英汉对照分子生物学导论 教学课件 ppt 作者 [美]西尔维恩 W.勒潘 王勇 编著Chapter 9.ppt

In the simplest cases, damage to a base can be reversed without having to remove the base. Pyrimidine dimers are one kind of lesion that can be directly repaired. The enzyme photolyase is used to break the covalent bonds between the pyrimidines, restoring the bases to their original states. Another example of direct reversal is the repair of a harmful alkylation to guanine. In this case, methyltransferase transfers the mutagenic alkyl (methyl) group onto one of its own amino acids. Methyltransferase is only good for one repair, and then is discarded by the cell. 在最简单的情形下，一个碱基的损伤可以在不一定要移去这一碱基的情况下得到逆转。嘧啶二聚体就是这样一种能够被直接修复的损伤。光解酶用来断开嘧啶二聚体之间的共价键，从而将这两个碱基恢复到它们原来的状态。另一个直接逆转的例子是对鸟嘌呤烷基化损伤的修复。在这一例子中，甲基转移酶将诱变性的烷基（甲基）转移到它自身的氨基酸上。一个甲基转移酶只能用于一次修复，用过之后细胞就将它丢弃了。 The two main errors that occur during DNA replication, mismatched bases and strand slippage, are both corrected by a mechanism called mismatch repair (MMR). This process exists in both prokaryotes and eukaryotes. We discuss the method in E. coli here because it is simpler and better understood. In a mismatched base pair, one base is correct and one base is abnormal. The incorrect base is almost always in the newly synthesized strand, as the composition of the template strand does not change during replication. MMR therefore specifically targets the mismatched base in the daughter strand for removal. GATC sequences within the E. coli genome provide a convenient way to distinguish parent strand from daughter strand. GATC sites are normally methylated on the adenine. However, for several minutes after synthesis of a new strand, the sequence remains unmethylated. MMR acts within this time frame, and is able to recognize the newly synthesized strand as the one with an unmethylated GATC sequence. 错配的碱基和链滑动这两种主要的错误都是由错配修复（MMR）这一机理来矫正的。这一过程在原核生物和真核生物中都存在。在此我们讨论大肠杆菌中的方法，因为它更简单一点、对它的了解也更多一点。在一对错配的碱基中一个碱基是正确的而另一个碱基是不正常的。不正确的碱基几乎总是在新合成的链上，因为模板链在复制中不发生变化。MMR因此特异性地将目标放在子链中错配的碱基上。基因组中的GATC序列提供了一种区别母链和子链的便利途径。GATC位点中的A一般都是会被甲基