of phage DNA. In the case of this type of virus (Figure 10-1), the 

 protein coat is left outside the infected bacterial ceU and only the DNA 

 core penetrates, where it undergoes replication and the products then 

 resynthesize the protein body prior to lysis of the infected cell. Correla- 

 tion of radioactive tag with genetic recombination has been invoked by 

 a number of workers in hopes of determining how replication takes 

 place, but so far no unequivocal evidence for any hypothesis has emerged 

 (Delbruck and Stent, 1957; Levinthal and Thomas, 1957). Presumably 



Figure 10-1. Electron Micrograph of Intact T2 Bacteriophage or Virus 

 Showing Polyhedral-shaped Head and Tail with Pronged Device at Its Ex- 

 tremity. (From Jacob, F. and WoUman, E. L., 1961. "Viruses and Genes," 

 Scientific American. 204, p. 92. Electron micrograph by Drs. S. Brenner and 

 R. W. Home, University of Cambridge.) 



in the case of the phage virus, the problem of replication of genetic ma- 

 terial may be synonymous with the problem of replication of the DNA 

 macromolecule. In organisms with more sophisticated chromosomes, the 

 problem includes that of replication of non-nucleic acid moieties as well, 

 so that, at present, one is not justified in equating molecular replication 

 of DNA with chromosome replication. Nonetheless, it does seem rather 

 probable that the basic mechanism of replication of genetic information 

 is the same in all organisms, including viruses. 



The mechanism of separation of the products of replication is another 

 matter. In the vast majority of cases where detailed analysis is possible, 

 the mechanism involves orientation of bichromatid chromosomes in a 



REPLICATION OF GENETIC MATERIAL IN MICROORGANISMS / 203 



