第11章转座子和逆转座子

1、第11章 转座子和逆转座子Transposons and retrotansposons,The sun1642 tomato variety produces an elongated fruit,which is produced by a duplication that arose as a result of the presence of the transposable element Rider.,转座子 ( transposon,Tn),基因组中可以从一个基因座转移到另一个基因座，具有移动能力的一段DNA片段。 A DNA sequence capable of moving

2、 ( transposing) from one location to another in a genome.,2,Discovery of Transposable Elements,McClintocks experiments in maize: the Ds element 1940-1950,3,转座因子的发现,印第安彩虹玉米，具有从最大到最小易于区分的10对染色体。,解离因子 ( Dissociation,Ds ): located at the site of the break 激活因子 ( Activator,Ac ) : unlinked genetic factor,

3、4,5,Transposable element in E. coli , 1960,6,转座子的分类,7,非复制性的转座 （占已经发现的多数）,复制性,逆转座,逆转座,病毒样的转座子,第一节 细菌的转座因子 Transposable elements in Bacteria,两大类: 简单转座因子：只包含和转座有关的基因序列。 Simple transposable elements which carry only the information required for movement. 复合转座因子：除了含有与转座相关的基因外还有其它基因（如抗生素抗性基因）。 Complex tra

4、nsposable elements which contain DNA sequences not directly related to transposition.,8,1.插入序列 (Insertion sequences，IS),9,不含任何宿主基因的可转位的DNA序列，是最简单的转座因子。,插入序列的结构 The structure of IS element,10,11,正向重复序列,反向重复序列,12,插入序列末端两重复序列高度相似但不完全相同。 An IS element ends in short inverted terminal repeats ; usually th

5、e two copies of the repeat are closely related rather than identical. 插入序列的中间序列常具有编码转座酶的功能。 IS elements code for the transposase. 插入序列转座后常导致靶位点的重复。 When an IS element transposes, a sequence of host DNA at the site of insertion is duplicated.,13,插入序列特点：,Duplication of a short sequence of DNA at the i

6、nsertion site,14,The IS elements are normal constituents of bacterial chromosomes and plasmids.,15,Formation of a conjugative R plasmid by recombination between IS elements,16,(Resistance transfer factore,RTF),细菌耐药性,2. 复合转座子 Composite Transposons Composite transposons contain a variety of genes that

7、 reside between two nearly identical IS elements . Composite transposon is designated by Tn, followed by a number.,17,四环素,18,氯霉素,四环素,卡拉霉素,一些复合转座子两端的IS序列完全相同的，如Tn9和Tn903。 In some cases, IS modules are identical, such as Tn9 (direct repeats of IS1) or Tn903 (inverted repeats of IS903). 另外一些转座子中，IS非常相似

8、但不完全相同，因而用R和L表示。 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.,复合转座子特点：,A schematic map of a plasmid carrying simple and composite transposon-resistance genes.,20,IS序列既可以自我转座又可以和复合转座子共同转座。 A functional IS module can t

9、ranspose either itself or the entire transposon. 当IS序列完全相同时，两者均可独立进行转座。 When the two IS modules are identical, either module can sponsor the transposition. 当IS序列不完全相同时，两者之一引发转座，L。 When the modules are different, transposition can depend on one of the modules,3. 非复合型转座子Noncomposite transposons,22,转座酶

10、,抑制酶,氨苄青霉 素抗性,反向重 复序列,23,共整合体,第二节 真核生物的转座因子Transposable Elements in Eukaryotes,DNA 转座子 (cut and paste transposons) Ac and Ds elements in maize P elements in Drosophila 反转座子 （Retrotransposons）,24,1.玉米的Ac-Ds 转座系统 （The Ac-Ds system in maize）,Barbara McClintock (1902-1992) 1940s 1950s 1983 Nobel Prize,2

11、5,26,Movie,玉米中转座因子效应,27,？,28,其它植物中的转座因子 Transposable element in other plants,peas,snapdragons,29,SBE,直链 支链,杂交劣育（Hybrid dysgenesis）: a condition causing sterility, elevated mutation rate, and a chromosome rearrangement in the offspring of crosses between certain strains of fruit flies.,30,2. 果蝇的P因子 P

12、 element transposons in Drosophilia,31,32,反向重 复序列,体细胞,生殖细胞,终止密码子,阻遏蛋白,转座酶,66KD的转座阻遏蛋白是造成现象差异的主要原因。（The 66KD repressor protein is provided as a maternal factor in the egg）,33,P-element-mediated gene transfer in Drosophila,34,3. 反转录病毒与反转座子 Retroviruses and Retroposons,反转录病毒：单链的RNA病毒，依靠独立的双链DNA中间体结构进行感

13、染。,Retroviruses are single-stranded RNA animal viruses that employ a double-stranded DNA intermediate for replication.,35,36,The life cycle of a retrovirus,37,The typical genome of a retrovirus contains gag, pol, and env genes,逆转录病毒基因表达,38,菱形衣 壳蛋白,外壳蛋白,逆转录酶,病毒侧翼序列,逆转录病毒可随机整合到宿主细胞。 The viral DNA inte

14、grates into the host genome at randomly selected sites. 有时（几率较小）逆转录病毒可能将宿主细胞转化为肿瘤细胞。 Sometimes (probably rather rarely), the integrated retrovirus can convert a host cell into a tumorigenic state through activating certain types of host genes.,39,40,启动子 增强子,逆转座子（Retroposon）,逆转录转座子指通过RNA为中介,反转录成DNA后进

15、行转座的转座因子。逆转座作用出现在真核生物。 （Transposable elements that utilize reverse transcriptase to transpose through an RNA intermediate are termed retrotransposons. ）,退化的逆病毒,42,逆转座子在真核生物中可分为两类： 病毒样逆转座子 (Viral retrotransposons): 与反转录病毒结构类似，不同之处在于不形成病毒颗粒。 如Yeast Ty elements, Drosophila copia elements 非病毒样逆转座子 (Nonv

16、iral retrotransposons ) : 不形成逆转录酶和整合酶，依靠外源提供的转座酶，真核生物基因组中所占比例最高。 LINEs (long interspersed elements) and SINEs in mammals,43,病毒与真核生物基因组中逆转座子结构比较,44,鼠白血病毒,酵母TY系列（Yeast Ty elements）,About 35 Ty1 copies in the yeast genome. sequences : 330 bp, about 100 copies. solo element.,45,Transposition through an

17、RNA intermediate,46,2.半乳糖诱导启动子,1,实验证明：,Drosophila copia elements,47,转座因子的频率,不同的转座因子的转座频率不同。 每个世代转座频率约为10-3 10-4 。 New germ-line transpositions are estimated to occur once in every 50 to 100 human births. 每个世代中转座插入靶位点使某个基因所引起的突变和自发突变概率相当，约为10-5 10-7 。,48,第三节 转座的遗传学效应 Genetic effect of transposition,4

18、9,Transposable elements as genetic parasites,Replication and spread of Tes may serve no purpose for the cell. The insertion of Tes into a gene will often destroy its function, with harmful consequences for the cell. The time and energy require to replicate large numbers of Tes are likely to place a

19、metabolic burden on the cell.,人类中的转座子,50,Repetitive elements found in the human gene (HGO) coding for homogentisate 1,2-dioxygenase(尿黑酸-1,2-氧化酶), the enzyme whose deficiency causes alkaptonuria（尿黑酸尿）.,The Alu sequences make up more than 10% of the human genome.,51,Transposable elements in grasses are responsible for genome size differences,其它生物中的转座因子,Green: transposons; Orange: genes,52,转座因子引起的遗传效应,基因突变（编码区） 染色体结构改变（非编码区）,Transposoble element can generate mutations in adjacent genes,About 50% of all spontaneous mutations in D