Modern Theories of Genetic Recombination
The direct experimental approach to the study of the mechanism of recombination, which became possible through progress in the development of molecular biology, has proved extremely fruitful (Chapter 4). The problem apparently was solved: it was shown that recombination takes place by breakages and reunions of the DNA molecules and that this process is controlled by appropriate enzymes. Filling in the details could be left to the biochemists. The final stage of work in this direction would be isolation of the corresponding enzymes and performance of the reaction in vitro. Without in any way seeking to minimize the necessity, the value, and the prospects of such an approach, the fruitfulness of which was demonstrated by the later work of Anraku and co-workers (Anraku and Lehman, 1969; Anruku et al., 1969) and of Cassuto et al. (1971) it must be pointed out that this would be adequate only if we knew nothing about intragenic recombination. The biochemical concept of recombination in the form in which it was described in Chapter 4 is sufficient to explain crossing-over. However, attempts to interpret phenomena connected with intragenic recombination in terms of this concept have so far proved unsuccessful (5.2).
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- Unwinding and pairing can, in principle, precede primary breaking. Spontaneous denaturation is known to take place at a physiological temperature in A-T ridge sites (page 142) within the molecule, leaving the molecule as a whole intact. The authors of several models of genetic recombination emphasize this point (Fogel and Hurst, 1967; Sermonti and Carere, 1968; Moore, 1972; Sobell, 1972).Google Scholar
- Integration crossing over takes place in the region of something of the order of 12 base pairs and it is not based on homology attΦ and att ß Vegetative recombination under the influence of the Int system is always reciprocal (1.18) and it also involves a very limited region of the chromosome, as a result of which the recombinants form a sharp peak in a density gradient (Nash and Robertson, 1971). Integration crossing over is not accompanied by additional DNA synthesis. The efficiency of transfer of the parental label to the recombinants may reach 50% of more (Kellenberger-Gujer and Weisberg, 1971). Parkinson (1971) showed that there is definite asymmetry and irreversibility of the recombination events taking place under the influence of the Int system.Google Scholar
- † The term “hybrid” is taken to mean a DNA molecule whose complementary strands are obtained from different parents-(heteroduplex).Google Scholar
- Many workers have emphasized that the situation illustrated in Fig. 76 must be taken as the basis for the explanation of nonreciprocity, regardless of whether they assume a role of the correction system or not. The situation may be the result of unequal participation of the parents in the recombination, connected with transformation-like transfer of material from the donor chromatid to the recipient chromatid. See the. See the “poisoned arrow” model of Hotchkiss (1971), the model of Stadler and To we (1971), and also the models of Paszewski (1970) and of Boon and Zinder (1969,1971). The same situation can result from the mechanism of strand assimilation proposed by Cassuto and Radding (1971) on the assumption that the gaps in the donor chromatid are repaired synchronously by DNA-polymerase. However, all these explanations are ad hoc. For example, generalization of the strand assimilation mechanism leads to types of models with which we are already familiar (5.2,5.3). The foregoing remarks do not rule out the possibility that conversion is effectively induced by fission of the P32 incorporated in only one gene of the yeast zygote (Korolev and Gracheva, 1972).Google Scholar
- † In “true” crossing over between alleles the hybrid DNA lies in the region between the mutant sites without affecting either of them, so that the possibility of conversion is ruled out (see also page 173).Google Scholar
- A step forward was made by Emerson (page 162) but it did not lead to any qualitative progress.Google Scholar
- In the model of Sigal and Alberts there is an elegant possibility of transition from state 3 (Fig. 70) to state 5 with a probability of 0.5. However, since this requires the rotation of one chromosome around the other and, in addition, no cytologically recordable chiasmata are formed as a result, I consider that this mechanism does not operate in vivo. Google Scholar
- For another explanation of this fact, see page 194.Google Scholar
- † In this case wild-type recombinants can appear in two-point crosses only because of the absence of correction, i.e., in asci of the 1+: 7- type.Google Scholar
- Kitani and Olive used this fact to explain polarity in terms of a model structurally analogous to that proposed by Taylor (5.3). It will be shown below (page 183) that polarity is the result of linked correction. In an analogous system (Stadler and Towe, 1971) the direction factor of correction reaches 50.Google Scholar