364 BACTERIOPHAGES 



posed, before bacterial infection, to doses of ultraviolet light 

 so slight as not to cause appreciable inactivation of the phages, 

 genetic recombination was very much increased. This phe- 

 nomenon was thoroughly studied in two and three factor crosses, 

 with equal and unequal multiplicities of the parents, either or 

 both irradiated. There are striking differences between the 

 kinetics of recombination with unirradiated and irradiated 

 phages, respectively. With unirradiated phages, recombina- 

 tion in phage lambda occurs relatively rarely and relatively 

 late during the intrabacterial growth cycle. In contrast, with 

 irradiated phage, recombination is much more frequent and it 

 occurs very early, producing large clones of recombinants. 

 It seems that the radiation produces lesions in the genetic 

 material that replication tends to bypass, resulting in an in- 

 creased number of recombinations. This is very similar to the 

 situation believed to prevail in multiplicity reactivation and 

 cross reactivation, with the difference that in the present case 

 the lesions are not lethal in single infection. 



