60 Information Storage and Neural Control 



strain of B. siibtilis which previously could not synthesize tryp- 

 tophan. 



Evidence that DNA is the genetic material in a DNA-protein 

 virus comes from studies of the infection of Escherichia coli with 

 bacteriophage T2. Virtually all of the DNA of the virus enters 

 the infected bacterium, and virtually none of the associated 

 protein enters. Finally, in bacterial conjugation, DNA is trans- 

 ferred from a donor to a recipient strain of E. coli. The amount 

 of DNA transferred is proportional to the number of genes trans- 

 ferred, again suggesting that the DNA carries the genetic in- 

 formation. 



There is, then, excellent evidence that DNA is the genetic 

 storage material in bacteria and some viruses (there are ribonucleic 

 acid (RNA) containing viruses in which the RNA has been shown 

 to be the genetic material). The generalization to higher organ- 

 isms of this picture of DNA as the storehouse of genetic information 

 rests largely upon the observations that the DNA content per cell 

 nucleus is proportional to chromosome number; haploid sperm 

 cells, for example, have one-half the DNA of diploid somatic 

 cells (3). Further, the chromosomal DNA is quite stable meta- 

 bolically as befits a genetic storage unit. At present, however, 

 much of our belief in the idea that genes are universally DNA 

 comes from a feeling that nature ought to be universal about 

 such things as the storage and transfer of genetic information, 

 so that what holds true for bacteria should hold true for man. 



If we accept DNA as the genetic material, we can then ask how 

 such a molecule stores genetic information. The simplest hypothesis 

 concerning this point follows from a consideration of the chemical 

 structure of DNA. DNA is a polymer of deoxyribonucleotides 

 linked together by phosphate bridges between deoxysugars to 

 give a sugar-phosphate "backbone" with purine and pyrimidine 

 side groups (Fig. la). The only topographic feature of this covalent 

 "primary" structure which forms a likely candidate for informa- 

 tion storage is the base sequence of the purines and pyrimidines. 



A consideration of the probable three-dimensional structure of 

 DNA tends to reinforce this view. The Watson-Crick model (4) 

 for DNA structure proposes that the molecule consists of two 

 chains forming a double helix with hydrogen bond pairing between 



