分子生物学（张海红）11 章Site-Specific Recombination.pptVIP

* Site-specific recombination differs from general recombination in that short specific sequences which are required for the recombination, are the only sites at which recombination occurs. These reactions invariably require specialized proteins to recognize these sites and to catalyze the recombination reaction at these sites. The steps and features of the general recombination reaction, however, still apply: strand exchange formation of a Holliday intermediate branch migration resolution. * * The chromosomes of most bacteria, plasmids and some viral genomes are circular. * * As a result of this process, for humans 95% of Y chromosome is unable to recombine, and Y chromosome contains only 70 to 300 working genes versus more than 1000 for X chromosome. For some other animals, the degradation of Y chromosome is even more severe. For example, the Y chromosome in kangaroos contains only the SRY gene. Figure 11-26 Chapter 11 Transposition of a poly-A retrotransposon by target site-primed reverse transcription Figure 11-26 Topic 4 Examples of transposable enements and their regulation Two types of regulation appear as recurring themes: Transposons control the number of their copies present in a given cell. Trnasposons control target site choice. Chapter 11 4-1 IS4-family transposons are compact elements with multiple mechanisms for copy number control Tn10 transposes via the cut-and paste mechanism, using the DNA hairpin strategy to cleave the nontransfered strands. Tn10 limits its copy number in any given cell by strategies that restrict its transposition frequency. One mechanism is the use of an antisense RNA to control the expression of the transposase gene. By this mechanism, cells that carry more copes of Tn10 will transcribe more of the antisense RNA, which in turn will limit expression of the transposase gene. The transposition frequency will, therefore, be very low in such a strain. Chapter 11 Tn10 Structure: Mechanism: cut-and-paste mechanism Chapter 11