522 - Heredity and Evolution 



macromolecule may be exceedingly long. Fre- 

 quently more than a thousand base pairs are 



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 \ C. H 



N N-H- VC NV „ 



Yn -* ' 



N— C. / 



N ^H 



HYDROGEN 

 BOND 



pjL-H- 



K r H 



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) — 



