Biochemical analysis of genetic recombination in eukaryotes

Abstract

SUMMARYRecent studies concerning molecular mechanisms of genetic recombination in eukaryotes are reviewed. Since many of these studies have focused on the testable predictions arising from the hybrid DNA theory of genetic recombination, this theory is summarised. Experiments to determine the time of meiotic crossing-over and the structure of the synaptonemal complex which facilitates meiotic crossing-over are described. Investigations of DNA nicking and repair events implicated in recombination are discussed. Properties of proteins which may facilitate hybrid DNA formation, and biochemical evidence for hybrid DNA formation are presented. Finally, a nuclease which has been implicated in gene conversion is described.STUDIES of the molecular events which result in recombination between chromosomes have been initiated in a wide variety of eukaryotic organisms. Since the meiocyte is a highly specialised cell which will produce specialised haploid progeny, the problem of identifying biochemical events which are strictly related to genetic recombination in the meiocyte are formidable indeed. In higher eukaryotes, the task of analysing genetic exchanges is further complicated by the striking evolutionary conservation of the number of reciprocal exchanges which occur in each meiosis. Although the amount of DNA can vary by orders of magnitude, the number of exchanges has remained quite limited.The hybrid DNA theory as proposed by Holliday (1964) and Whitehouse and Hastings (1965) has become the focal point for experimental approaches. The scheme proposed by Holliday (1964) is still compatible with the available genetic data, and thus will be used as a basis for the discussion, in Section 1, of likely biochemical events during meiotic recombination. The first requirement which must be met to study the biochemical predictions which arise from this theory is to determine the time of meiotic crossing-over. The evidence, discussed in Section 2, strongly suggests that genetic exchange occurs during the pachytene stage of meiotic prophase. The prominent feature of the pachytene nucleus, which provides the framework for crossingover, is the synaptonemal complex, whose structure is reviewed in Section 3.DNA nicking and repair events implicated in hybrid DNA formation and recombination are evaluated in Section 4, and properties of DNA binding proteins which may facilitate this process are reviewed in Section 5. Biochemical evidence for hybrid DNA formation is presented in Section 6 and a nuclease implicated in gene conversion is described in Section 7. 's model (1964) nicks are introduced in single strands of the same polarity at corresponding sites in two homologous chromatids. These strands unravel, exchange positions, and reanneal with the complementary unnicked strand in the homologous chromatid, forming a half-chromatid chiasma and generating hybrid DNA on the two chromatids. The following events do not necessarily occur in the order in which they are described. The nicks at the origin of the hybrid DNA are ligated. The ...

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