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  Section: Genetics » Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes)
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Mechanism of genetic recombination

Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes)
Coupling and repulsion hypothesis
A testcross in maize
Crossing over and meiosis 
Crossing over and chiasma formation
Mechanism of genetic recombination
Crossing over and linkage maps 
Recombination frequencies from a test-cross
Recombination frequencies from F2 data
Interference and coincidence
Linkage maps
Mapping function and poisson distribution
Linkage groups
Chi-square test 
Cytological basis of crossing over
Creighton and McClintock's experiment in corn
Meselson and Weigle's experiment using lambda (λ) phage
Crossing over at four strand stage

In classical theory and chiasmatype theory discussed above, there was an emphasis on causal relationship between chiasmata and crossing over. It has, however, been difficult to explain how exchange of segments or recombination of gene clusters is brought about. There were at least two theories available initially to explain this, but newer hypotheses were proposed later. Only these two earlier theories and the more recent hybrid DNA models will be discussed here in brief. Molecular Mechanisms of Recombination should be consulted for a more detailed study of the mechanism of genetic recombination at the molecular level. Such a detailed account is also available in the book 'Towards an Understanding of the Mechanism of Heredity' by H.L.K. Whitehouse, who had himself put forward a popular Polaron Hybrid DNA Model to explain the mechanism of crossing over at the molecular level.
Precocity theory. Precocity theory of CD. Darlington is ordinarily discussed in context with meiotic chromosome pairing. However, Darlington extended this theory to explain recombination also. The theory assumes that prophase is precocious in meiotic cell division and therefore involves homologous pairing to satisfy the pairing need, which is achieved in mitosis due to duplication of chromosomes. This theory is now untenable in the light of recent information that DNA synthesis really takes place before the onset of prophase I in meiosis, although synthesis of a very small fraction of DNA called zygotene DNA (zyg DNA) (0.1 to 0.2%) is delayed till zygotene. Precocity theory presumes that DNA synthesis or chromosome duplication takes place later in pachytene or diplotene and then results in separation of homologous chromosomes (terminalization). On the basis of precocity theory, Darlington explained crossing over to be the result of strain or torsion produced due to coiling of homologous chromosomes and sister chromatids.

Belling's hypothesis and copy choice model. In making a distinction between chiasmata and crossing over, we concluded earlier that crossing over or exchange of chromosome segments (which leads to genetic recombination) is the result of a breakage and reunion process. One may ask the question, whether it is actually a breakage-reunion process or only appear to be so, as envisaged in a hypothesis proposed by John Belling in 1928. While studying meiosis in some plant species, Belling visualized genes as beads (described as chromomeres), connected by nongenic inter-chromomeric regions.

During duplication of chromosomes, the chromomeres are duplicated first and the newly formed chromomeres remain tightly juxtaposed, to the old ones, without any interchromomeric regions found between them (Fig. 10.4a). When these interchromomeric regions or linking elements are synthesized, they may switch from a newly made chromomere on one homologous chromosome to an adjacent chromomere on the other newly synthesized homologue. One can now visualize how it can generate a crossover chromatid that would only seem to have arisen from a physical breakage and reunion of chromosomes (Fig. 10.4b). Bellirtg's hypothesis, from its very nature also suggests that the events producing recombinant chromosomes can take place only between newly made chromatids, so that only two strand double and multiple crossovers are possible and no three strand or four strand multiple crossovers are possible.
Belling's model for crossing over showing duplication of chromomeres (a) followed by joining of chromomeres newly synthesized on two different homologous chromosomes (b).
Fig. 10.4. Belling's model for crossing over showing duplication of chromomeres (a) followed by joining of chromomeres newly synthesized on two different homologous chromosomes (b).

The above model of crossing over or recombination became known as the copy choice model, which was used by Joshua Lederberg in 1955 to explain recombination in microbial systems. In other words, copy choice model meant that a newly synthesized daughter chromatid is derived due to copying of one chromosome upto a certain distance and then switching on to the other homologous chromosome for copying the remaining distance or region of the chromosome (Fig. 10.5). This copy choice model, as a mechanism of recombination, was inadequate for the following reasons : (i) Copy choice model assumes conservative mode of DNA replication, but all experimental evidence suggests that DNA replicates in a semi-conservative manner, (ii) Copy choice model predicts that in every meiosis, when multiple crossovers occur, only two of the four chromatids would be involved. The experimental evidence suggests that three strands or four strands may actually be involved in multiple crossovers, as shown in tetrad analysis, where the four products of a meiosis can be recovered and analysed (see Tetrad Analysis, Mitotic Recombination and Gene Conversion in Haploid Organisms (Fungi and Single Celled Algae) for details). For instance, a single meiotic division in a hybrid ABC/abc may give ABc, AbC, aBC and abc, which can result only from three strand double crossovers, an event not explainable by copy choice model (Fig. 10.6).

Mechanism of genetic recombination based on copy choice mechanism.
Fig. 10.5. Mechanism of genetic recombination based on copy choice mechanism.
A possible three strands double crossover, making Belling’s copy-choice (switch) model very unlikely.
Fig. 10.6. A possible three strands double crossover, making Belling’s copy-choice (switch) model very unlikely.

Hybrid DNA models. During 1960s, hybrid DNA models had become-very popular, since these models best explained the results obtained in certain microbial systems. In these models, only one strand in each of two DNA duplexes belonging to non-sister chromatids (from homologues) breaks. The single strands released from these breaks then pair crosswise with unbroken strands by complementary base pairing. This results in the formation of hybrid DNA segments, and hence the name hybrid DNA models.

Several hybrid DNA models have been proposed during the last three decades. Two of the earliest models included the one proposed in 1963 by H.L.K. Whitehouse of Cambridge, England and the other in 1964 by R. Holliday of London. The two models mainly differ in so far as Whitehouse believed that breaks would occur in single strands having opposite polarity, while Holliday proposed that breaks would occur in sirands having same polarity. The details of Holliday's model, which is relatively simple and has been widely accepted, are presented in Figure 10.7. In contrast to these models involving single strand breaks, there are also hybrid DNA models assuming double strand breaks, which are discussed in Molecular Mechanisms of Recombination.
Mechanism of recombination as explained on the basis of hybrid DNA model of R. Holliday (1964).
Fig. 10.7. Mechanism of recombination as explained on the basis of hybrid DNA model of R. Holliday (1964).

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