SEJARAH PERKEMBANGAN

DNA damage, repair and recombinationTM-06RECOMBINATIONHomologous recombinationCP 6.4: menjelaskan rekombinasi DNAFLASH 4. Homologous recombinationHomologous recombinationThe exchange of homologous regions between two DNA molecules occurs extensively in eukaryotes during meiosis. In prokaryotes, recA-dependent recombination involves a four-stranded Holliday intermediate which can resolve in two ways. The integrity of DNA containing unrepaired lesions can be maintained during replication by homologous recombination.

Also known as general recombination, this process involves the exchange of homologous regions between two DNA molecules. In diploid eukaryotes, this commonly occurs during meiosis when the homologous duplicated chromosomes line up in parallel in metaphase I and the nonsister chromatids exchange equivalent sections by crossing over. After meiosis is complete, the resulting haploid gametes contain information derived from both maternally and paternally inherited chromosomes, thus ensuring that an individual will inherit genes from all four grandparents. Haploid bacteria also perform recombination, for example between the replicated portions of a partially duplicated DNA or between the chromosomal DNA and acquired foreign DNA such as plasmids or phages. In E. coli, the two homologous DNA duplexes (double helices) align with each other (Fig. 1a) and nicks are made in a pair of sister strands (ones with identical sequence) near a specific sequence called chi (GCTGGTGG), which occurs approximately every 4 kb, by a nuclease associated with the RecBCD protein complex (Fig. 1b). The ssDNA carrying the 5-ends of the nicks becomes coated in RecA protein to form RecAssDNA filaments. These cross over and search the opposite DNA duplex for the corresponding sequence (invasion, Fig. 1c), after which the nicks are sealed and a four-branched Holliday structure is formed (Fig. 1d). This structure is dynamic and the cross-over point can move a considerable distance in either direction (branch migration, Fig. 1e). The Holliday intermediate can be resolved into two DNA duplexes in one of two ways. If the two invading strands are cut, the resulting recombinants are similar to the original molecules apart from the exchange of a section in the middle (Fig. 1f). If the noninvading strands are cut, the products have one half from one parental duplex and one half from the other, with a hybrid (heteroduplex) section in between (Fig. 1g). Homologous recombination is also important for DNA repair. When a replication fork encounters an unrepaired, noncoding lesion it can skip the damaged section of DNA and re-initiate on the other side, leaving a daughter strand gap. This gap can be filled by replacing it with the corresponding section from the parental sister strand by recombination. The resulting gap in the parental sister strand can be filled easily since it is not opposite a lesion. The original lesion can be removed later by normal excision repair. This mechanism has also been called post-replication repair.

2CP 6.4: menjelaskan rekombinasi DNARECOMBINATIONHomologous recombinationINTRODUCTION

In this tutoral, we examine a type of DNA exchange called general, or homologous recombination that occurs between two molecules of DNA that are similar in sequence. Although we describe a bacterial system, homologous recombination also occurs in eukaryotes. It is perhaps most striking during meiosis, in which two homologous chromosomes undergo crossing over, producing diverse gametes with unique combination of alleles along the recombined DNA. Homologous recombination requires the action of several enzymes and enzyme complexes. In E. coli, the two main enzymes are recA and recBCD. CP 6.4: menjelaskan rekombinasi DNARECOMBINATIONHomologous recombination

CP 6.4: menjelaskan rekombinasi DNARECOMBINATIONHomologous recombination

CP 6.4: menjelaskan rekombinasi DNARECOMBINATIONHomologous recombination

CP 6.4: menjelaskan rekombinasi DNARECOMBINATIONHomologous recombination

CP 6.4: menjelaskan rekombinasi DNARECOMBINATIONHomologous recombination

CP 6.4: menjelaskan rekombinasi DNARECOMBINATIONHomologous recombination