分子生物学-第五讲DNA的损伤、修复和DNA转座.pptVIP

* 科学家简介9 * A composite transposon has a central region carrying markers (such as drug resistance) flanked by IS modules. The modules have short inverted terminal repeats. If the modules themselves are in inverted orientation (as drawn), the short inverted terminal repeats at the ends of the transposon are identical. * Figure 15.13 Mu transposition generates a crossover structure, which is converted by replication into a cointegrate. * Nonreplicative transposition results when a crossover structure is released by nicking. This inserts the transposon into the target DNA, flanked by the direct repeats of the target, and the donor is left with a double-strand break * Conservative transposition involves direct movement with no loss of nucleotide bonds; compare with lambda integration and excision. * Several mechanisms restrain the frequency of Tn10 transposition, by affecting either the synthesis or function of transposase protein. Transposition of an individual transposon is restricted by methylation to occur only after replication. In multicopy situations, cis-preference restricts the choice of target, and OUT/IN RNA pairing inhibits synthesis of transposase. 图18.18：许多机制通过影响抑制Tn10转座，转座酶蛋白的功能或化合，单独转座子的转座被半甲基化限制发生，在复制后条件下，优先cis限制目标的选择，OUT/IN　ＲＮＡ对抑制转座酶化合。 * 不依赖供体序列与靶位点间序列的同源性 (非同源重组过程 ，不依赖 recA 酶) Regional preference ( 在3kb区域内的随机插入)---------------有的转座元倾向于优先插入某些染色体上的某些大的区段，这些区段可长达3Kb左右 第三节 细菌中的转座成分 一、转座成分概述 1、转座子(元)或转座元件(transposon or transposable element): 基因组上不必借助于宿主基因就可以自主复制和移动的DNA片段，它们可以直接从基因组的一个位点移到另一个位点（供体和受体） 转座（transposition）：转座元的转移过程（不十分确切） 2、发现和发展 ? 1914 A. Emerson 1936 Marcus . M. Rhoabes 玉米果皮、糊粉层花斑突变 1983. Barbara McClintock (86y) DNA transposable element ? 1947 冷泉港实验室（美） Barbara McClintock 玉米籽粒糊粉层色素不稳定遗传机理跳跃基因（jumping gene） ? 基因转座现象的再次发现与证实 二、Prok.转座子种类 ? 两种类型： 简单转座子（simple transposon） （插入序列 insertion sequence IS ） 复合转座子（composite transposon） ? 共同特征： a）两端有20～40bp的IR