276 



( II M'TER 20 



acids. Digestion of the DNA at those 

 points where it is joined to amino acid pro- 

 duces extensive depolymerization. Such re- 

 sults indicate that the amino acid sequence 



is short and. therefore, must sometimes be 

 located internally rather than always being 

 at the end of a DNA strand. On the aver- 

 age, there is a sequenee of three amino acids 

 per thousand nucleotides. The amino acid 

 sequences appear bound to the phosphate of 

 the DNA. not as a side ehain. but as an 

 integral part of the molecule. Thus, the 

 backbone of the DNA strand appears to be 

 interrupted by short amino add sequences. 

 About 33 c '( of the amino acids in DNA 

 are the hydroxyl amino acids, serine and 

 threonine, and 10 to 15% are glutamic acid. 

 As expected, available evidence indicates 

 that the amino acid attached to the phos- 

 phate of DNA is often serine. The occur- 

 rence of amino acids in the DNA of human 

 leucocytes, calf thymus, and the vaccinia 

 virus has also been reported. The DNA 



■"■See A. Bendich and H. S. Rosencranz (1963). 



in the larval salivary gland chromosomes of 

 Drosophila also seems to be interrupted." 



If confirmed, these results are significant 

 since they involve sequences of amino acids 

 which arc so short that, when interstitial, 

 they may be immune to the action of certain 

 digestive enzymes. Such amino acid groups 

 may be involved in: 



1. The bending of DNA double-helices 

 (which are rather rigid) especially 

 where DNA is much coiled on itself 



2. The mechanism by which strand sep- 

 aration leading to replication occurs 



3. The separation of functional DNA 

 units 



4. The functioning of individual DNA 

 units 



5. The mechanism of crossing over 



6. Mutagenesis by agents capable of af- 

 fecting amino acids. 7 



,; See reference to D. M. Steffensen (1963) on p. 



162. 



7 See 1. A. Rapoport and R. G. Kostyanovskii 



(1959). 



SUMMARY AND CONCLUSIONS 



DNA /'// vivo usually exists in the Watson-Crick double helix configuration and usually 

 replicates, after the strands separate, by the formation oi complementary strands. 



In certain viruses, <2>X174 and <2>S13 for example, the DNA is single-stranded. In 

 bacteria and bacterial viruses, the DNA exists uncombined with protein. 



In ordinary chromosomes, the following are found: a basic 40 A thick fibril con- 

 taining protamine and one DNA double helix; a basic 100 A thick fibril composed of 

 histone and two DNA double helices. A chromatid probably contains a number of 

 100 A fibrils. 



The DNA molecule seems to be interrupted periodically by short amino acid 

 sequences. 



