DNA, deoxyribonucleic acid, is the genetic substance of all living cells and many viruses. The hereditary information transmitted from each generation to the next is encoded in the structures of the DNA molecules. These molecules are very long, essentially one-dimensional chains, composed of four type of subunits; the genetic information is conveyed in the sequence of the subunits (see NUCLEIC ACIDS).

DNA，脱氧核糖核酸，是所有活细胞和许多病毒的遗传物质。这种代代相传的遗传信息被编码在DNA分子的结构中。这些分子都非常长，本质上是一维链，由四类亚基构成；遗传信息是以亚基的序列传递（可参阅核酸词条）。

   DNA was first discovered by the German biochemist Friedrich Miescher in 1869. However, it was not until 1953 that the actual structure of the DNA molecules was finally determined. This was done in great part by the combined efforts of the American biochemist James D. Watson, the British molecular biologist Francis Crick, and the British biophysicist Maurice Wilkins.

    1869年，德国生物化学家弗里德里希·米歇尔首次发现了DNA。然而，直到1953年才最终确定了DNA分子的实际结构。在很大程度上，这是由美国生物化学家詹姆斯·D. 沃森，英国分子生物学家弗朗西斯·克里克和英国生物物理学家莫里斯·威尔金斯协作的结果。

   Structure.  The subunits of DNA, which are called nucleotides, are each composed of a carbohydrate residue (deoxyribose), a phosphate group, and a heterocyclic base. The chain is formed by coupling each deoxyribose to the deoxyribose of the next subunit through a phosphate group. There are four heterocyclic bases in DNA—adenine, guanine, cytosine, and thymine.

    结构。DNA的亚基被称为核苷酸，每个都是由一个碳水化合物的残留（脱氧核糖），一个磷酸基和一个杂环的碱基组成。该链是通过一个磷酸基将每个脱氧核糖与下一个亚基的脱氧核连接形成的。在DNA中有四个杂环的碱基---腺嘌呤、鸟嘌呤、胞嘧啶和胸腺嘧啶。

   DNA molecules are most often found as double polynucleotide chains, intertwined to form a double helix. The nucleotides in the two chains of typical DNA are matched in such a way that adenine in one chain is always paired with thymine in the other, while guanine in one chain is always paired with cytosine in the other. Thus the two chains bear a precisely complementary relationship to each other, and the nucleotide sequence of either chain directly specifies the sequence of the other chain. In some viruses, however, DNA is present in a single-stranded form.

    DNA分子最常见的是缠绕形成的一个双螺旋的双多核苷酸链。在典型的DNA两个链中的核苷酸以这样一种方式相匹配，即一个链上的腺嘌呤总是与另一个链上的胸腺嘧啶成对，而一个链上的鸟嘌呤总是与另一个链上的胞嘧啶成对。因此，两个链彼此承担着一种精确的互补关系，而且任何一个链的核苷酸序列直接指定了另一个链的序列。然而，在一些病毒中，DNA是以单链形式存在的。

   A virus particle contains only one DNA molecule, which ranges in length from 5,000 to over 200,000 subunits, depending on the virus. The number of molecules of DNA in the cell of a higher organism is unknown; the total length of DNA in a human cell would include 5 billion subunits. Over 99% of this DNA is found in the chromosomes in the cell nucleus. A small but distinct fraction is found in the mitochondria, and in plant cells DNA is also found in the chloroplasts.

    一个病毒粒子只包含一个DNA分子，根据病毒的情况，长度范围从5千到20万个亚基不等。在更高级的有机体细胞中，DNA的分子数量是未知的。在人类细胞中DNA的总长度将包含50亿个亚基。在细胞核的染色体中发现有超过99%的这种DNA。在线粒体中发现了一种小而不同的部分，并且在叶绿体中也发现了植物细胞的DNA。

   Replication.  As DNA molecules contain the hereditary instructions, a copy of each must be transmitted to each daughter cell at cell division. Replication of the DNA molecule is accomplished prior to division by a progressive separation of the two strands of the double helix, followed by the enzymatically catalyzed synthesis, from subunits of complementary strands about each separated strand. The single strands act as a template during this process. The over-all result is the formation of two double helices, each identical to the original.

    复制。由于DNA分子包含了遗传指令，因此在细胞分裂时，每个子细胞都必定要传递一个副本。DNA分子的复制是在分裂前通过逐步分离双螺旋的两个链，接着从每个分离链的互补链亚基进行酶催化合成完成的。在这个过程中单链担当了一个模板。总的结果是形成两个双螺旋线，每个都与原先的完全相同。

   A partial replication can be used for repair of DNA. Maintenance of the fidelity of the hereditary instructions is of paramount importance to the cell and its progeny. If the damage is restricted to one strand of the double helix, is can be repaired in a two-stage process in which the damaged section is first removed by an enzyme and then replaced by re-synthesis on the corresponding section of the undamaged complementary strand. Mechanisms for such repair are present in many types of cells.

    部分的复制可用于对DNA的修复。对遗传指令精确性的维护对于细胞及其子代是至关重要的。如果损坏仅限于双螺旋的一个链，就可在一个两阶段的过程中得以修复，其中，首先用一种酶祛除受损的部分，然后在未受损的互补链相应部分上通过再合成来替代。许多细胞类型都有这种修复机制。

   In some viruses, DNA is present in a single-stranded form. During the replication a complementary double-stranded intermediate is formed, which replicates in the normal manner. A final stage of asymmetric replication provides the single strands for the progeny particles.

    在一些病毒中，DNA是以一种单链形式存在的。在复制过程中，会形成一种互补的双链中间体，它会以正常的方式复制。不对称复制的最终阶段为子代粒子提供了单链。

   Mutation.  Occasional mistakes occur in DNA replication as a consequence of radiation effects, thermal modifications of structure, or the presence in the cell of certain chemical substances. These result in a change in the nucleotide pattern and thereby in a genetic mutation. In the course of evolution increments of DNA content, accompanied by mutational changes in the DNA, have provided the hereditary variations from which new species could arise.

    突变。由于辐射影响，结构的热改变，或细胞中出现的特定化学物质的结果，在DNA复制中偶尔会出现错误。这会导致核苷酸模式的改变，从而导致基因突变。在进化过程中，伴随着DNA中突变变化的DNA含量的增长，为可能产生的新物种提供了遗传变异。

   Function.  A major portion of hereditary instructions of DNA is the set of specifications for the sequences of the amino acids of polypeptide chains. The polypeptides make up the protein molecules, which comprise the structural elements and catalytic agents (enzymes) of the cell. Each polypeptide chain is specified by a sequence of nucleotides in DNA molecule. The nucleotides, taken consecutively in groups of THREE, specify through a code common to all forms of life the corresponding amino acid sequence. An elaborate machinery exists within the cell to synthesize protein according to the DNA instructions and to exert a variety of specific controls upon this process. This machinery involves RNA, ribonucleic acid, which is synthesized on the DNA template (see RNA).

    功能。DNA遗传指令的主要部分是对多肽链的氨基酸序列的一套规范。多肽构成了蛋白质分子，它包含结构要素和细胞的催化剂（酶）。每种多肽链都是由DNA分子中的核苷酸序列指定的。采用三个一组连续不断的核苷酸，通过对所有生命形式的通用代码指定对应的氨基酸序列。根据DNA指令，细胞内存在的一种复杂机制进行合成蛋白质，并对这个过程进行各种指定控制。这种机制包括在DNA模板上合成的RNA，核糖核酸。

   Another portion of the total DNA blueprint is directly related to the process of control and specification of the initiation and termination points of message sequences for particular proteins. Although all cells of a higher organism contain the complete DNA complements, in any specialized cell only a small percentage of the messages are employed at any time. It is believed that means exist within the cell for the recognition of segments of DNA that in turn act as on-off switches of the function of the adjacent DNA tracts; controls are exerted through complex interactions of cellular components with such “operator” segments.

    DNA总图谱的其它部分直接与控制过程和规范启动，以及特定蛋白质信息序列的端点有关。尽管所有更高级有机体的细胞都包含了完整的DNA补充物，但在任何专门细胞中，在任何时候，只能用到很小比例的信息。人们认为，在细胞内存在识别DNA片断的手段，反过来这些手段充当了毗连的DNA束开关的功能；通过具有这种“运算符号”片段的复杂细胞的相互作用来实现控制。

                                                 ROBERT L. SINSHEIMER

                                             California Institute of Technology

                                                 罗伯特·L. 辛斯海默

                                                 加利弗尼亚理工学院

                                                  2022年10月19日译

（译者注：该词条位列《大美百科全书》1985年第9卷，第222页）