CHROMOSOME DUPLICATION AND GENETIC RECOMBINATION 205 



somes begins. Yet the correlation of genetic crossing over with chiasma 

 formation is good and supports the view that reciprocal exchanges do 

 occur. One proposed escape from this dilemma is based on the idea of 

 transient pairing within short, localized regions during DNA replication. 

 On this scheme, recombination would be by copy-choice, with sister- 

 strand exchanges also occurring, and complete pairing of homologous 

 chromosomes taking place at meiotic prophase, as observed by cytologists. 

 A model of this scheme is shown in Figure 7.10. 



The two-mechanism hypothesis which at present appears more likely 

 requires that the chromosome provide a suitable structural basis for both 

 copy-choice and reciprocal exchange. A model of such a chromosome 

 has been proposed by Freese and by Taylor, and adapted to the bacterial 

 chromosome bv Kellenberger. The essential feature of this model is 

 that DNA molecules are held together by links in such a way that copy- 

 choice recombination may occur between homologous DNA molecules 

 during replication, and that reciprocal exchanges may occur at the 

 links, then or later. A diagram of this model is shown in Figure 7.11, 

 and a proposed scheme for nonreciprocal recombination within a locus is 

 shown in Figure 7.12 



In summary, then, this intriguing and fundamentally important area of 

 recombination mechanics is in need of a far better understanding of the 

 molecular organization of the chromosome than is now available. Gene- 

 tic evidence does not exclude such diverse possibilities as: (1) that copy- 

 choice is responsible only for rare events which mar the otherwise classi- 



%)O0Or\/XXX)C ' 



FIGURE 7.12. Proposed model of copy-choice replication of DNA. Assume that 

 A and 6 represent the DNA double helices of homologous chromosomes, and that 1 

 and 2 are the complementary strands of each. The newly replicating strands may 

 switch from A to 6 or from 6 to A, and as a result each of the new strands will 

 be recombinant. 



