98 Information Storage and Neural Control 



Chamberlin and Berg (7) suggest that genetic information can 

 be transcribed from either of the two DNA strands to infor- 

 mational-RNA. Thus, when single stranded 0X174 DNA was 

 used as a template for RNA polymerase, an RNA strand having 

 a composition complementary to the 0X174 DNA was formed. 

 However, when single stranded 0X174 DNA was used as a tem- 

 plate for DNA polymerase, double stranded DNA was formed. 

 The double stranded 0X174 DNA could then be used as a primer 

 for RNA polymerase and, in this case, RNA was formed having 

 a composition identical to that of double stranded 0X174 DNA. 

 If both of the complementary DNA strands are ultimately trans- 

 lated from the same fixed starting point, the foregoing experi- 

 ments would suggest that complementary nucleotide triplets code 

 the same amino acid. On the other hand, if the complementary 

 DNA strands are translated from opposite starting points, the 

 results would point to one of two possibilities: 1) that two dif- 

 ferent nucleotide triplets code the same amino acid, or 2) that the 

 complementary DNA strands are identical even though they have 

 opposite polarities. The second of these possibilities would further 

 require that one half of a given DNA strand be complementary to 

 the opposite half. The latter restrictions do not seem to apply 

 to the 0X174 DNA studied by Chamberlin and Berg (7, 65). 

 Davern's experiments (14) also suggest that these restrictions are 

 unlikely as a general proposition. Hence, some form of a "degen- 

 erate" genetic code seems to be the most appealing hypothesis at 

 this time. 



Unimodal and Bimodal Distributions 



The DNA from almost every species so far examined forms one 

 discrete band after density gradient centrifugation. Streptomyces 

 viridochrornogenes DNA (Figs. 6, 7) illustrates this unimodal dis- 

 tribution. Several examples have now been found in which DNA 

 manifests a bimodal distribution. Mouse DNA illustrates the bimodal 

 DNA distribution (36). Mouse DNA manifests a major component 

 having a density of 1.701 gcm~^ and a second minor component, 

 comprising about 8 per cent of the total DNA, having a density 

 of 1.690 gcm~^ (Figs. 6, 7). A bimodal DNA distribution is also 

 observed with guinea pig DNA. In this case, the major component 



