194 



S. S. COHEN 



the structure of nucleoliistone has been obtamed by X-ray analysis. However, 

 Wilkins (1956) beheves that in this instance histone is wrapped around both 

 grooves of the DNA, as indicated in Fig. 28b. 



FWffiffHwg Arginine 



side-chains 

 (q) ooo Polypeptide chain 



(b) 



Fig. 28. (A) shows how protamine binds to DNA. The polypeptide chain wmds around 

 the small groove on the DNA helix. Phosphate groups are at the black circles and co- 

 incide with the basic ends of the arginine side chains (Wilkins, 1956). 



(B) shows a possible way histone might bmd to DNA. The luies trace the polypeptide 

 chain, the folds in it, and the side chains of basic amino acids which are associated with 

 phosphate groups (Wilkins, 1956). 



Starting with this view of deoxyribonucleoprotein structure, Bloch (1955) 

 has proposed a mechanism whereby there are a separation and rotation of 

 bases prior to repHcation, with the histones holding the separated deoxy- 

 nucleotide chains of the parent duplex in position during the alignment and 

 polymerization of complementary nucleotides. It will be recalled that it has 

 been shown that the synthesis of histone occurs simultaneously with that of 

 DNA (Bloch and Godman, 1955; Alfert, 1957), and this would lead to the 

 formation of four polynucleotide plus polypeptide strands. It is suggested 

 that the specific configuration of the DNA chain may also thereby determine 

 the structure of the newly synthesized histones. Separation of these relation- 

 ally coiled complexes organized in two twisted strips can be effected as a 

 result of phenomena comparable to the chromosome coiling which takes 



