IV. CELLULAR CONTROL OF DNA BIOSYNTHESIS 



161 



UMP 



TMP 



TDP 



\ 



dCTP 



TTP J 



+ 



"PRIMER" DNA 



conserved units? 



DNA 



semi- 



conserved 



un i t 



DNA 



semi -conserved 



Fig. 5. Schematic representation of the biochemical sequence of events involved 

 in the biosynthesis of DNA. The ribonucleoside monophosphates — adenylic acid 

 (AMP), cyt3^dilic acid (CMP), guanyhc acid (GMP), or uridylic acid (UMP)— are 

 converted to deoxyribotides dAMP, dGMP, dCMP, or TMP (thymidylic acid) 

 which are, in turn, converted to the di- and triphosphate derivatives (dADP, dGDP, 

 dCDP, TDP, and dATP, dGTP, dCTP, and dTTP). These are polymerized under 

 the direction of "primer" DNA to form new DNA. The primer DNA is derived 

 from pre-existing DNA and may represent the conserved unit of this semi- 

 conserved molecule. Thus, if the pre-existent DNA carries a semi-conserved density 

 label such as N", the product DNA will carry a density label distribution (as 

 shown), demonstrating the semi-conservative nature of DNA replication. 



primer and a template in the formation of new material (Romberg, 

 1959). In doing this, the old and new polynucleotide strands are not 

 confused; there is one for one pairing of strands and these remain as 

 separate entities which may be distinguished by a variety of physical 

 techniques (Levinthal, 1956; Taylor et al., 1957; Meselson and Stahl, 

 1958; Mendelsohn, 1960; Sueoka, 1960; Forro, 1960; Simon, 1961). This 

 "semi-conservative" method of replication appears to extend to aggre- 

 gates of DNA molecules which may be as large as chromosomes (the 

 evidence for which has been discussed in Chapter II). 



