Genes: Nature and Mode of Action - 527 



Table 27-1— The Genetic Code after Transfer from DNA to RNA 



After M. W. Nirenberg. 



* Not absolutely certain; data scanty. 



double infection. Thus each bacterial cell 

 may receive two kinds of viruses. When repli- 

 cation has been completed (p. 524), the lib- 

 erated viruses may be (1) all of one kind; (2) 

 some of one kind and some of the other; or 

 (3) a recombination of the original two kinds 

 — indicating that some kind of exchange has 

 occurred between the strands of DNA ma- 

 terial. The viruses are very favorable for such 

 genetic studies, because millions of individual 

 viruses are produced in each experiment and 



the production of each new generation re- 

 quires less than half an hour (p. 524). 



Mutations, whether "spontaneous" or in- 

 duced experimentally (p. 530), result ap- 

 parently from one or more structural changes 

 in the genie DNA. Such changes may involve 

 (1) substitution in one or more of the base 

 pairs; (2) subtraction of one or more base 

 pairs; (3) addition of one or more base pairs; 

 or (4) combinations of the foregoing. Such 

 changes are heritable — provided they are not 



