RECOMBINATION IN VIRUSES AND BACTERIA 121 



high probabihty that there will be another of the same type; but the 

 probability that the reciprocal type will come from the same cell is low. 

 The prediction of a simultaneous reduction in self- and reciprocal corre- 

 lation is not borne out. Therefore, reciprocal recombination products 

 are not formed simultaneously by a single event. 



Rather, all results are consistent with the partial-replica theory which 

 accounts for recombination by another means than crossing over. This 

 theory, which fits nicely with the Watson-Crick model of DNA, assumes 

 that duplication of the genetic material occurs progressively along a 

 template. On occasion a partial replica of the first template separates 

 from it and is completed on another (Figure 5.4). Thus, if every new 

 and old DNA molecule can act as a template, multiplication can be ex- 

 ponential, but every mating results in the formation of only one re- 

 combinant product. It is even possible that a single replica may copy 

 more than two templates by a process of group mating. How this partial 

 replica or copy-choice mechanism can be related to DNA structure will 

 be discussed in Chapter 7. 



In addition to accounting for the types of recombination that occur, 

 the partial-replica hypothesis provides a model for a peculiar kind of 

 heterozygous phage that is formed in crosses with a frequency of a few 

 per cent. This phage is a single particle but it does not reproduce its 

 kind. When it multiplies it gives rise to haploid phage containing one or 

 the other genes for which it was heterozygous and, once again, to a few 

 per cent of heterozygotes. The region of heterozygosity in the DNA 

 molecule must be small because only when genes are closely linked are 

 phages found which are heterozygous for both of them. When two dis- 

 tant genes are involved in a cross, the phages may be heterozygous for 

 one of them. Upon reproduction two types are formed; one is an old 

 combination and the other a recombination of the two genes. Figure 



Switch 



FIGURE 5.4. Diagram of a possible mode of partial replica formation and recombina- 

 tion in phages. See also Figure 7.12. 



