Molecular Mechanisms of Recombination

However, it should be recognized that recombination covers a much wider spectrum of events including, (i) crossing over, both meiotic and mitotic (reciprocal exchange of segments), (ii) gene conversion (non- reciprocal events), (iii) exchange between sister chromatids (even though this does not lead to a change in genotype), (iv) repair of DNA damage, etc. For the purpose of this section, in the treatment of recombination, we have excluded processes, in which different DNA sequences are simply ligated together to produce, what is commonly called recombination DNA. The recombinant DNA and gene cloning will be separately discussed in Genetic Engineering and Biotechnology 1.  Recombinant DNA and PCR (Cloning and Amplification of DNA). As earlier discussed in Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes), the mechanism of genetic recombination is mainly based on breakage and reunion process, which has been demonstrated experimentally both at the level of chromosomes as well as at the level of DNA molecules (Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes)).

The term recombination has often been treated as a synonym to crossing over discussed in Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes). However, this simple breakage and reunion of chromaTids or DNA molecules can not explain gene conversion and associated phenomena. Gene conversion leads to the production of rare octads with ratios like 6 : 2, 2 : 6, 5 : 3, etc. among 8 ascospores of a single ascus in ascomycetes like Neurospora. These results could not be explained even on the basis of copy choice model used earlier for explaining recombination events in bacteria and viruses, because copy choice does not take into account the semi-conservative method of DNA replication. Therefore, formation of hybrid DNA or heteroduplex DNA (DNA having two strands from different sources) has been assumed in several recent models proposed to explain the mechanism of genetic recombination at the molecular level. These models will be discussed in a relatively greater detail in this section.