38 



Nature of the Genetic Material 



the presence of long chains, the backbone of which consists of alter- 

 nate phosphate and sugar groups (fig. 4). To each sugar is attached 

 a nitrogenous base which can be the purines adenine and guanine 

 and the pyrimidines thymine and cytosine. The sequence of these 

 bases along the chain is irregular. The individual monomere (the 

 nucleotide) consists of phosphate, sugar, and base. The important 

 new point is that the molecule consists of two such helical chains, 



Fig. 5. Watson and Cricks model 

 of DNA molecule with two chains 

 arranged in hehces consisting of sugars 

 (pentagons) and phosphates in be- 

 tween (not marked especially), con- 

 nected by a stack of bases, one purine, 

 one pyrimidine on each level, with 

 hydrogen bonds between the pair of 

 bases. (From Crick, 1954; redrawn 

 from photo of wire model in Scientific 

 American, by permission of the author 

 and the publisher.) 



coiled around a common axis (fig. 5). They are held together by 

 H-bonds between the bases; thus the bases are joined in pairs. Only 

 certain pairs of bases will fit into this structure. One must be a 

 purine; the other, a pyrimidine: adenine must join with thymine and 

 guanine with cytosine, independently of their position in one or the 

 other chain (fig. 5). The backbone of the molecule is regular, but 

 any sequence of the pairs of bases fits into the structure. This means 



