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CELL HEREDITY 



ct'lls of a culture. The difference l)etween bacteria, on tfie one hand, 

 and plant and animal cells, on the other, in timing of DNA replication 

 is striking, and proliably reflects the difference in structural integration 

 of DNA in the two systems. DNA replication itself is most likely an 

 identical process in bacteria and in other organisms, as has been shown 

 beautifulK in the following experiments. 



The replication of DNA 



DNA in solution exists in the form of double helices, pairs of poly- 

 nucleotide chains coiled around each other in a precise manner such 

 that hydrogen-bonding is possible between adjacent nitrogen bases of the 

 two strands (Figure 1.2). The pairing is selective, adenine with thymine 

 and guanine with cytosine. Thus the nucleotide sequence of either 

 strand specifies precisely the sequence of the other strand which is com- 

 plementary to it. This situation provides a formal basis for a template 

 mechanism of replication, in which the two strands come apart, each 

 determining the nucleotide sequence in a newly forming complementary 

 strand (Figure 7.3). 



In the few years since this replication model for DNA was proposed, 

 a number of lines of evidence have been obtained which provide strong 

 support for it. The enzymatic mechanism of DNA synthesis has been 

 discovered and shown to require DNA as a primer or template. By the 

 use of isotopically labeled DNA to distinguish between old and newly 

 formed strands, the process of DNA replication has been followed in a 

 number of organisms from bacteria to higher plants. V'irtually identical 

 results have been obtained with the use of entirely different methods, 

 one employing DNA alone and the other autoradiographs of intact 

 chromosomes. 



In 1956, Kornberg and co-workers described an enzyme preparation 

 from E. coli which was capable of synthesizing DNA. Since that time, 

 intensive work with the system has clarified many of its features. The 

 over-all reaction is: 



n TPPP 



n dCPPP 



n dAPPP 



n dCPPP 



+ DNA. 



TP 

 dGP 

 dAP 

 dCP 



DNA + 4(n)PP 



The substrates for the reaction are the four deoxynucleosidc triphos- 

 phates of adenine, guanine, cytosine, and thymine. All four are required 

 for any detectable synthesis to occur, and a further requirement is the 



