300 CELL HEREDITY 



hast's ill DNA, guaiiiiu' is the hca\i('st, and so it was postulatt'd that the 

 (k'nsitics niiiiht torrclatc with giiaiiiiu' content. An excellent correlation 

 was found, as shown in l'iu;ure lO.l.'l Subsequent analyses of the sedi- 

 mentation data demonstrate \{'r\ little density heterogeneity among the 

 molecules. ( ;onse(|uentl\ , it may he inferred that \irtually e\'ery DNA 

 molecule of Mycohactciiinn. for example, contains more tiuanine than 

 any of the D\.\ molecules from E. coli. 



On the unitary Inpothesis, one would expect considerable similarity in 

 base ratios of DNA molecules from different species, since by and large 

 they contain so many enzymes w hich are functionally identical and struc- 

 turally yerv similar. A number of interpretations haye been proposed to 

 account for these findings, among them: (1) there is some species-specific 

 noncoding DNA in e\ery molecule; (2) that the code differs from species 

 to species; (3) that the DNA codes only a small species-specific part of 

 the protein molecule, the rest being coded by RNA; and (4) that the code 

 is based on a two-letter alphabet in which the 6-keto-containing bases, 

 guanine and thymine, are equiyalent, as are the 6-amino compounds, 

 adenine and cytosine. This is an unresoKed question. 



RESUME 



In this chapter \ye haye focused attention upon the selection of par- 

 ticular traits well suited to an intensive analysis of gene action. The 

 process of selection included: first, the choice of a microorganism instead 

 of a multitissued plant or animal; second, the selection of a particular 

 class of mutants, the auxotrophs, blocked at a particular step in some 

 biosynthetic pathway; and third, the selection of mutant strains contain- 

 ing altered enzymes differing from the wild type in a manner accounting 

 for the phenotypic difference between strains. With the aid of these 

 criteria, about a dozen systems have been found in which the mode of 

 gene action involves an influence upon protein specificity. 



We must now retrace our steps to inquire how special or how general 

 these selected systems mav be. hi addition to the few mutants in which 

 altered enzymes have been identified, there are about three to four times 

 as many mutants in which the absence of an enzyme corresponding to the 

 blocked step has been reported. These instances are somewhat ambigu- 

 ous, for the effect of the mutation may be to interfere with enzyme 

 formation, or to inactivate the formed enzyme, or to alter its specificity 

 so greatly that it is unrecognized. It is evident then, that, even in the 

 best analyzed material, there is still a serious question about the precise 

 mechanism of gene action. Nonetheless, it does appear likely that the 



