DNA中的点突变（一个碱基被另一个碱基取代）可能导致一个氨基酸_爱下问搜题

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问题问答题简答题

DNA中的点突变（一个碱基被另一个碱基取代）可能导致一个氨基酸被另一个氨基酸替换。但在某些情况下，由于密码子的简并性，基因编码的氨基酸序列也可能不会改变。一种细菌生产的胞外蛋白酶在其活性位点上（―Gly―Leu―Cys―Arg―）有一个半胱氨酸残基。紫外线照射过后，分离到两个突变菌株。菌株1生产以Ser取代了活性部位Cys的无活性酶（―Gly―Leu―Ser―Arg―））；而在菌株2内，合成了一条C－末端结束在活性部位内的以―Gly―Leu―COO结尾的截断了的肽链，指出在每一种菌株中可能发生的突变。

答案

参考答案：

Cys密码子（UGU，UGC）与几个Ser密码子（AGU、AGC或UCUUCC）只有一个碱基的差别。单独的一个点突变能够将Cys转变为Ser，例如UGU（Cys）→AGU（Ser）或UGC→AGC，基因中的突变可以是一个A/T碱基对取代T/A碱基对或者是C/G取代G/C。第二个蛋白质被提前终止，因而该突变的基因在Leu密码子之后含有一个终止密码子，假设只有一个核苷酸发生了改变，有下述三种可能性。Ser→STOPTCA→TGATCA→TAATCG→TAG

完形填空

Everyone in the village knew who Ugly was. Ugly was a_1_with many injuries. His left foot appeared to have been badly broken .Ugly only loved two things in this world: eating garbage and ,shall we say ,__2 __.However, every time someone saw Ugly there was the__3__reaction. All the children were warned not to___4_him, and the adult threw rocks at him when he tried to follow them. Whenever he found children, he would come running and putting his head against their hands. If you ever _5___him up he would immediately begin suckling（允吸）.on your shirt, fingers, whatever he could find . One day, Ugly shared his love with the neighbour’s___6_dog . They did not respond __7__,and Ugly was badly treated . Even from the end of the__8_I could hear his screams, and I tired to rush to his aid. By the time I got to where he was __9__,it was clear Ugly’s life was almost at an end. It must be_10__him terribly, I thought . Then I felt a familiar sucking feeling on my _11__. Ugly, in so much ___12_, was trying to suckle my ear. I pulled him closer to me , and he_13___suckled my fingers and watched me .Even in the greatest pain, that ugly cat was asking only for a little love ,perhaps some ___14_.At that moment I thought Ugly was the most 15____and loving creature I had ever seen .Never once did he bite me, or even try to get away from me. He just _16___me completely trusting in me to relieve his pain. Ugly died in my __17__before I could get inside. He taught me more about __18__than a thousand books, lectures, or talk show ever could . He had been __19__on the outside ,__20 I was on the inside .To give my pity and love to those I cared for.

A．interesting

小题11:A face B. hand C. ear D. finger
小题12:A pain B. excitement C. happiness D. worry

C．looked up at

单项选择题

王某系某学院某课程负责人，为完成学校下达的工作任务而组织教员编写教材一部，王某任主编。下列说法正确的是( )。

A．该教材的著作权由全体作者享有

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