细胞学_ppt课件.ppt

2. 转座重组（transposition recombination ）: Another type of event sponsors recombination between specific pairs of sequences, and has been best characterized in prokaryotes. Transposition provides a means by which certain elements move from one chromosomal location to another. The mechanisms involved in transposition depend upon breakage and reunion of DNA strands, and thus are related to the processes of recombination. ; 3. 位点专一性重组（site-specific recombination）: A different type of event allows one DNA sequence to be inserted into another without reliance on sequence homology. Site-specific recombination is responsible for the integration of phage genomes into the bacterial chromosome. The recombination event involves specific sequences of the phage DNA and bacterial DNA, which include a short stretch of homology. The enzymes involved in this event act only on the particular pair of target sequences. Related reactions are responsible for inverting specific regions of the bacterial chromosome. ; 4. 异常重组（illegitimate recombination ）: Another type of recombination is used by RNA viruses, in which the polymerase switches from one template to another while it is synthesizing RNA. As a result, the newly synthesized molecule joins sequence information from two different parents. This type of mechanism for recombination is called copy choice, and is discussed briefly in 16 Retroviruses and retroposons. It is not used for recombination between duplex DNA substrates. ;Figure 14.1 Recombination occurs during the first meiotic prophase. The stages of prophase are defined by the appearance of the chromosomes, each of which consists of two replicas (sister chromatids), although the duplicated state becomes visible only at the end. The molecular interactions of any individual crossing-over event involve two of the four duplex DNAs. ;Figure 14.2 Recombination between two paired duplex DNAs could involve reciprocal single-