118 CELL HEREDITY 



about 40 per cent of the DNA remained in the form of hirge molecules; 

 the rest was present after release in very small pieces. These small pieces 

 have an unknown function, but the large piece clearly passed from one 

 generation to the next. After a cycle of infection, the stars formed by the 

 progeny had some 20 per cent as many arms as the stars formed by the 

 original infecting DNA particle; the number remained constant at this 

 level after the next cycle. These observations are consistent with the 

 notion that the infecting DNA contains a large molecule making up 

 about 40 per cent of the total. This molecule may be in the form of a 

 double helix, replicating according to the Watson-Crick model. Thus 

 the first cycle phages have 20 per cent of the total radioactivity and, 

 since the two strands of the double helix each remain intact, all future 

 cycles yield progeny with the same amount of radioactivity. These re- 

 sults are comparable to those found with the chromosomes of other 

 organisms (Chapter 7), and indicate not onlv a doubleness of the genetic 

 material but also a semiconservation of its organization. That the large 

 DNA molecule is genetic material is demonstrated by the fact that at 

 least some genetic markers pass with it from cycle to cycle. 



With this information in mind we may turn our attention to the 

 recombination that occurs when different phage mutants multiply in the 

 same cell. If two related strains of phages, say of genotypes AB and 

 ab, are mixed in excess with a suspension of sensitive bacteria, nearly 

 every bacterium will be infected by both types. The phage progeny of 

 such a cross will be of four genotypes: AB, ab, Ab, and aB. The re- 

 ciprocal cross, Ab X aB, gives the same kinds of progeny, but this time 

 the recombinants are AB and ab; they appear with the same frequency 

 as did Ab and aB in the first cross. Thus the coupling of genes in the 

 parents does not determine the frequency of recombinants; the particular 

 genes involved in the cross determine the rate of exchange. Evidently 

 a linkage of genes exists in phages just as in other organisms; the fre- 

 quency of recombination between different genes is roughly additive, and 

 linear genetic maps have been constructed (Table 5.2). When closely 

 linked genes are studied, however, evidence for negative interference is 

 obtained: the occurrence of one genetic exchange increases the likelihood 

 of an adjacent one. 



It must be appreciated that the progeny of a phage cross do not result 

 from one generation of multiplication. No matter how few phages infect 

 a cell, some hundred progeny may be recovered. Several generations of 

 multiplication take place within each cell, and the multiplying phages 

 have a chance to mate again and again. When many phages infect a 

 single cell, there is no assurance that each one of them will mate or, 

 conversely, that all progeny will result from a mating and not just from 



