524 - Heredity and Evolution 



in Cold Spring Harbor, N.Y., and Seymour 

 Ben/.er at Purdue University. 



Kornberg demonstrated self-templated syn- 

 thesis of DNA in a medium prepared from 

 disrupted bacterial cells. This preparation 

 contained a good supply of the four varieties 

 of free nucleotides and an enzyme designated 

 as DNA polymerase, which catalyzes the 

 formation of linkages in the sugar-phosphate 

 chain. No synthesis of DNA occurred, how- 

 ever, unless some preformed DNA was added 

 to the system. But when just a small amount 

 of preformed DNA was added as a primer, 

 new DNA was synthesized vigorously. And, 

 most important of all, the "new" DNA ap- 

 peared to be identical to the "old." A sta- 

 tistical analysis showed that the base-pair 

 constitution of the newly synthesized DNA 

 was exactly the same as that in the priming 

 sample — regardless of the source and nature 

 of this sample. In some cases, DNA from the 

 same bacterial species was used; but in other 

 cases the primer DNA was of viral origin or 

 even more remarkable, DNA extracted from 

 animal cells. 



Replication of Viral DNA. Viruses, of 

 course, display many resemblances to genes. 

 Some biologists, indeed, regard viruses as 

 "parasitic genes." In any event, the mecha- 

 nisms of replication in genes and viruses are 

 very similar, if not identical. Consequently 

 the recent experiments on virus replication 

 are highly significant. 



Most of the experiments were done with 

 the bacteriophage virus (strain T 2 ), which is 

 shown in Figure 1-4; and the bacterial host 

 was one of the human colon bacilli, Escher- 

 ichia coli, also shown in Figure 1-4. The 

 DNA fraction of the virus was labeled by 

 incorporating radioactive phosphorus (P 32 ) 

 into the phosphate parts of the molecule, and 

 the protein fraction was identified by the in- 

 corporation of radioactive sulfur (S 35 ). In 

 some experiments the viral DNA and pro- 

 tein were radioactively labeled in advance; 

 but in others, the precursory compounds 

 (phosphate and amino acids) containing 

 radioactive atoms were added to the culture 



medium in which the bacteria (and viruses) 

 were growing. 



A bacterium infected by one particle of 

 virus is destined to survive for only 24 min- 

 utes (at 20°C). Then the bacterial cell sud- 

 denly disintegrates. This liberates about 200 

 new virus particles into the culture medium, 

 each ready to carry the infection to another 

 bacterium. It is important, therefore, to de- 

 termine precisely how this remarkable pro- 

 cess of replication is achieved. 



A study of the virus particle shows that all 

 its content of DNA lies in the so-called 

 "head" (Fig. 1-4B), a swollen rounded part; 

 whereas the "tail" is formed solely of pro- 

 tein, which also forms a thin skin covering 

 the DNA core of the head. When infection 

 occurs, the tail makes contact with the sur- 

 face of the bacterium, but only the DNA part 

 of the virus actually enters into the bacterial 

 cell. The protein part remains attached to 

 the cell surface and eventually weakens this 

 surface, by virtue of an enzymic activity. The 

 whole responsibility for achieving a redupli- 

 cation of the virus is assumed by the DNA 

 fraction, which penetrates into the interior 

 of the bacterial cell. The protein is impor- 

 tant, however, in spreading the infection and 

 in causing a lysis of the cells, but it is not 

 essential to the actual process of replica- 

 tion. 



The fate of the DNA, which woes into the 

 bacterium, can also be followed, at least to 

 some extent. Within 12 minutes, the viral 

 DNA increases to about 200 times its original 

 amount. About one third of this increase is 

 at the expense of preformed nucleotides pres- 

 ent in the bacterium, but mainly it represents 

 new synthesis that utilizes inorganic phos- 

 phate from the culture medium. And while 

 the viral DNA increases quickly, viral pro- 

 tein slowly begins to appear inside the bac- 

 terium, where at first it was completely ab- 

 sent. Indeed, no complete new virus particles 

 (viral DNA plus viral protein) can be found 

 in the bacterium until the very last part of 

 the infection cycle. Bv this time, however, the 

 viral protein has also increased about two 



