RECOMBINATION ANALYSIS IN MICROBIAL SYSTEMS 57 



very soon, the leading chromatid will be pulled away from the lower 

 parental chromosome (Figure 7, A) and will resume its growth along 

 the upper parental chromosome. The lagging chromatid will be synthe- 

 sized as though no pairing had occurred. This will lead to non- 

 reciprocal recombination. If pairing lasts a little longer, the lagging 

 chromosome will reach the cross-over point ( Figure 7, B ) and will be 

 obliged to shift to the upper parent in order to copy from an existing 

 structure. If pairing is disrupted after the lagging chromosome has 

 copied beyond the position of a-z, both new 'chromatids will be obliged 

 to resume copying along their initial templates. The result will be a 

 reciprocal interallelic recombination. If, however, eflPective pairing 

 lasts somewhat longer, it may become relatively stable, giving rise to 

 multiple reciprocal cross-over events. As a consequence of the stability 

 of the paired region, a restriction is imposed on the formation of further 

 paired regions. Thus Stadler's results can be explained by supposing 

 that the pairing which gives rise to all events is the same, but that its 

 early interruption releases the chromosomes from the constraint that 

 prevents further cross-overs. It is thereby possible to reconcile experi- 



paired separated 



ai + a i + 



A. O O 



L 



-> 



+ -f 



O: 



j::^ 



-h ao -\- ^2 



a 



B. a 



1 



-h 



■> 



+ 



-H 



y \l \/ ^ separated at first 



a 



J\ /\ /\ meiotic division 



Figure 7. Model of reciprocal and non-reciprocal cross-over events suppos- 

 ing that effective pairing is at first unstable. A and B: early rupture of pair- 

 ing. C; persistence of pairing, giving rise to a stable, effectively paired region. 



