Postscript 



Progress in the field of molecular genetics has been so sweeping in the 

 interval since this book was written, that with the reprinting of the first 

 edition, we feel that some account of recent advances, however brief, is 

 mandatory. This postscript is limited to an extension of the discussion 

 in Chapters 11 and 13 on mechanisms of gene action and will deal specif- 

 ically with (1) messenger RNA, (2) protein synthesis and the genetic code, 

 and (3) the relation of specific amino acid alterations in particular pro- 

 teins to the corresponding nucleotide changes in DNA. It is hoped that 

 this account will aid the reader in assimilating the new findings which 

 are revolutionizing both concepts and experimental methods in biology. 



I. MESSENGER RNA 



Several independent lines of evidence have led to the formulation of the 

 messenger RNA hypothesis, which postulates the existence of a class of 

 RNA molecules, complementary to DNA, which carry genetic information 

 from DNA to the ribosomes where they control protein synthesis. In 1956 

 Volkin and Astrachan detected, in phage-infected E. coli, the synthesis of 

 an RNA fraction with a base composition mimicking that of the phage 

 DNA. Subsequently, an analogous RNA fraction whose base composition 

 mimicked that of the DNA of the same strain, was detected in uninfected 

 cultures of several bacteria. In the phage-infected system, Brenner, Jacob 

 and Meselson showed in 1961 that an RNA fraction, newly synthesized 

 after phage infection, could be found attached to pre-existing ribosomes. 

 Taken together, these observations support the messenger hypothesis. 



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