122 CELL HEREDITY 



5.4 shows how this is possible and, in atlditioii, accounts for the results 

 of a three-factor cross in which some phages become heterozygous for 

 the middle marker. In this case they are recombinant for the distal 

 markers, as the model predicts. The structure and frequency of hetero- 

 zygous phages are sufficient to account by their reproduction for all re- 

 combinants found. 



There are other phenomena that have an explanation in the partial- 

 replica hypothesis. For example, ultraviolet light increases the fre- 

 quency of recombination in phages, probably by damaging some short 

 region of the DNA molecule. When replication reaches that site it can- 

 not progress. Unable to complete itself, the partial replica may leave 

 its damaged template and finish its replication on another template, un- 

 damaged at the site in question. In this way some genes on a damaged 

 phage may be "rescued" by replication in a recombinent particle. Genes 

 in phages inactivated by P^^ disintegration, which probably breaks the 

 sugar-phosphate backbone of DNA by transmutation to S^^ or through 

 the recoil energv sustained by the decaying P^"' nucleus, may be rescued 

 in a similar way. This, and the fact that linked loci are lost more often 

 together, is fine evidence that nucleic acids are actually the carriers of 

 genetic information. 



Clearly, phage genes are exchanged by a mechanism which can hardly 

 be called sexual. There is no true zygote, and there is no evidence of 

 processes like mitosis or meiosis. The phage DNA molecule, although 

 not a nucleoprotein, is sometimes referred to as a chromosome because it 

 comprises the assemblage of genes linked in a linear order. 



LYSOGENIC BACTERIA AND PROPHAGE 



The bacteriophages discussed thus far are called virulent because they 

 invariably kill their hosts. Although the virulent phages have been 

 studied most intensively, they are much less prevalent in nature than 

 another class of bacteriophages, called temperate, which do not neces- 

 sarily kill the bacteria they infect. The DNA of temperate phages may 

 take up a symbiotic relationship with the bacterial host; in such an event 

 the bacterium is not destroyed, and may even benefit from the presence 

 of the phage. Bacteria which carry such phages are called lysogenic. 

 When lysogenic bacteria are disrupted, no infectious phages are found 

 within them. The noninfectious unit which is carried by lysogenic cells 

 and which contains the information for phage formation is called pro- 

 phage. During the growth of a lysogenic strain, some cells, at a rate 

 that is usually from 10"" to 10~'^ per generation, lyse and release in- 



