. Principles of modern biology. Biology. Fig. 27-1. Diagram of the Watson-Crick model of a section of a DNA molecule. Note that the double helix is formed by two sugar-phosphate chains (—S—P—S—P—), which run parallel to each other in opposite directions. The "cross bars" or "rungs," which hold the coiling sides of the "ladder" together, are constituted by base pairs, adenine=thymine (A=T) and guanine=cytosine (G=C), bound to each other by hydrogen bonds (=). The base pairs may occur in any sequence, but the specific sequence in any one DNA molecule serves to deter
. Principles of modern biology. Biology. Fig. 27-1. Diagram of the Watson-Crick model of a section of a DNA molecule. Note that the double helix is formed by two sugar-phosphate chains (—S—P—S—P—), which run parallel to each other in opposite directions. The "cross bars" or "rungs," which hold the coiling sides of the "ladder" together, are constituted by base pairs, adenine=thymine (A=T) and guanine=cytosine (G=C), bound to each other by hydrogen bonds (=). The base pairs may occur in any sequence, but the specific sequence in any one DNA molecule serves to determine the triplet code of protein synthesis. (After Watson and Crick.). HYDROGEN THYMINE H BONDx u / ADENINE \0 NN-H- VCNV „ Yn -* ' N—C. / N^H HYDROGEN BOND pjL-H- K rH H GUANINE >Ns /H H J i CYTOSINE v<( -N. Fig. 27-2. Base pairs that form the "cross bars" of a DNA molecule. The size and shape of these purine and pyrimidine units determine their capacity to be bound together by hydrogen bonds, which stabilize the macromolecular structure. (After Crick.) represented. Moreover, a different molecule occurs each time the sequence of the base pairs is shifted. Therefore it is safe to say that an almost infinite variety of DNA molecules exist—corresponding to the almost infinite variety of genes that have arisen during the evolution of our many species. Self-templated Replication of DNA. A self- guided type of replication must be a prime requirement for any genie material. Accord- ingly, a mechanism for the replication of DNA has been proposed by Crick and others. This is diagramed in Figure 27-3. The double helix presumably uncoils and separates into single strands, on which the bases of the successively arranged nucleotides occupy exposed positions. Then, if an ade- quate supply of the proper kinds of free nucleotides is present in the cell, these nu- cleotides become bonded in proper order— by virtue of the base-pair rule (p. 134)—. Please
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