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UNITY AND DIVERSITY IN BIOCHEMISTRY 



3*4 A, and the repeating unit which is made up of ten nucleotides occupies 

 34 A. The angle between two adjacent nucleotides in the same chain is 

 36°. The purine and pyrimidine bases, being directed to the interior, are 

 perpendicular to the axis of the helix. They are associated in pairs, one 

 from each chain by means of hydrogen bonds. 



As there is' not enough room to allow two purines end to end, and since 

 two pyrimidines would form too short a bridge, the only possible linkages 



Adenine 



Thymin* 



Guanine 



Crlo«<nt 



Fig. 15 (Watson and Crick) — Hydrogen bonds compatible with the formula of Watson 



and Crick. 



are adenine-thymine and guanine-cytosine linkages. The shape of these 

 links is represented in Fig. 15. 



The desoxyribonucleic are more stable than the ribonucleic acids and, 

 consequently, a greater number of results have been obtained in deter- 

 minations of their molecular weight. They are very elongated, threadlike 

 molecules, whose molecular weight is in the region of six million. 



The composition of the desoxyribonucleic acids varies from one species 

 to another, but their composition appears to be the same in different organs 

 and does not appear to be influenced by the environment. 



