BACTERIOPHAGE GENETICS 357 



tion of different nucleotides for that present in the wild type 

 genetic material. On this hypothesis there is clearly a modest 

 limit to the number of possible phenotypes that could arise as a 

 result of mutations at a single point. The four phenotypes 

 (wild type plus three mutant types) found by Benzer at a single 

 point might be attributable to the four known nucleotides of 

 T4. Substituents on the nucleotides, such as glucose or hydroxy- 

 methylcytosine, might permit a few more types but the total 

 number of variations that can occur at a single point locus is 

 clearly limited if the muton and recon are restricted to a single 

 nucleotide pair. 



6. Radiation Genetics 



The various effects of ultraviolet light and ionizing radiations 

 on bacteriophages have been discussed in Chapter VI. We 

 will consider here the genetic significance of studies involving 

 radiation and radioactive decay. Certain types of radiation 

 damage are reversible by light (photoreactivation) and by cer- 

 tain chemicals (chemoreactivation) without involving genetic 

 interaction. However, the reactivation of inactivated phages 

 under conditions of multiple infection (multiplicity reactivation), 

 or the rescue of markers of inactivated phages by mixed infec- 

 tion with active phages (cross-reactivation) has features sugges- 

 tive of genetic interaction between two or more phage particles 

 within an infected cell. As a result radiation has become a 

 tool in the study of the genetics of phages. 



a. Multiplicity Reactivation of Phage Inactivated by Ultraviolet Light 



Delbriick and Bailey, while assaying ultraviolet-inactivated 

 phage stocks, noted that the plaque counts of surviving phage 

 particles were quite variable and depended on the relative propor- 

 tions of phage lysate and bacteria in the plating mixtures. 

 In investigating this titration anomaly, Luria (1947) discovered 

 the phenomenon known as multiplicity reactivation. A fuller dis- 

 cussion of the experimental methods and results was given by 

 Luria and Dulbecco (1949). An ultraviolet-inactivated phage 



