154 BIG MOLECULES 



partly because one thinks of a cylindrical cam with coding on its walls (Fig- 

 ure 6-13), and partly because it is a degeneration of Gamovl 



The other suggestion, made by Crick, Orgel, and Griffiths in 1957, was 

 that in a linear arrangement of only 4 characters, only about 20 unique 

 groups of 3 could be written, provided that no character be counted as be- 

 longing to more than one group of three — that is, if there is no overlap. We 

 think here of a helical molecule with 20 arrangements of 3 bases which de- 

 fine the code information along the chain. Partly because the process re- 

 sembles the meshing of carefully fitted gears, and partly because of the 

 initials of the inventors of the theory, let us call it the cog theory. Figure 6-13 

 is a schematic representation of the cam and the cog. 



triplet base code 



sugar 

 ridge 



cam cog 



Figure 6-13. Cogs and Cams for Coding on DNA. Each spot represents a 



pyridine or pyrimidine base. 



Both theories have serious drawbacks, not yet resolved. In the Crick 

 model, the amino acids in solution must "know" that they are forbidden to 

 indulge in overlap; while in the, Gamov model a severe geometric restriction 

 exists, viz., the DNA molecule (and hence the RNA whose shape is deter- 

 mined by DNA) must always hold a very specific and rigid helical structure 

 if the diamond arrays are to persist on the surface. 



However, successes in a flurry of investigation, genetic and biochemical, 

 have engendered the belief that the basic facts of the amino-acid code car- 

 ped by DNA may be completely known by 1963! There have been three 

 recent remarkable disclosures. First, Nirenburg startled the International 

 Biochemical Congress in Moscow in the Summer of 1961 by announcing 

 that polyphenylalanine (a polypeptide) could be produced by adding poly- 

 uridylic acid (i.e., an RNA, the pyrimidine bases of which are all uracil) to 

 a cell-free solution of phenylalanine. This showed that a sequence of uracils 

 (probably three of them) codes phenylalanine. Secondly, from elegant 

 genetic studies, Crick et al. argued that: 



