重組與轉(zhuǎn)座打印稿

1、第六章第六章 重組與轉(zhuǎn)座重組與轉(zhuǎn)座Recombination and Transposition Two events may induce the changes of genetic materialMutation （突變）：基因或染色體結(jié)構(gòu)的改變Recombination （重組）：染色體序列的重新組合Different types of recombination Homologous recombination（同源重組） or generalized recombination（普遍性重組） Site-specific recombination（位點特異性重組） Transpo

2、sitional recombination（轉(zhuǎn)座重組） 第一節(jié) 同源重組 Homologous recombination1. Breakage and reunion of DNA molecules 噬菌體噬菌體DNA的斷裂和重接的斷裂和重接 2. The Holliday Model Evidence supporting this model the electron microscopic visualization of chi-form the discovery of RecA protein in E.coli g+（black spore） g（gray spore） 2

3、00,000 ascus4g+：4g：mostly5g+：3g：0.06%6g+：2g：0.05%3g+：g：g+：3g：0.008%3. Gene conversion（基因轉(zhuǎn)變基因轉(zhuǎn)變 ） Initially found in yeast and Neurospora. Gene conversion is a nonreciprocal genetic exchange between two closely linked genes Gene conversion is a consequence of DNA recombination It results from the mis

4、match repair process of heteroduplex DNA. Gene conversion is a meiotic process of directed change in which one allele directs the conversion of a partner allele into its own form4. Meselson-Radding model (single-strand break and repair model) 5. Recombination is directed by specific enzymes In E.col

5、i RecA mediated the exchange of strands Rec BCD involved in the nicking and unwinding of DNA RuvC cleave Holliday junction RuvAB promote branch migration 第二節(jié) 位點專一性重組位點專一性重組 Site-specific recombination The integration and excision of DNA involves site specific recombination Attachment sites (att) are

6、 the loci on a phage and the bacterial chromosome at which Site-specific recombination occurs When the att site is deleted from the E. coli chromosome, phage can integrate elsewhere, although the efficiency is 0.1% of the frequency of integration at att. This inefficient integration occurs at second

7、ary attachment sites.The sequence O is common to attB and attP. It is called the core sequence; and the recombination event occurs within it.第三節(jié) 轉(zhuǎn)座因子與轉(zhuǎn)座重組Transposable elements and transpositional recombination Transposable elements ( transposon): A DNA sequence capable of moving ( transposing) from

8、one location to another in a genome. 1. Insertion sequences ( IS) ( 插入序列）插入序列） Insertion sequences ( IS) are the simplest transposons An IS element ends in short inverted terminal repeats; usually the two copies of the repeat are closely related rather than identical. IS elements code for the transp

9、osase The IS elements are normal constituents of bacterial chromosomes and plasmids. When an IS element transposes, a sequence of host DNA at the site of insertion is duplicated. The frequency of transposition The frequency varies among different elements. The overall rate of transposition is 10-3 1

10、0-4 per element per generation. Insertions in individual targets occur at a level comparable with the spontaneous mutation rate, usually 10-5 10-7 per generation. Precise excision of the IS element is usually infrequent, with a range of rates of 10-6 to 10-10 per generation, 103 times less frequent

11、than insertion. 2. Bacterial transposons (Tn) （細菌轉(zhuǎn)座子）（細菌轉(zhuǎn)座子） Tn carry drug resistance (or other) gene in addition to the functions concerned with transposition. One class of larger transposons are called composite elements, because a central region carrying the drug marker(s) is flanked by IS elemen

12、ts In some cases, IS modules are identical, such as Tn9 (direct repeats of IS1) or Tn903 (inverted repeats of IS903). In other cases, the modules are closely related, but not identical. So we can distinguish the L and R modules in Tn10 or in Tn5. A functional IS module can transpose either itself or

13、 the entire transposon. When the two IS modules are identical, either module can sponsor the transposition. When the modules are different, transposition can depend on one of the modules3. The Ac-Ds system in maize (玉米的Ac-Ds 轉(zhuǎn)座系統(tǒng)）Barbara McClintock1940s1950s1983 Nobel Prize4. P element tranposons in

14、 Drosophilia 果蠅的P因子 Hybrid dysgenesis: a condition causing sterility, elevated mutation rate, and a chromosome rearrangement in the offspring of crosses between certain strains of fruit flies. 6. Genetic effect of tranposition （轉(zhuǎn)座的遺傳學效應(yīng)（轉(zhuǎn)座的遺傳學效應(yīng) ） Transposoble element can generate mutations in adjac

15、ent genes Transposable elements can promote rearrangements of the genome Chromosomal rearrangements might contribute to evolution5. Mechanism of transposition （轉(zhuǎn)座機制） replicative transposition conservative (nonreplicative) transposition第四節(jié) 逆轉(zhuǎn)錄病毒與逆轉(zhuǎn)座子 Retroviruses and Retroposons 1. Retroviruses Retro

16、viruses are single-stranded RNA animal viruses that employ a double-stranded DNA intermediate for replication. The viral DNA integrates into the host genome at randomly selected sites. Sometimes (probably rather rarely), the integrated retrovirus can convert a host cell into a tumorigenic state thro

17、ugh activating certain types of host genes.2. Retroposon Transposable elements that utilize reverse transcriptase to transpose through an RNA intermediate are termed retrotransposons. （逆轉(zhuǎn)錄轉(zhuǎn)座子是指通過RNA為中介,反轉(zhuǎn)錄成DNA后進行轉(zhuǎn)座的轉(zhuǎn)座因子。逆轉(zhuǎn)座作用出現(xiàn)在真核生物 ） Retroposons in eukaryotes are generally divided into two classes.

18、 Viral retrotransposons(病毒樣逆轉(zhuǎn)座子）: resembles retroviruses, but differ from them in not forming viral particle Yeast Ty elements, Drosophila copia elements Nonviral retrotransposons (非病毒樣逆轉(zhuǎn)座子） : do not code for reverse transcriptase and/or integrase, transposition depends on enzymes coded elsewhere. LINEs (long interspersed elements) and SINEs in mammals Yeast Ty elements About 35 Ty1 copies in the yeast genome. sequences : 330 bp, about 100 copi