Pyrimidine Moieties in Animals, Plants, and Bacteria 97 



population of molecules differing only slightly in density and in 

 molar per cent (G + C), the DNA bands will overlap, and there 

 will be an increase in the overall standard deviation of the band. 

 So long as the total variance of a band and the number average 

 molecular weight of a DNA preparation are known, it is possible 

 to calculate the contribution which heterogeneity of composition 

 makes to the band width (16, 36, 54). An independent estimate 

 of the heterogeneity of composition of a DNA preparation may 

 also be made from the DNA thermal denaturation curves. These 

 independent estimates agree satisfactorily. 



Nonoverlapping Bands 



Density gradient centrifugation experiments have permitted a 

 number of interesting conclusions concerning DNA. First, as 

 already mentioned, the DNA obtained from many bacterial 

 species form discrete bands which do not overlap. These observa- 

 tions indicate that the respective organisms have no DNA mole- 

 cules with common density, and, by inference, that they have no 

 DNA molecules with common nucleotide base composition (16). 

 Since the metabolism and replication of bacteria do have much in 

 common, it is generally thought that many of their proteins should 

 be identical or very similar. Yet all current discussions of the 

 way in which DNA can control the sequence of amino acids in 

 proteins require a direct correlation between the composition of 

 the DNA and of the protein. There are a number of ways in 

 which this dilemma can be resolved. The simplest approach is to 

 assume that some amino acids are coded by more than one nucleo- 

 tide triplet. For example, each of the triplets UCC and UAC 

 might specify the amino acid threonine. Thus, the dependence of 

 the amino acid composition of proteins on the DNA nucleotide 

 composition would not be exacting and DNA molecules having 

 different compositions could code very similar proteins. A genetic 

 code in which two or more nucleotide triplets are used to code a 

 particular amino acid is called a '"degenerate" code (13). 



There are a number of arguments which can be advanced in 

 favor of this hypothesis. By studying nitrous acid induced mutants 

 of bacteriophage, Tessman (73) demonstrated that each of the 

 complementary DNA strands is functional. The experiments "* of 



