272 ELLIOT VOLKIN 



answer to iiiolccular prohlcins of iiilicritaiicc ami >p('cilic <:i()\vtli. Since 

 their discovoiy by Miescher in 1871, the nueh'ic acids have been vari- 

 ously inipHcated as occupyin<f a central I'ole in the processes of genetic 

 continuity, though historically such conjectuix' has l)een based on evi- 

 dence of circumstance in time and place (iirachet, 1933, 1937, 1940a; 

 (^aspersson, 1941). As with so many scientific endeavois, research in the 

 nucleic acids acquired a renewed impetus with breakthroughs in meth- 

 odology, specifically the development of chromatographic i)iocedures 

 (Vischer and Chargaf!\ 1947, 1948a,b; C'ohn, 1949, 19o0a,b, 1951) and 

 the availability of radioactive elements. The establislmient of deoxyribo- 

 nucleic acid (DNA) as the entity i-esponsible for cellular heritability 

 has been thoroughly documented (see Chapters I, II, and IV). This 

 chapter will be largely concerned with one aspect of ribonucleic acid 

 (RNA) metabolism, i.e., the role of RNA in the translation of genetic 

 information from DNA to the sites of protein synthesis. 



On the basis of rigorous and detailed in vivo and in vitro experi- 

 mentation, originally done by Zamecnik and colleagues (see review by 

 Zamecnik, 1960), such sites for i)rotcin synthesis have been demon- 

 strated to be the RNA-rich subcellular components of cells — the micro- 

 somes of highly developed tissues, the ribosomes of microbial systems 

 (see Chapter VHI). Altliough the function of the bulk of RNA in these 

 particles is still open to question, the role of another species of RNA 

 has been disclosed by the use of cell-free systems that synthesize protein. 

 This is the "transfer RNA," which is involved in cariying amino acids 

 in their aminoacyl form to the particles where they are utilized for 

 protein synthesis. Purification and separation of transfer RNA's have 

 revealed that a high degree of specificity exists between the transfer 

 RNA species and particular amino acids (see Chapter VIII). This 

 phase of specific interaction, however, is not considered to occujw the 

 key role in detennining the ultimate explicit structure of the protein 

 product. For this purpose, another species of RNA has been invoked. 

 The generally accepted term for it is "messenger RNA" (Jacob and 

 Monod, 1961), so named because of its putative intermediary role in 

 relaying the genetic code of DNA to functional sites. Within the span 

 of just a few years, research in this area has quickly passed from that 

 which purported to demonstrate the biologic existence of such an RNA 

 to that which now utilizes RNA in vitro systems to establish the exact 

 coding relations between specific nucleotide triplets and specific amino 

 acids incorporated into protein. At this point, the speed with which 

 such coding information becomes available in the literature may make 

 this writing archaic by the time it reaches final printing. 



