single-ring structures called pyrimidines. The molecule made up of a base plus a sugar is termed a nucleoside. In each molecule of DNA a phosphate group links the five-carbon sugar of one nucleoside to the five-carbon sugar of the next nucleoside in the chain. This phosphate bonding creates a sugar-phosphate backbone.
每个核苷酸都含有一个五碳脱氧核糖，分别连接4个碱基，即：腺嘌呤，鸟嘌呤，胞嘧啶，胸腺嘧啶。碱基连接糖称核苷。磷酸键形成磷酸骨架。
Chargaff’s rules describe the fact that (1) the amount of adenine is equal to the amount of thymine in DNA, with amount of cytosine equal to that of guanine, and (2) the ratios of A to T and of C to G vary with different species.
（1）腺嘌呤与胸腺嘧啶，胞嘧啶与鸟嘌呤相等；（2）腺嘌呤与胸腺嘧啶，胞嘧啶与鸟嘌呤的比例随物种不同而不同。 The Research Race for the Molecular Structure of DNA
In the late 1940s and early 1950s, researchers looking for the structure of DNA drew upon Chargaff s insight, Levene's ideas on DNA components, and two other lines of evidence. One was the suggestion of Linus Pauling that DNA might have a helical structure held in place by hydrogen bonds, and the other was X-ray diffraction photos of DNA, showing a helical structure with distance between the coils, taken by Franklin and Wilkins. 直到40年代末50年代初，研究者在寻求DNA结构过程中，确立了Chargaff 的观点和, Levene的组成理论以及其他两个线索。一个是Linus Pauling的假设，DNA可能具有螺旋结构，通过氢键连接。另一个是X-衍射图片，Franklin and Wilkins提供。
Based on this information Watson and Crick proposed the double helix model of DNA-A twisted ladder-like molecule with two outer sugar phosphate chains and rungs formed by nucleotide pairs. Paired nucleotides, which always occur as A-T or G-C, are linked by hydrogen bonds. Watson and Crick also proposed that
genetic information is encoded by the sequence of base pairs along the DNA molecule.
基于这些信息，Watson 和 Crick提出了双螺旋结构模型，成对的核苷酸通过氢键相连，遗传信息就贮藏在碱基对中。 How DNA Replicates
In their model of DNA structure and function, Watson and Crick hypothesized that DNA replicates itself by \along the hydrogen bonds joining A to T and C to G. This process would produce two opposite halves that could then serve as templates for the construction of new, complementary strands. This model of semiconservative replication conservative because each new molecule has one half of the former parent molecule-was later confirmed by the work of
DNA进行复制是以拉链方式自我复制，产生的两个二分体分别为模板生成互补链，即半保留复制。并由Meselson 和 Stahl验证。
In E. coli DNA replication begins with the formation of a bubblelike structure on the circular chromosome that is produced by replication forks. Studies of bacterial DNA replication have shown that a growing DNA chain lengthens only in the 5' to 3' direction (from the 5' carbon of one sugar to the 3' carbon of the next). The leading strand is synthesized continuously, while the lagging strand is synthesized in short stretches known as Okazaki fragments. The enzyme DNA polymerase links free nucleotides as they line up on the template formed by the original strand of the parent molecule.
In eukaryotes DNA replication follows the same general principles as in prokarotes. On the long DNA molecules replication proceeds (in two directions at once) from hundreds or thousands of points of origin.
大肠杆菌复制开始时形成泡样复制叉，链生长方向由5′向3′端，前导链连续生成，后随链由冈崎片段组成，由DNA聚合酶催化。真核生物复制与原核生物复制相似，但有几百到几千个复制原点（原核一般只有一个复制原点）。
课后作业：第一篇阅读材料 答案：1C,2C,3B,4A,5D,6D,7B
Glossary
one-gene-one-enzyme hypothesis 一基因一酶假说
The hypothesis that a large class of gene exist in which each gene controls the synthesis or activity of but a single enzyme. Since enzymes and other proteins whose syntheses are controlled by more than one gene are now known, this hypothesis has been replaced by the one-gene-one polypeptide hypothesis. one-gene-one-polypeptide hypothesis一基因一多肽假说
The hypothesis that a large class of genes exist in which each gene controls the synthesis of a single polypeptide. The polypeptide may function independently or as a subunit of a more complex protein.
adenine 腺嘌呤
A purine base that is an essential constituent of the nucleic acids and also of such coenzymes as NAD and FAD. guanine 鸟嘌呤
A double-ring nitrogenous base molecule in DNA and RNA. It is the complementary base of cytosine. cytosine胞嘧啶
A single-ring nitrogenous base molecule in DNA and RNA. It is complementary to guanine. thymine 胸腺嘧啶
A single-ring nitrogenous base molecule of DNA but not RNA. It is complementary to adenine. purine 嘌呤
One of a group of nitrogenous bases whose parent compound has the formula shown. The two most abundant purines are adenine and guanine, which are constituents of nucleic acids and coenzymes.
pyrimidine 嘧啶
A heterocyclic organic compound, C4N4N2 the fundamental form of pyrimidine bases. Some of these bases are constituents of nucleic acid. nucleoside 核苷
A purine or pyrimidine base attached to ribose or deoxyribose. The nucleosides commonly found in DNA and RNA are: cytidine, cytosine deoxyriboside, thymidine, uridine, adenosine, adenine deoxyriboside, guanosine, and guanine deoxyriboside. Note that thymidine is a deoxyriboside and cytidine, uridine, adenosine,and guanosine are ribosides.
X-ray diffraction X射线衍射
A technique for determining the arrangement of atoms in a crystalline substance by analysing the diffraction patterns produced when a narrow beam of X-rays is passed through the substance. X-ray diffraction has contributed to the elucidation of structure of many biological molecules, including some important macromolecules, e.g., DNA, haemoglobin, myoglobin. DNA double helix 双螺旋
The three-dimensional structure of doublestranded DNA. semiconservative replication 半保留复制
The method of replication of DNA in which the molecule divides longitudinally, each half being conserved and acting as a template for the formation of a new strand. replication fork 复制叉
A point at which the two strands of a DNA double helix are unwound and separated during replication. Okazaki fragment 冈琦片断
A short sequence of DNA that is the primary product of DNA polymerase during DNA replication. polymerase DNA 聚合酶
An enzyme that brings new DNA triphosphate nucleotides into position for bonding on another DNA molecule.
The Origin and Diversity of Life