THE CHEMICAL BASIS OF HEREDITY 



21 



Relation between the gene and the DNA molecule 



A further clarification of the physical meaning of linkage observed in 

 transformation experiments, and of the relation between unit deter- 

 minants or genes and DNA molecules, has come from recent studies of 

 the physical chemistry of DNA. It was discovered that the DNA double 

 helix collapses upon heating, as a re':ult of breaking of hydrogen bonds 

 between the nitrogen bases, and that the collapsed molecules lose their 

 transforming activity. As may be recalled from Figure 1.3, there are 

 two hydrogen bonds between adenine and thymine, and three between 

 cytosine and guanine. Probably for this reason, the temperature at 

 which DNA molecules collapse is directly related to their guanine- 

 cytosine content; the higher the G-C content, the higher is the so-called 

 melting out temperature. 



By carefully controlling the temperature to which a purified prepara- 

 tion of pneumococcal DNA is heated, it has been found possible to 

 inactivate one genetic marker before another, as shown in Figure 1.7. 

 For example, there is a great loss of transforming activit; by the gene 

 for resistance to the drug, micrococcin, at a temperature vvhich hardly 

 affects the gene for streptomycin resistance at all. However, the activitv 

 of two genes involved in sulfanilamide resistance and that of the gene for 



80 



85 90 



Temperature, °C 



FIGURE 1.7. Inactivation temperatures of diflFerent transforming factors. Under 

 standardized conditions, transforming ability of purified DNA extracts is lost after 

 heating to temperatures whicfi are characteristic for particular genes in pneumococcus. 



(o). Micrococcin resistance is lost at 88° C. 



(b). Resistance to streptomycin and two closely linked genes which determine sul- 

 fonamide resistance {Fa and Fad) are lost together at 89.5° C. 



(c). Amethopterin resistance is lost at 91.5° C. (from M. Roger, unpublished). 



