2 - Studies on Extrachromosomal Homologous Recombination in Mammalian Cells: Implications for Chromosomal Recombination and Gene Targeting

Abstract

This chapter presents studies on extrachromosomal homologous recombination in mammalian cells and its implications for chromosomal recombination and gene targeting. The chapter presents the studies of extrachromosomal recombination in both mouse and human cells. The results of these studies indicate that linear DNA is a much better substrate for homologous recombination than is circular DNA. The chapter also discusses whether the class of homologous recombination can serve as a model for chromosomal recombination and gene targeting. The experiments on extrachromosomal recombination in the chapter show that circles are extremely poor substrates for recombination, even when only one of the two substrates in an intermolecular reaction is circular. One can think of two possible reasons for this. First, DNA ends are needed to initiate the recombination process and must be present on both of the substrates that undergo intermolecular reactions. This possibility is more easily accommodated by the SSA model than the DSBR model. Another reason for the nonrecombinogenic nature of circular DNA may be because of the fact that, it becomes refractory to recombination more rapidly than does linear DNA after either DNA is introduced into mammalian cells. If one hypothesizes that this refractory state is associated with, for example, the conversion of naked DNA to chromatin, these results would imply that circular DNA is converted to chromatin more rapidly than linear DNA following transformation.