4 SEX IN MICROORGANISMS 



ing statement: "Perhaps one might dispute the propriety of calhng 

 the observed changes induced mutations. In some respects they look 

 more hke transfers or even exchanges of genetical materials." At 

 the same symposium, Hershey gave convincing evidence for the in- 

 dependent occurrence of h and r mutations in phage T2. The field 

 was open for genetic investigation. A two-factor cross, hr by h'^r'^, 

 could now be attempted. 



In 1949 Hershey and Rotman published a paper in which they 

 described the main features of phage genetics. The principal finding 

 is the fact that in any two-factor cross, with equal multiplicity of 

 the two parents, recombinants are obtained in different frequencies 

 depending on the linkage relation between the two markers. Within 

 the two parental types and within two recombinant types the fre- 

 quencies are the same. What changes is the ratio of recombinants 

 to the total. Shortly after the discovery of the r mutant, Hershey did 

 some single-factor crosses to control the results of mixed infection. 



The cross hr'^ x ^+r+ (original experiment of Luria) is a one- 

 factor cross, and obviously no recombinants can be found. Also hr x 

 /; + r is a single-factor cross. From his strain J? Hershey isolated an 

 r mutant (the frequency of the r mutation is quite high), and he 

 proceeded to cross hr by r. The amazing result of this simple experi- 

 ment was the discovery in the yield of a high percentage of f+. 

 As mentioned before, the hr was isolated from an r mutant in the 

 strain h. However, this strain can be prepared the other way round: 

 an /; mutant can be isolated from the r strain. Now we have again 

 two strains, hr and r. We repeat the cross as before; the result is 

 perfectly regular. No f+ is found. Conclusion: Different mutations 

 occurring at different loci give the same phenotype, r. Let us take 

 rl and r2. The following genotype will be obtained: flr2+, rl+r2 

 (parental types); rlrl, rl+r2+ (recombinant types). Only rl+f2 + 

 can be distinguished from the others. The existence of rlr2 can only 

 be demonstrated by doing separate crosses with rl and r2. In neither 

 case is the normal type found in the yield. All the r's that Hershey 

 isolated proved to be at different loci. 



THE INTRACELLULAR LIFE CYCLE OF BACTERIOPHAGE 



The first step in the growth cycle of a phage is the adsorption 

 of the phage particle to the bacterial surface. If the bacterial cell is 



