Pyrimidine Moieties in Animals, Plants, and Bacteria 99 



is the lighter one (1.697 gcm~^) and the minor component is the 

 heavier one (1.703 gcm"^^) (36). 



Since the density of DNA is linearly related to the molar per 

 cent (G+C), it is possible that the mouse and guinea pig minor 

 components consist of a population of DNA molecules differing 

 in molar per cent (G+C:). However, alternate explanations for 

 the minor component may be offered. This point will be resolved 

 when the guinea pig and mouse minor DNA components are 

 isolated in pure forin. 



An interesting bimocial distribution has been found by Sueoka 

 (72) in crab testes DNA (Table V). The major DNA component 

 has a density of approximately 1.705 gcm"^ However, a very 

 light minor component also occurs which is indistinguishable from 

 a double stranded polynucleotide in which only two of the four 

 nucleotide bases are present. The bases involved are adenine and 

 thymine and the polymer is called the (deoxy-A-T) polynucleotide. 

 The function of this unusual polynucleotide, deoxy-A-T, is 

 unknown. 



Heterogeneity of Composition of DNA 



Since the DNx^ of any species is quite heterogeneous, it is of 

 interest to compute an upper bound on the standard deviation 

 {aac) of the distribution of the guanine-cytosine base pairs over 

 the population of DNA molecules. The upper bound of {ctgc) is 

 given as: 



[4] O-GC max = 10 O- density 



where o- density is the standard deviation of the DNA distribution 

 in the CsCl density gradient (54). It should be pointed out that 

 the actual value of (tqc lies considerably below the calculated 

 upper bound because thermal motion of the DNA molecules con- 

 tributes significantly to band width. 



The DNA's of nine bacterial species form bands in the density 

 gradient with o- density in no case greater than 0.003 gcm~^ The 

 corresponding upper bound on the standard deviation, (tqc, of 

 the molecular content of guanine plus cytosine is therefore in no 

 case greater than 0.03. It is remarkable that the standard deviation 

 of guanine-cytosine content within the molecular population of 



