5 The Nucleases of Genetic Recombination

Abstract

I. INTRODUCTION
Genetic recombination plays two fundamental roles in the cell: It provides (1) a mechanism for the generation of genetic diversity and (2) a route for the repair of DNA lesions caused by irradiation or chemical damage. Nucleases play important roles in genetic recombination, and this chapter focuses on those enzymes that are directly implicated in the process. Although recombination can take two forms, generalized and site-specific, this chapter concentrates on general genetic recombination, i.e., recombination processes that occur between two homologous chromosomes. For excellent reviews on the mechanisms and enzymes of site-specific and transpositional recombination, see Craig (1988), Cox (1988), Hatfull and Grindley (1988), Landy (1989), and Mizuuchi (1992).

Nucleases are required both for the initiation of crossover events between two interacting chromosomes and for the resolution of crossovers to allow the separation of recombinant DNA molecules. Because of the difficulty in directly ascribing many eukaryotic nucleases to a defined role in recombination (due to the lack of mutants), most of our knowledge has come from studies of bacterial and bacteriophage nucleases. However, even in bacteria, the situation is not totally clear, since mutants frequently have little or no phenotype due to the ability of one nuclease to take over in the absence of another. In this case, recombination-defective phenotypes are only observed when two or more nucleases of a given type have been inactivated by mutation. It is possible that the observed redundancy reflects the important role that nucleases play in the recombination process.

II. RECOMBINATION MODELS...