATP, adenosine triphosphate; GTP, guanosine triphosphate; tRNA, transfer RNA.
图 3-12。蛋白质分子形成中的化学事件。AMP，腺苷单磷酸；ATP，腺苷三磷酸；GTP，鸟苷三磷酸；tRNA，转运 RNA。
controlled; otherwise, some parts of the cell might overgrow or some chemical reactions might overact until they kill the cell. Each cell has powerful internal feedback control mechanisms that keep the various functional operations of the cell in step with one another. For each gene ( ≈ 20 , 000 − 25 , 000 ( ≈ 20 , 000 − 25 , 000 (~~20,000-25,000(\approx 20,000-25,000 genes in all), at least one such feedback mechanism exists.
受控；否则，细胞的某些部分可能会过度生长，或者某些化学反应可能会过度反应，直到它们杀死细胞。每个细胞都有强大的内部反馈控制机制，使细胞的各种功能操作相互协调。对于每个基因（总共 ( ≈ 20 , 000 − 25 , 000 ( ≈ 20 , 000 − 25 , 000 (~~20,000-25,000(\approx 20,000-25,000 个基因），至少存在一个这样的反馈机制。
There are basically two methods whereby the biochemical activities in the cell are controlled: (1) genetic regulation, in which the degree of activation of the genes and the formation of gene products are themselves controlled, and (2) enzyme regulation, in which the activity levels of already formed enzymes in the cell are controlled.
细胞内的生化活动基本上有两种控制方法：（1）基因调控，控制基因的激活程度和基因产物的形成；（2）酶调控，控制细胞内已形成酶的活性水平。

GENETIC REGULATION 基因调控

Genetic regulation, or regulation of gene expression, covers the entire process from transcription of the genetic code in the nucleus to the formation of proteins in the cytoplasm. Regulation of gene expression provides all living organisms with the ability to respond to changes in their environment. In animals that have many different types of cells, tissues, and organs, differential regulation of gene expression also permits the different cell types in the body to each perform their specialized functions. Although a cardiac myocyte contains the same genetic code as a renal tubular epithelial cell, many genes are expressed in cardiac cells that are not expressed in renal tubular cells. The ultimate measure of gene “expression” is whether (and how much) of the gene products (proteins) are produced because proteins carry out cell functions specified by the genes. Regulation of gene expression can occur at any point in the pathways of transcription, RNA processing, and translation.
基因调控或基因表达调控涵盖了从细胞核中遗传密码的转录到细胞质中蛋白质的形成的整个过程。基因表达的调控使所有生物体能够对环境变化做出反应。在具有多种不同类型细胞、组织和器官的动物中，基因表达的差异性调控还允许体内不同类型的细胞各自执行其特定功能。尽管心肌细胞与肾小管上皮细胞具有相同的遗传密码，但心脏细胞中表达的许多基因在肾小管细胞中并不表达。基因“表达”的最终衡量标准是基因产物（蛋白质）是否（以及多少）被生产，因为蛋白质执行由基因指定的细胞功能。基因表达的调控可以在转录、RNA 加工和翻译的任何环节发生。
The Promoter Controls Gene Expression. Synthesis of cellular proteins is a complex process that starts with transcription of DNA into RNA. Transcription of DNA is
启动子控制基因表达。细胞蛋白质的合成是一个复杂的过程，始于 DNA 转录为 RNA。DNA 的转录是
Figure 3-13. Gene transcription in eukaryotic cells. A complex arrangement of multiple clustered enhancer modules is interspersed with insulator elements, which can be located upstream or downstream of a basal promoter containing TATA box (TATA), proximal promoter elements (response elements, RE), and initiator sequences (INR).
图 3-13. 真核细胞中的基因转录。多个聚集的增强子模块以复杂的方式排列，并夹杂着绝缘子元素，这些元素可以位于含有 TATA 盒（TATA）的基础启动子上游或下游，以及近端启动子元素（响应元素，RE）和启动子序列（INR）。
controlled by regulatory elements found in the promoter of a gene (Figure 3-13). In eukaryotes, which includes all mammals, the basal promoter consists of a sequence of bases (TATAAA) called the TATA box, the binding site for the TATA-binding protein and several other important transcription factors that are collectively referred to as the transcription factor IID complex. In addition to the transcription factor IID complex, this region is where transcription factor IIB binds to both the DNA and RNA polymerase 2 to facilitate transcription of the DNA into RNA. This basal promoter is found in all protein-coding genes, and the polymerase must bind with this basal promoter before it can begin traveling along the DNA strand to synthesize RNA. The upstream promoter is located farther upstream from the transcription start site and contains several binding sites for positive or negative transcription factors that can affect transcription through interactions with proteins bound to the basal promoter. The structure and transcription factor binding sites in the
由基因启动子中的调控元件控制（图 3-13）。在真核生物中，包括所有哺乳动物，基础启动子由一段碱基序列（TATAAA）组成，称为 TATA 框，是 TATA 结合蛋白和几个其他重要转录因子的结合位点，这些转录因子统称为转录因子 IID 复合体。除了转录因子 IID 复合体外，该区域是转录因子 IIB 与 DNA 和 RNA 聚合酶 2 结合的地方，以促进 DNA 转录为 RNA。这个基础启动子在所有编码蛋白质的基因中都存在，聚合酶必须与这个基础启动子结合，才能开始沿着 DNA 链移动以合成 RNA。上游启动子位于转录起始位点的更上游位置，包含多个正向或负向转录因子的结合位点，这些因子可以通过与结合在基础启动子上的蛋白质的相互作用影响转录。
upstream promoter vary from gene to gene to give rise to the different expression patterns of genes in different tissues.
上游启动子因基因而异，从而导致不同组织中基因的不同表达模式。
Transcription of genes in eukaryotes is also influenced by enhancers, which are regions of DNA that can bind transcription factors. Enhancers can be located a great distance from the gene they act on or even on a different chromosome. They can also be located upstream or downstream of the gene that they regulate. Although enhancers may be located far from their target gene, they may be relatively close when DNA is coiled in the nucleus. It is estimated that there are more than 100,000 gene enhancer sequences in the human genome.
真核生物中基因的转录也受到增强子的影响，增强子是可以结合转录因子的 DNA 区域。增强子可以位于它们作用的基因的很远处，甚至在不同的染色体上。它们也可以位于调控基因的上游或下游。尽管增强子可能远离其目标基因，但当 DNA 在细胞核中卷曲时，它们可能相对接近。估计人类基因组中有超过 100,000 个基因增强子序列。
In the organization of the chromosome, it is important to separate active genes that are being transcribed from genes that are repressed. This separation can be challenging because multiple genes may be located close together on the chromosome. The separation is achieved by chromosomal insulators. These insulators are gene sequences that provide a barrier so that a specific gene is isolated against transcriptional influences from surrounding genes. Insulators can vary greatly in their DNA sequence and the proteins that bind to them. One way an insulator activity can be modulated is by DNA methylation, which is the case for the mammalian insulin-like growth factor 2 (IGF2) gene. The mother’s allele has an insulator between the enhancer and promoter of the gene that allows for the binding of a transcriptional repressor. However, the paternal DNA sequence is methylated such that the transcriptional repressor cannot bind to the insulator, and the IGF-2 gene is expressed from the paternal copy of the gene.
在染色体的组织中，重要的是将正在转录的活跃基因与被抑制的基因分开。这种分离可能具有挑战性，因为多个基因可能紧密位于染色体上。分离是通过染色体绝缘体实现的。这些绝缘体是基因序列，提供了一种屏障，使特定基因能够隔离于周围基因的转录影响。绝缘体的 DNA 序列和与之结合的蛋白质可能有很大差异。调节绝缘体活性的一种方式是 DNA 甲基化，这在哺乳动物的胰岛素样生长因子 2（IGF2）基因中就是这种情况。母亲的等位基因在基因的增强子和启动子之间有一个绝缘体，允许转录抑制因子的结合。然而，父亲的 DNA 序列被甲基化，使得转录抑制因子无法结合到绝缘体上，从而 IGF-2 基因从父亲的基因拷贝中表达。
Other Mechanisms for Control of Transcription by the Promoter. Variations in the basic mechanism for control of the promoter have been discovered in the past three decades. Without giving details, let us list some of them:
其他启动子转录控制机制。在过去三十年中，发现了启动子控制基本机制的变异。无需详细说明，让我们列出其中的一些：
  1. A promoter is frequently controlled by transcription factors located elsewhere in the genome. That is, the regulatory gene causes the formation of a regulatory protein that in turn acts as an activator or repressor of transcription.
    启动子通常受到位于基因组其他地方的转录因子的控制。也就是说，调控基因导致调控蛋白的形成，而调控蛋白则作为转录的激活因子或抑制因子。
  2. Occasionally, many different promoters are controlled at the same time by the same regulatory protein. In some cases, the same regulatory protein functions as an activator for one promoter and as a repressor for another promoter.
    偶尔，许多不同的启动子会同时受到同一调节蛋白的控制。在某些情况下，同一调节蛋白在一个启动子中作为激活因子，而在另一个启动子中作为抑制因子。
  3. Some proteins are controlled not at the starting point of transcription on the DNA strand but farther along the strand. Sometimes, the control is not even at the DNA strand itself but occurs during the processing of the RNA molecules in the nucleus before they are released into the cytoplasm. Control may also occur at the level of protein formation in the cytoplasm during RNA translation by the ribosomes.
    一些蛋白质的控制并不是在 DNA 链的转录起始点，而是在链的更远处。有时，控制甚至不在 DNA 链本身，而是在 RNA 分子在细胞核中处理的过程中发生，然后它们被释放到细胞质中。控制也可能发生在细胞质中 RNA 翻译为蛋白质的过程中，由核糖体进行。
  4. In nucleated cells, the nuclear DNA is packaged in specific structural units, the chromosomes. Within each chromosome, the DNA is wound around small proteins called histones, which in turn are held tightly together in a compacted state by still other proteins. As long as the DNA is in this compacted state, it cannot function to form RNA. However, multiple control mechanisms are being discovered that can cause selected areas of chromosomes to become decompacted one part at a time, so that partial RNA transcription can occur. Even then, specific transcriptor factors control the actual rate of transcription by the promoter in the chromosome. Thus, still higher orders of control are used to establish proper cell function. In addition, signals from outside the cell, such as some of the body’s hormones, can activate specific chromosomal areas and specific transcription factors, therefore controlling the chemical machinery for function of the cell.
    在有核细胞中，核 DNA 被包装在特定的结构单元中，即染色体。在每条染色体内，DNA 缠绕在称为组蛋白的小蛋白质周围，而这些组蛋白又被其他蛋白质紧密结合在一起，形成压缩状态。只要 DNA 处于这种压缩状态，就无法发挥形成 RNA 的功能。然而，正在发现多种控制机制，可以使染色体的特定区域逐步解压，从而发生部分 RNA 转录。即便如此，特定的转录因子控制着染色体中启动子的实际转录速率。因此，还使用更高层次的控制来建立适当的细胞功能。此外，来自细胞外部的信号，如身体的一些激素，可以激活特定的染色体区域和特定的转录因子，从而控制细胞功能的化学机制。
    Because there are many thousands of different genes in each human cell, the large number of ways in which genetic activity can be controlled is not surprising. The gene control systems are especially important for controlling intracellular concentrations of amino acids, amino acid derivatives, and intermediate substrates and products of carbohydrate, lipid, and protein metabolism.
    因为每个人类细胞中有成千上万种不同的基因，基因活动控制的方式多种多样并不令人惊讶。基因控制系统对于调节细胞内氨基酸、氨基酸衍生物以及碳水化合物、脂质和蛋白质代谢的中间底物和产物的浓度尤其重要。

CONTROL OF INTRACELLULAR FUNCTION BY ENZYME REGULATION
酶调控对细胞内功能的控制

In addition to control of cell function by genetic regulation, cell activities are also controlled by intracellular inhibitors or activators that act directly on specific intracellular enzymes. Thus, enzyme regulation represents a second category of mechanisms whereby cellular biochemical functions can be controlled.
除了通过基因调控控制细胞功能外，细胞活动还受到作用于特定细胞内酶的细胞内抑制剂或激活剂的控制。因此，酶的调控代表了细胞生化功能控制的第二类机制。
Enzyme Inhibition. Some chemical substances formed in the cell have direct feedback effects to inhibit the specific enzyme systems that synthesize them. Almost always, the synthesized product acts on the first enzyme in a sequence, rather than on the subsequent enzymes, usually binding directly with the enzyme and causing an allosteric conformational change that inactivates it. One can readily recognize the importance of inactivating the first enzyme because this prevents buildup of intermediary products that are not used.
酶抑制。细胞内形成的一些化学物质对合成它们的特定酶系统具有直接的反馈抑制作用。合成的产物几乎总是作用于序列中的第一个酶，而不是后续的酶，通常直接与酶结合并导致变构构象变化，从而使其失活。人们很容易认识到使第一个酶失活的重要性，因为这可以防止未被使用的中间产物的积累。
Enzyme inhibition is another example of negative feedback control. It is responsible for controlling intracellular concentrations of multiple amino acids, purines, pyrimidines, vitamins, and other substances.
酶抑制是负反馈控制的另一个例子。它负责控制细胞内多种氨基酸、嘌呤、嘧啶、维生素和其他物质的浓度。
Enzyme Activation. Enzymes that are normally inactive often can be activated when needed. An example of this phenomenon occurs when most of the ATP has been depleted in a cell. In this case, a considerable amount of cyclic adenosine monophosphate (cAMP) begins to be formed as a breakdown product of ATP. The presence of this cAMP, in turn, immediately activates the glycogensplitting enzyme phosphorylase, liberating glucose mole-
酶的激活。通常处于非活性状态的酶在需要时往往可以被激活。这个现象的一个例子发生在细胞中大部分 ATP 被耗尽时。在这种情况下，作为 ATP 的分解产物，开始形成相当数量的环腺苷单磷酸（cAMP）。cAMP 的存在反过来立即激活糖原分解酶磷酸化酶，释放出葡萄糖分子。
cules that are rapidly metabolized, with their energy used for replenishment of the ATP stores. Thus, cAMP acts as an enzyme activator for the enzyme phosphorylase and thereby helps control intracellular ATP concentration.
快速代谢的物质，其能量用于补充 ATP 储存。因此，cAMP 作为酶激活剂，激活酶磷酸化酶，从而帮助控制细胞内 ATP 浓度。
Another interesting example of both enzyme inhibition and enzyme activation occurs in the formation of the purines and pyrimidines. These substances are needed by the cell in approximately equal quantities for the formation of DNA and RNA. When purines are formed, they inhibit the enzymes that are required for formation of additional purines. However, they activate the enzymes for formation of pyrimidines. Conversely, the pyrimidines inhibit their own enzymes but activate the purine enzymes. In this way, there is continual cross-talk between the synthesizing systems for these two substances, resulting in almost exactly equal amounts of the two substances in the cells at all times.
另一个有趣的酶抑制和酶激活的例子发生在嘌呤和嘧啶的形成中。这些物质在细胞中以大致相等的数量用于 DNA 和 RNA 的形成。当嘌呤形成时，它们抑制形成额外嘌呤所需的酶。然而，它们激活形成嘧啶的酶。相反，嘧啶抑制它们自己的酶，但激活嘌呤酶。通过这种方式，这两种物质的合成系统之间不断进行交互，导致细胞中这两种物质的数量始终几乎完全相等。
Summary. There are two principal mechanisms whereby cells control proper proportions and quantities of different cellular constituents: (1) genetic regulation; and (2) enzyme regulation. The genes can be activated or inhibited, and likewise, the enzyme systems can be activated or inhibited. These regulatory mechanisms usually function as feedback control systems that continually monitor the cell’s biochemical composition and make corrections as needed. However, on occasion, substances from outside the cell (especially some of the hormones discussed in this text) also control the intracellular biochemical reactions by activating or inhibiting one or more of the intracellular control systems.
总结。细胞控制不同细胞成分的适当比例和数量主要有两种机制：（1）基因调控；（2）酶调控。基因可以被激活或抑制，同样，酶系统也可以被激活或抑制。这些调控机制通常作为反馈控制系统运行，持续监测细胞的生化成分，并根据需要进行修正。然而，有时来自细胞外的物质（特别是本文讨论的一些激素）也通过激活或抑制一个或多个细胞内控制系统来控制细胞内的生化反应。

THE DNA-GENETIC SYSTEM CONTROLS CELL REPRODUCTION
DNA-遗传系统控制细胞繁殖

Cell reproduction is another example of the ubiquitous role that the DNA-genetic system plays in all life processes. The genes and their regulatory mechanisms determine cell growth characteristics and when or whether cells will divide to form new cells. In this way, the allimportant genetic system controls each stage in the development of the human, from the single-cell fertilized ovum to the whole functioning body. Thus, if there is any central theme to life, it is the DNA-genetic system.
细胞繁殖是 DNA-遗传系统在所有生命过程中普遍作用的另一个例子。基因及其调控机制决定了细胞的生长特征，以及细胞何时或是否会分裂形成新细胞。通过这种方式，至关重要的遗传系统控制着人类发展的每一个阶段，从单细胞受精卵到完整的功能身体。因此，如果生命有任何中心主题，那就是 DNA-遗传系统。

Life Cycle of the Cell
细胞的生命周期

The life cycle of a cell is the period from cell reproduction to the next cell reproduction. When mammalian cells are not inhibited and are reproducing as rapidly as they can, this life cycle may be as little as 10 to 30 hours. It is terminated by a series of distinct physical events called mitosis that cause division of the cell into two new daughter cells. The events of mitosis are shown in Figure 3-14 and described later. The actual stage of mitosis, however, lasts for only about 30 minutes, and thus more than 95 % 95 % 95%95 \% of the life cycle of even rapidly reproducing cells is represented by the interval between mitosis, called interphase.
细胞的生命周期是从细胞繁殖到下一个细胞繁殖的时期。当哺乳动物细胞没有受到抑制并尽可能快速地繁殖时，这个生命周期可能短至 10 到 30 小时。它通过一系列称为有丝分裂的明显物理事件终止，这些事件导致细胞分裂成两个新的子细胞。有丝分裂的事件在图 3-14 中显示，并在后面描述。然而，有丝分裂的实际阶段仅持续约 30 分钟，因此即使是快速繁殖细胞的生命周期中，超过 95 % 95 % 95%95 \% 的时间都由有丝分裂之间的间隔（称为间期）所代表。
Except in special conditions of rapid cellular reproduction, inhibitory factors almost always slow or stop the
除非在细胞快速繁殖的特殊条件下，抑制因素几乎总是会减缓或停止

Figure 3-14. Stages of cell reproduction. A, B, C, Prophase. D, Prometaphase. E, Metaphase. F, Anaphase. G, H, Telophase.
图 3-14. 细胞繁殖的阶段。A, B, C, 前期。D, 前中期。E, 中期。F, 后期。G, H, 末期。
uninhibited life cycle of the cell. Therefore, different cells of the body actually have life cycle periods that vary from as little as 10 hours for highly stimulated bone marrow cells to an entire lifetime of the human body for many nerve cells.
细胞的无拘无束的生命周期。因此，身体的不同细胞实际上具有不同的生命周期，从高度刺激的骨髓细胞的 10 小时到许多神经细胞的整个人生。

Cell Reproduction Begins with Replication of DNA
细胞繁殖始于 DNA 的复制

The first step of cell reproduction is replication (duplication) of all DNA in the chromosomes. It is only after this replication has occurred that mitosis can take place.
细胞繁殖的第一步是染色体中所有 DNA 的复制（重复）。只有在这种复制发生后，才能进行有丝分裂。
The DNA begins to be duplicated 5 to 10 hours before mitosis, and the duplication is completed in 4 to 8 hours. The net result is two exact replicas of all DNA. These replicas become the DNA in the two new daughter cells that will be formed at mitosis. After replication of the DNA, there is another period of 1 to 2 hours before mitosis begins abruptly. Even during this period, preliminary changes that will lead to the mitotic process are beginning to take place.
DNA 在有丝分裂前 5 到 10 小时开始复制，复制在 4 到 8 小时内完成。最终结果是所有 DNA 的两个精确复制品。这些复制品成为在有丝分裂时形成的两个新子细胞中的 DNA。在 DNA 复制后，还有 1 到 2 小时的时间，然后有丝分裂突然开始。即使在这个期间，导致有丝分裂过程的初步变化也开始发生。
DNA Replication. DNA is replicated in much the same way that RNA is transcribed from DNA, except for a few important differences:
DNA 复制。DNA 的复制方式与 RNA 从 DNA 转录的方式非常相似，但有一些重要的区别：
  1. Both strands of the DNA in each chromosome are replicated, not just one of them.
    每条染色体中的 DNA 双链都会被复制，而不仅仅是其中一条。
Figure 3-15. DNA replication, showing the replication fork and leading and lagging strands of DNA.
图 3-15. DNA 复制，显示复制叉以及 DNA 的领先链和滞后链。
2. Both entire strands of the DNA helix are replicated from end to end, rather than small portions of them, as occurs in the transcription of RNA.
DNA 双螺旋的两个完整链条是从头到尾复制的，而不是像 RNA 转录中那样只复制小部分。
3. Multiple enzymes called DNA polymerase, which is comparable to RNA polymerase, are essential for replicating DNA. DNA polymerase attaches to and moves along the DNA template strand, adding nucleotides in the 5 ′ 5 ′ 5^(')5^{\prime} to 3 ′ 3 ′ 3^(')3^{\prime} direction. Another enzyme, DNA ligase, causes bonding of successive DNA nucleotides to one another, using high-energy phosphate bonds to energize these attachments.
3. 多种酶称为 DNA 聚合酶，与 RNA 聚合酶相似，对于复制 DNA 至关重要。DNA 聚合酶附着并沿着 DNA 模板链移动，按 5 ′ 5 ′ 5^(')5^{\prime} 到 3 ′ 3 ′ 3^(')3^{\prime} 的方向添加核苷酸。另一种酶，DNA 连接酶，促使连续的 DNA 核苷酸相互结合，利用高能磷酸键为这些连接提供能量。
4. Replication fork formation. Before DNA can be replicated, the double-stranded molecule must be “unzipped” into two single strands (Figure 3-15). Because the DNA helixes in each chromosome are approximately 6 centimeters in length and have millions of helical turns, it would be impossible for the two newly formed DNA helixes to uncoil from each other were it not for some special mechanism. This uncoiling is achieved by DNA helicase enzymes that break the hydrogen bonding between the base pairs of the DNA, permitting the two strands to separate into a Y shape known as the replication fork, the area that will be the template for replication to begin.
4. 复制叉形成。在 DNA 复制之前，双链分子必须被“解开”成两条单链（图 3-15）。由于每条染色体中的 DNA 螺旋大约长 6 厘米，并且有数百万个螺旋转动，如果没有某种特殊机制，两个新形成的 DNA 螺旋将无法相互解开。这种解开是通过 DNA 解旋酶实现的，它们打破 DNA 碱基对之间的氢键，使两条链分开成一个 Y 形，称为复制叉，这是复制开始的模板区域。
DNA is directional in both strands, signified by a 5 ′ 5 ′ 5^(')5^{\prime} and 3’ end (see Figure 3-15). Replication progresses only in the 5 ′ 5 ′ 5^(')5^{\prime} to 3 ′ 3 ′ 3^(')3^{\prime} direction. At the replication fork one strand, the leading strand, is oriented in the 3 ′ 3 ′ 3^(')3^{\prime} to 5 ′ 5 ′ 5^(')5^{\prime} direction, toward the replication fork, while the lagging strand is oriented 5 ′ 5 ′ 5^(')5^{\prime} to 3 ′ 3 ′ 3^(')3^{\prime}, away from the replication fork. Because of their different orientations, the two strands are replicated differently.
DNA 在两个链中都是定向的，以 5 ′ 5 ′ 5^(')5^{\prime} 和 3'末端表示（见图 3-15）。复制仅在 5 ′ 5 ′ 5^(')5^{\prime} 到 3 ′ 3 ′ 3^(')3^{\prime} 方向进行。在复制叉处，一条链，即领先链，朝向复制叉的 3 ′ 3 ′ 3^(')3^{\prime} 到 5 ′ 5 ′ 5^(')5^{\prime} 方向，而滞后链则朝 5 ′ 5 ′ 5^(')5^{\prime} 到 3 ′ 3 ′ 3^(')3^{\prime} 方向，远离复制叉。由于它们的不同方向，这两条链的复制方式也不同。
5. Primer binding. Once the DNA strands have been separated, a short piece of RNA called an RNA primer binds to the 3 ′ 3 ′ 3^(')3^{\prime} end of the leading strand. Primers are generated by the enzyme D N A D N A DNAD N A primase.
5. 引物结合。一旦 DNA 链被分开，一段称为 RNA 引物的短 RNA 片段会结合到 3 ′ 3 ′ 3^(')3^{\prime} 链的前导端。引物是由酶 D N A D N A DNAD N A 引物酶生成的。
Primers always bind as the starting point for DNA replication.
引物总是作为 DNA 复制的起始点结合。
6. Elongation. DNA polymerases are responsible for creating the new strand by a process called elongation. Because replication proceeds in the 5 ′ 5 ′ 5^(')5^{\prime} to 3 ′ 3 ′ 3^(')3^{\prime} direction on the leading strand, the newly formed strand is continuous. The lagging strand begins replication by binding with multiple primers that are only several bases apart. DNA polymerase then adds pieces of DNA, called Okazaki fragments, to the strand between primers. This process of replication is discontinuous because the newly created Okazaki fragments are not yet connected. An enzyme, DNA ligase, joins the Okazaki fragments to form a single unified strand.
6. 延伸。DNA 聚合酶负责通过一种称为延伸的过程来创建新链。由于在前导链上复制是从 5 ′ 5 ′ 5^(')5^{\prime} 到 3 ′ 3 ′ 3^(')3^{\prime} 方向进行的，新形成的链是连续的。滞后链通过与多个相距仅几碱基的引物结合开始复制。然后，DNA 聚合酶在引物之间向链中添加称为冈崎片段的 DNA 片段。这个复制过程是不连续的，因为新创建的冈崎片段尚未连接。一个酶，DNA 连接酶，将冈崎片段连接起来形成一个统一的单链。
7. Termination. After the continuous and discontinuous strands are both formed, the enzyme exonuclease removes the RNA primers from the original strands, and the primers are replaced with appropriate bases. Another exonuclease “proofreads” the newly formed DNA, checking and clipping off any mismatched or unpaired residues.
7. 终止。在连续和不连续链都形成后，酶外切酶从原始链中去除 RNA 引物，并用适当的碱基替换引物。另一个外切酶对新形成的 DNA 进行“校对”，检查并剪切掉任何不匹配或未配对的残基。
Another enzyme, topoisomerase, can transiently break the phosphodiester bond in the backbone of the DNA strand to prevent the DNA in front of the replication fork from being overwound. This reaction is reversible, and the phosphodiester bond reforms as the topoisomerase leaves.
另一种酶，拓扑异构酶，可以暂时断开 DNA 链主链中的磷酸二酯键，以防止复制叉前面的 DNA 被过度扭绕。这个反应是可逆的，当拓扑异构酶离开时，磷酸二酯键会重新形成。
Once completed, the parent strand and its complementary DNA strand coils into the double helix shape. The process of replication therefore produces two DNA molecules, each with one strand from the parent DNA and one new strand. For this reason, DNA replication is often described as semiconservative; half of the chain is part of the original DNA molecule and half is brand new.
一旦完成，母链及其互补 DNA 链会卷曲成双螺旋形状。因此，复制过程产生两个 DNA 分子，每个分子都有一条来自母 DNA 的链和一条新的链。因此，DNA 复制通常被描述为半保留的；链的一半是原始 DNA 分子的一部分，另一半是全新的。
DNA Repair, DNA “Proofreading,” and “Mutation.” During the hour or so between DNA replication and
DNA 修复、DNA“校对”和“突变”。在 DNA 复制和之间的一个小时左右
the beginning of mitosis, there is a period of active repair and “proofreading” of the DNA strands. Wherever inappropriate DNA nucleotides have been matched up with the nucleotides of the original template strand, special enzymes cut out the defective areas and replace them with appropriate complementary nucleotides. This repair process, which is achieved by the same DNA polymerases and DNA ligases that are used in replication, is referred to as DNA proofreading.
在有丝分裂的开始阶段，DNA 链会经历一个活跃的修复和“校对”时期。无论在哪里，不适当的 DNA 核苷酸与原模板链的核苷酸配对，特殊的酶会切除缺陷区域，并用适当的互补核苷酸替换它们。这个修复过程是由与复制中使用的相同的 DNA 聚合酶和 DNA 连接酶完成的，称为 DNA 校对。
Because of repair and proofreading, mistakes are rarely made in the DNA replication process. When a mistake is made, it is called a mutation. The mutation may cause formation of some abnormal protein in the cell rather than a needed protein, which may lead to abnormal cellular function and sometimes even cell death. Given that many thousands of genes exist in the human genome, and that the period from one human generation to another is about 30 years, one would expect as many as 10 or many more mutations in the passage of the genome from parent to offspring. As a further protection, however, each human genome is represented by two separate sets of chromosomes, one derived from each parent, with almost identical genes. Therefore, one functional gene of each pair is almost always available to the child, despite mutations.
由于修复和校对，DNA 复制过程中很少出现错误。当出现错误时，这被称为突变。突变可能导致细胞中形成一些异常蛋白，而不是所需的蛋白，这可能导致细胞功能异常，有时甚至导致细胞死亡。考虑到人类基因组中存在数千个基因，以及从一代人到另一代人的时间约为 30 年，可以预期在基因组从父母传递给后代的过程中会出现多达 10 个或更多的突变。然而，作为进一步的保护，每个人类基因组由两组独立的染色体组成，一组来自每个父母，几乎具有相同的基因。因此，尽管存在突变，每对基因中的一个功能基因几乎总是可以提供给孩子。

CHROMOSOMES AND THEIR REPLICATION
染色体及其复制

The DNA helixes of the nucleus are packaged in chromosomes. The human cell contains 46 chromosomes arranged in 23 pairs. Most of the genes in the two chromosomes of each pair are identical or almost identical to each other, so it is usually stated that the different genes also exist in pairs, although occasionally this is not the case.
细胞核中的 DNA 螺旋被包装在染色体中。人类细胞包含 46 条染色体，分成 23 对。每对染色体中的大多数基因是相同或几乎相同的，因此通常认为不同的基因也成对存在，尽管偶尔并非如此。
In addition to DNA, there is a large amount of protein in the chromosome, composed mainly of many small molecules of electropositively charged histones. The histones are organized into vast numbers of small, bobbinlike cores. Small segments of each DNA helix are coiled sequentially around one core after another.
除了 DNA，染色体中还有大量的蛋白质，主要由许多带正电的小分子组蛋白组成。组蛋白被组织成大量的小型纺锤状核心。每个 DNA 螺旋的小段依次缠绕在一个又一个核心周围。
The histone cores play an important role in regulation of DNA activity because as long as the DNA is packaged tightly, it cannot function as a template for formation of RNA or replication of new DNA. Furthermore, some of the regulatory proteins decondense the histone packaging of the DNA and allow small segments at a time to form RNA.
组蛋白核心在调节 DNA 活性中发挥着重要作用，因为只要 DNA 被紧密包装，它就无法作为 RNA 形成或新 DNA 复制的模板。此外，一些调节蛋白会解开 DNA 的组蛋白包装，允许小段 DNA 逐次形成 RNA。
Several nonhistone proteins are also major components of chromosomes, functioning as chromosomal structural proteins and, in connection with the genetic regulatory machinery, as activators, inhibitors, and enzymes.
几种非组蛋白也是染色体的主要成分，作为染色体结构蛋白发挥作用，并与遗传调控机制相结合，充当激活剂、抑制剂和酶。
Replication of the chromosomes in their entirety occurs during the next few minutes after replication of the DNA helixes has been completed; the new DNA helixes collect new protein molecules as needed. The two newly formed chromosomes remain attached to each other (until time for mitosis) at a point called the centromere located near their center. These duplicated but still attached chromosomes are called chromatids.
在 DNA 螺旋复制完成后的几分钟内，染色体的完整复制发生；新的 DNA 螺旋根据需要收集新的蛋白质分子。两个新形成的染色体在一个称为着丝点的地方相互连接（直到有丝分裂的时刻），该着丝点位于它们的中心附近。这些复制但仍然连接的染色体被称为姐妹染色单体。

CELL MITOSIS 细胞有丝分裂

The actual process whereby the cell splits into two new cells is called mitosis. Once each chromosome has been replicated to form the two chromatids, mitosis follows automatically within 1 or 2 hours in many cells.
细胞分裂成两个新细胞的实际过程称为有丝分裂。一旦每条染色体被复制形成两个染色单体，有丝分裂通常会在许多细胞中自动发生，时间为 1 到 2 小时。
Mitotic Apparatus: Function of the Centrioles. One of the first events of mitosis takes place in the cytoplasm in or around the small structures called centrioles during the latter part of interphase. As shown in Figure 3-14, two pairs of centrioles lie close to each other near one pole of the nucleus. These centrioles, like the DNA and chromosomes, are also replicated during interphase, usually shortly before replication of the DNA. Each centriole is a small cylindrical body about 0.4 micrometer long and about 0.15 micrometer in diameter, consisting mainly of nine parallel tubular structures arranged in the form of a cylinder. The two centrioles of each pair lie at right angles to each other. Each pair of centrioles, along with attached pericentriolar material, is called a centrosome.
有丝分裂装置：中心粒的功能。有丝分裂的第一个事件发生在细胞质中，发生在称为中心粒的小结构周围，通常在间期的后期。如图 3-14 所示，两个中心粒对靠近细胞核的一极相互靠近。这些中心粒与 DNA 和染色体一样，在间期也会复制，通常在 DNA 复制之前不久。每个中心粒是一个约 0.4 微米长、直径约 0.15 微米的小圆柱体，主要由九个平行的管状结构以圆柱形排列而成。每对中心粒之间呈直角排列。每对中心粒及其附着的周围中心粒物质称为中心体。
Shortly before mitosis takes place, the two pairs of centrioles begin to move apart from each other. This movement is caused by polymerization of protein microtubules growing between the respective centriole pairs and actually pushing them apart. At the same time, other microtubules grow radially away from each of the centriole pairs, forming a spiny star called the aster, in each end of the cell. Some of the spines of the aster penetrate the nuclear membrane and help separate the two sets of chromatids during mitosis. The complex of microtubules extending between the two new centriole pairs is called the spindle, and the entire set of microtubules plus the two pairs of centrioles is called the mitotic apparatus.
在有丝分裂发生之前不久，两个中心粒对开始相互分离。这种运动是由于在各自的中心粒对之间生长的蛋白质微管的聚合，实际上将它们推开。同时，其他微管从每对中心粒向外放射生长，形成一个称为星体的刺状星形结构，位于细胞的两端。星体的一些刺穿透核膜，并在有丝分裂过程中帮助分离两组染色单体。延伸在两个新中心粒对之间的微管复合体称为纺锤体，而整个微管集合加上这两对中心粒被称为有丝分裂装置。
Prophase. The first stage of mitosis, called prophase, is shown in Figure 3-14 A , B A , B A,B\boldsymbol{A}, \boldsymbol{B}, and C C C\boldsymbol{C}. While the spindle is forming, the chromosomes of the nucleus (which in interphase consist of loosely coiled strands) become condensed into well-defined chromosomes.
前期。减数分裂的第一阶段称为前期，如图 3-14 所示。在纺锤体形成的同时，细胞核中的染色体（在间期时呈松散卷曲的状态）变得浓缩成明确的染色体。
Prometaphase. During the prometaphase stage (see Figure 3-14D), the growing microtubular spines of the aster fragment the nuclear envelope. At the same time, multiple microtubules from the aster attach to the chromatids at the centromeres, where the paired chromatids are still bound to each other. The tubules then pull one chromatid of each pair toward one cellular pole and its partner toward the opposite pole.
前中期。在前中期阶段（见图 3-14D），星体的生长微管刺破核膜。与此同时，来自星体的多个微管附着在着丝粒处的染色单体上，此时成对的染色单体仍然相互结合。然后，微管将每对中的一个染色单体拉向一个细胞极，而其伴侣则拉向相对的极。
Metaphase. During the metaphase stage (see Figure 3-14E), the two asters of the mitotic apparatus are pushed farther apart. This pushing is believed to occur because the microtubular spines from the two asters, where they interdigitate with each other to form the mitotic spindle,
中期。在中期阶段（见图 3-14E），有丝分裂装置的两个星体被推得更远。这种推挤被认为是由于来自两个星体的微管脊相互交错形成有丝分裂纺锤体。
push each other away. Minute contractile protein molecules called “molecular motors,” which may be composed of the muscle protein actin, extend between the respective spines and, using a stepping action as in muscle, actively slide the spines in a reverse direction along each other. Simultaneously, the chromatids are pulled tightly by their attached microtubules to the very center of the cell, lining up to form the equatorial plate of the mitotic spindle.
相互推开。称为“分子马达”的微小收缩蛋白分子，可能由肌肉蛋白肌动蛋白组成，延伸在各自的脊柱之间，并像肌肉一样以踏步动作主动滑动脊柱，使其沿相反方向相互滑动。同时，染色单体通过其附着的微管被紧紧拉向细胞的中心，排列成有丝分裂纺锤体的赤道板。
Anaphase. During the anaphase stage (see Figure 3 − 1 4 F 3 − 1 4 F 3-14F3-\mathbf{1 4 F} ), the two chromatids of each chromosome are pulled apart at the centromere. All 46 pairs of chromatids are separated, forming two separate sets of 46 daughter chromosomes. One of these sets is pulled toward one mitotic aster, and the other is pulled toward the other aster, as the two respective poles of the dividing cell are pushed still farther apart.
后期。在后期阶段（见图 3 − 1 4 F 3 − 1 4 F 3-14F3-\mathbf{1 4 F} ），每条染色体的两个染色单体在着丝点处被拉开。所有 46 对染色单体被分开，形成两组各 46 条的子染色体。这两组中的一组被拉向一个有丝分裂星体，另一组被拉向另一个星体，因为分裂细胞的两个极被进一步推开。
Telophase. In the telophase stage (see Figure 3-14G and H H HH ), the two sets of daughter chromosomes are pushed completely apart. Then, the mitotic apparatus dissipates, and a new nuclear membrane develops around each set of chromosomes. This membrane is formed from portions of the endoplasmic reticulum that are already present in the cytoplasm. Shortly thereafter, the cell pinches in two, midway between the two nuclei. This pinching is caused by the formation of a contractile ring of microfilaments composed of actin and probably myosin (the two contractile proteins of muscle) at the juncture of the newly developing cells that pinches them off from each other.
末期。在末期阶段（见图 3-14G 和 H H HH ），两组子染色体被完全推开。然后，纺锤体消散，新的核膜在每组染色体周围形成。这个膜是由已经存在于细胞质中的内质网部分形成的。随后，细胞在两个细胞核之间的中间位置收缩成两部分。这种收缩是由于在新形成的细胞交界处形成了由肌动蛋白和可能的肌球蛋白（肌肉的两种收缩蛋白）组成的收缩环微丝，导致它们相互分离。

CONTROL OF CELL GROWTH AND CELL REPRODUCTION
细胞生长和细胞繁殖的控制

Some cells grow and reproduce all the time, such as the blood-forming cells of the bone marrow, the germinal layers of the skin, and the epithelium of the gut. Many other cells, however, such as smooth muscle cells, may not reproduce for many years. A few cells, such as the neurons and most striated muscle cells, do not reproduce during the entire life of a person, except during the original period of fetal life.
一些细胞一直在生长和繁殖，例如骨髓中的造血细胞、皮肤的生殖层和肠道的上皮细胞。然而，许多其他细胞，如平滑肌细胞，可能多年不再繁殖。一些细胞，如神经元和大多数横纹肌细胞，在一个人的整个生命中都不再繁殖，除了在胎儿生活的最初阶段。
In certain tissues, an insufficiency of some types of cells causes them to grow and reproduce rapidly until appropriate numbers of these cells are again available. For example, in some young animals, seven-eighths of the liver can be removed surgically, and the cells of the remaining one-eighth will grow and divide until the liver mass returns to almost normal. The same phenomenon occurs for many glandular cells and most cells of the bone marrow, subcutaneous tissue, intestinal epithelium, and almost any other tissue except highly differentiated cells such as nerve and muscle cells.
在某些组织中，某些类型细胞的不足导致它们迅速生长和繁殖，直到再次有适当数量的这些细胞可用。例如，在一些年轻动物中，可以外科手术切除七分之八的肝脏，剩下的八分之一的细胞会生长和分裂，直到肝脏的质量几乎恢复到正常。许多腺体细胞和大多数骨髓、皮下组织、肠上皮以及几乎任何其他组织（除了高度分化的细胞，如神经细胞和肌肉细胞）也会出现相同的现象。
The mechanisms that maintain proper numbers of the different types of cells in the body are still poorly understood. However, experiments have shown at least three ways in which growth can be controlled. First, growth often is controlled by growth factors that come from other parts of the body. Some of these growth factors circulate in the blood, but others originate in adjacent tissues. For example, the epithelial cells of some glands, such as the pancreas, fail to grow without a growth factor from the underlying connective tissue of the gland. Second, most normal cells stop growing when they have run out of space for growth. This phenomenon occurs when cells are grown in tissue culture; the cells grow until they contact a solid object, and then growth stops. Third, cells grown in tissue culture often stop growing when minute amounts of their own secretions are allowed to collect in the culture medium. This mechanism, too, could provide a means for negative feedback control of growth.
维持身体中不同类型细胞适当数量的机制仍然不太清楚。然而，实验表明至少有三种方式可以控制生长。首先，生长通常受到来自身体其他部分的生长因子的控制。这些生长因子中有些在血液中循环，但其他一些则源自相邻组织。例如，某些腺体（如胰腺）的上皮细胞在没有来自腺体下方结缔组织的生长因子时无法生长。其次，大多数正常细胞在没有生长空间时会停止生长。当细胞在组织培养中生长时，这种现象会发生；细胞生长直到接触到固体物体，然后生长停止。第三，在组织培养中生长的细胞通常在其自身分泌物的微量积聚在培养基中时停止生长。这种机制也可能为生长提供负反馈控制的手段。
Telomeres Prevent the Degradation of Chromosomes. A telomere is a region of repetitive nucleotide sequences located at each end of a chromatid (Figure 3-16). Telomeres serve as protective caps that prevent the chromosome from deterioration during cell division. During cell division, a short piece of “primer” RNA attaches to the DNA strand to start the replication. However, because the primer does not attach at the very end of the DNA strand, the copy is missing a small section of the DNA. With each cell division, the copied DNA loses additional nucleotides from the telomere region. The nucleotide sequences provided by the telomeres therefore prevent the degradation of genes near the ends of chromosomes. Without telomeres, the genomes would progressively lose information and be truncated after each cell division. Thus, the telomeres can be considered to be disposable chromosomal buffers that help maintain stability of the genes but are gradually consumed during repeated cell divisions.
端粒防止染色体的降解。端粒是位于每条染色单体两端的重复核苷酸序列区域（图 3-16）。端粒作为保护帽，防止染色体在细胞分裂过程中退化。在细胞分裂期间，一小段“引物”RNA 附着在 DNA 链上以开始复制。然而，由于引物并未附着在 DNA 链的最末端，复制的 DNA 缺少了一小部分。随着每次细胞分裂，复制的 DNA 会从端粒区域失去额外的核苷酸。因此，端粒提供的核苷酸序列可以防止靠近染色体末端的基因降解。如果没有端粒，基因组将逐渐失去信息，并在每次细胞分裂后被截断。因此，端粒可以被视为一次性染色体缓冲区，帮助维持基因的稳定性，但在重复的细胞分裂过程中逐渐被消耗。
Figure 3-16. Control of cell replication by telomeres and telomerase. The cells’ chromosomes are capped by telomeres, which, in the absence of telomerase activity, shorten with each cell division until the cell stops replicating. Therefore, most cells of the body cannot replicate indefinitely. In cancer cells, telomerase is activated, and telomere length is maintained so that the cells continue to replicate themselves uncontrollably.
图 3-16. 端粒和端粒酶对细胞复制的控制。细胞的染色体由端粒封顶，在缺乏端粒酶活性的情况下，随着每次细胞分裂而缩短，直到细胞停止复制。因此，身体的大多数细胞不能无限制地复制。在癌细胞中，端粒酶被激活，端粒长度得以维持，从而使细胞继续不受控制地复制自身。