REPLICATION AND CODE-SCRIPTS 



153 



A. In 0.002 M-Mg ++ :m A 305070100 



I I I I I 



B 



++ 



I I I 



B.mo.oiM-Mg -.m B 305070 i oo 



(b) 



Figure 6-12. (b) Two Sedimentation Patterns (A and B) of the Ribosomes shown in (a). 

 Note how the binding of these little particles is so dependent upon the 

 medium. The numbers are the sedimentation rates (in svedberg units) of 

 the different particles in the ultracentrifuge: the larger particles fall faster. 

 (Photographs (a) and (b), courtesy of S. T. Bayley, National Research 

 Council's Biophysics Section, Ottawa.) 



"Cogs" and "Cams" 



It is generally assumed that the code is contained in the arrangement 

 of the four basic (2 pyridine and 2 pyrimidine) groups in the nucleic acid 

 chain. There are at least 20 amino acids which must be distinguished. The 

 smallest number of 4 basic groups which could be arranged in enough differ- 

 ent ways to distinguish 20 amino acids is 3; and 3 in principle could dis- 

 tinguish as many as 64 amino acids (4 1 ). 



Two suggestions have been made in which it is shown that, of the 64 pos- 

 sible ways or arrangements, only about 20 are unique in a chain. One sug- 

 gestion was made by Gamov, Rich, and Yeas in 1953, who postulated that 

 the cyclic, helical structure of DNA would give rise to arrangements in which 

 the 4 pyridine and pyrimidine bases jut out from the helix to form the 

 4 corners of a diamond on the external surface of the helix. Only 20 unique 

 arrangements of the 4 bases could exist. Let us call this the cam theory 



