56 MOLECULES, VIRUSES, AND BACTERLV 



advanced chromatid separates from the Parent 2 template, leaving the 

 latter free for the delayed chromatid to copy along the same region. 



When non-reciprocal allelic recombination occurs, outside markers 

 may recombine. When they do, it is always reciprocally. Using three 

 outside markers, Stadler ( 1959 ) has recently performed an experiment 

 in Netirospora designed to test whether reciprocal and non-reciprocal 

 recombination can be considered to arise from a single event. It will be 

 recalled that when reciprocal recombination occurs at one point along 

 a pair of chromosomes, we observe the suppression of crossing-over at 

 distant sites. In terms of the partial pairing theory, this means that 

 when a region of eflFective pairing is formed, there will be few or no 

 additional paired regions at other points of the two chromosomes in- 

 volved. This interference with crossing-over was used by Stadler to per- 

 form his test. His marking of chromosomes is essentially as in Figure 6, 

 except that an additional marker z, to the right of y, is present. Re- 

 combinant chromosomes containing a normal a locus are selected and 

 divided into two classes. In one class, the recombination between ai 

 and fl2, which yielded a normal a locus, has not been accompanied by 

 reciprocal recombination of x and y, while in the second class, x and y 

 are recombined. The two categories obtained are then examined to see 

 if crossing-over between y and z is normal or reduced. It is found that 

 interference in the y-z segment occurs only if x and y have been recom- 

 bined (as well as ai and og). Now, the majority of recombinations 

 yielding a normal a locus are non-reciprocal. Thus Stadler's experiment 

 shows that if allelic recombination, which is essentially non-reciprocal, 

 is the only recombination occurring in the x-y interval, interference is 

 not observed. This result suggests that reciprocal and non-reciprocal 

 crossing-over do not result from the same primary event. In terms of 

 the partial pairing theory, this would mean that non-reciprocal recom- 

 bination does not require effective pairing. We find ourselves in the 

 situation of supposing either (1) that non-reciprocal recombination 

 results from the diffusion of an extra copy of a small segment of one 

 rephcating chromosome to a second replicating chromosome, where it 

 is incorporated in the linear structure of the new chromatid, or ( 2 ) that 

 some kind of pairing other than "effective pairing" occurs. 



One simple way out of this dilemma is to introduce the factor of 

 time into models in a much more significant way than has been done in 

 the past. We can, for example, suppose that the lack of synchrony in 

 the synthesis of the new chromatids, postulated in Figure 6, B, creates 

 the essential condition for effective pairing. Pairing occurs as a conse- 

 quence of lack of synchrony, a cross-over takes place as shown in Fig- 

 ure 6, B, and this is the initial phase of all recombination within chro- 

 mosomes. Effective pairing may then be supposed to be highly unstable 

 at the outset and to tend to end shortly after it has begun. If it ends 



