The genetics of recombination / D.G. Catcheside.

  • Catcheside, D. G.
Date:
[1977]
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: The genetics of recombination / D.G. Catcheside. Source: Wellcome Collection.

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    8 Introduction in a short segment (negative interference) by supposing that pairing was intermittent, as indeed it must be, and that in paired regions exchanges could be clustered. Lederberg (1955) suggested that recombination in bacteria might be related to the process of replication of the hereditary material, which might be copied first from one parent and then, further along, from the other parent. This copy choice hypothesis was a revival of Belling's (1928) hypothesis of crossing over. Freese (1957) applied the idea to allelic recombination in JVeurospora crassa. The hypothesis assumed that replication was conservative so that, by copying the hereditary material oí a parental chromosome so as to leave the latter intact, a process of switching between two homologues might operate when these were close together. Some lack of synchrony between the synthesis of two daughters might allow both to copy from the same parent over a short interval, so generating 3:1 ratios. Repeated switching would account for multiple exchanges. This hypothesis suffers from serious faults. For example it takes no account of the evidence that both DNA (Meselson and Stahl, 1958) and chromosomes (Taylor, et al., 1957) replicate semiconserva- tively. It does not explain post-meiotic segregation, nor the inequality of 6:2 and 2:6 ratios. It does not explain why recombination occurs with molecular precision and that actual exchanges of material occur. It does not predict that different pairs of chromatids are involved, apparently at random at different places along a bivalent. Modifications to the copy choice theory can be made to accommodate these and other objections, but it becomes cumbersome, inelegant and hardly plausible. All subsequent theories propose the formation of a hybrid overlap (as a joint molecule or a heteroduplex) between a segment of a chain from one DNA molecule and its complement from another DNA molecule. The joint molecule is held together by hydrogen bonding. The manner by which such a joint molecule is established may not be the same in all organisms, but it seems to be a common ingredient in all recombination processes. The idea that recombination involved the formation of a hybrid , segment occurred independently to several people at about the same time, notably Holliday (1962, 1964), Meselson (1964), Whitehouse (1963) and Taylor et al. (1962). All made more or less detailed proposals. The notion has the explicit merit of assuming that the process is precise at the molecular level, so that the breaks, later inferred, appear to be between exactly corresponding nucleotides of the two parents. No other processes offer the appearance of such exactness. The hypothesis also has the result that the joint molecule may have mispaired bases, due to genetic differences between the parents. Correction of these by removal and replacement might occur either by excision of just one of the mispaired bases or by excision of a length of one strand including a mispaired base. Repair of the gaps would complete the conversion. Failure of correction of a mispaired base during the meiotic stages would lead to post-meiotic segregation, following replication of the chromosome or chromosomes that