CHAPTER 42 



FIGURE 42-4. The hypothesized synthesis of over- 

 lapping partial replicas. The phage cross is 

 a+(xi)b- X a-(x2)b+. 



(x,) 



b _ 



(x,) ^ (x, 



■ b^ b- 



(xJ 



(xJ 



(x,) 



PARTIALLY DIPLOID 

 RECOMBINANT 



MATING 



PARTIAL 

 REPLICATION 



nism of phage recombination is based largely 

 on experiments employing virulent T phages. 

 In other experiments, ^° crosses are made, in 

 unlabeled host cells, between different mu- 

 tants of the temperate phage, lambda, of 

 which one is unlabeled and the other is 

 labeled with the isotopes C^^ and N'\ 

 Density-gradient ultracentrifugation is then 

 used to determine the distribution of labeled 

 parental DNA among both parental and 

 recombinant genotypes. It is found, from 

 this and other experiments, that discrete 

 amounts of the original parental DNA appear 

 in the recombinant phages. The simplest 

 model accounting for the results is that 

 recombinants are formed following breaks in 

 the parental DNA strands. Such results do 

 not support, but do not exclude, the usual 

 form of the hypothesis of phage recombina- 

 tion by a copy-choice mechanism, according 

 to which the recombinant phage DNA should 

 be made of entirely new, unlabeled, material. 



1° M. Meselson and J. J. Weigle (1961), and G. Kel- 

 lenberger, M. L. Zichichi, and J. J. Weigle (1961). 



(x,) 



b . 



PARENTAL 



A. D. Hershey, about 1960. 



