MECHANISMS OF GENE ACTION 



299 



DNA BASE RATIOS AND GENE ACTION 



The unitary hypothesis of gene action proposes that genes control 

 cellular functions by determining protein specificity, and that this, in 

 turn, is accomplished by the translation of a linear code from the nucleo- 

 tide sequence of DNA to the amino acid sequence of protein. Recent 

 studies of the physical properties of DNA extracted from a large number 

 of different species have provided data not easily reconciled with the 

 unitary hypothesis. 



A method for determining the density of DNA by equilibrium sedimen- 

 tation in cesium chloride was described in Chapter 7. When DNA ex- 

 tracts from different organisms were centrifuged by this method, it was 

 discovered that they varied considerably in density. Of the four nitrogen 



100 



80 - 



60 



r 40 



20 



<t>^M. phlei 



Serratia- 



E. coli 



Calf thymus 

 Salmon sperm 



Denatured 



*Z). pneumoniae 



AT-DNA 



1.68 



1.69 



1.70 



1.71 1.72 



Density 



1.73 



1.74 



1.75 



FIGURE 10.13. Relationship of density to the guanine-cytosine content of various 

 samples of DNA. The density as determined by equilibrium centrifugation in cesium 

 chloride (see Chapter 7) was correlated with base composition of both native and de- 

 natured samples of the DNA of a number of species (from Doty, Marmur, and Sueoka, 

 1959, BroofchavenSymp. Biology, 12:10). 



