问题 问答题

试述你了解的信号肽假说。

答案

参考答案:(1)Salatini D.和Blobel G.提出信号肽概念,分泌蛋白和一些膜蛋白在信号肽的引导下进入内质网,分泌蛋白最终被分泌到胞外,线粒体蛋白和细胞核糖也在导肽和核定位序列指导下到达各自部位。(2)信号肽序列通常在多肽链的氨基端,一般由10~40个氨基酸残基组成,氨基端至少有一个带正电荷的氨基酸残基,在中部有一段长度为10~15个高度疏水性的氨基酸残基组成的肽段,常见的为Ala、Leu、Val、Ile和Phe,该疏水区极为重要,可引导蛋白质通过膜性结构。在信号肽C端有一个可被信号肽酶识别的位点,此位点上游常有一段疏水性较强的五肽。信号肽位置也不一定在新生肽链的N端,有些蛋白质如卵清蛋白的信号肽位于多肽链的中部,但其功能相同。(3)识别信号肽的是一种核糖核蛋白复合物,称为信号肽识别颗粒(signal recognition particle,SRP),SRP由一分子小7SL rRNA和6种蛋白质组成,它有两个功能域,一个用以识别信号肽,另一个用以干扰进入的氨酰-tRNA和转肽酶的反应,以终止多肽链的延伸作用。信号肽与SRP的结合发生在蛋白质刚一开始时,即N端的新生肽链刚一出现时,一旦SRP与带有新生肽链的核糖体相结合,肽链的延伸作用暂时终止,或延伸速度大大减低。SRP-核糖体复合体移动到内质网上并与SRP受体停泊蛋白相结合。一旦与受体结合,蛋白质合成的延伸作用又重新开始。

阅读理解

My topic today is the Internet. It was prompted(引出)by the first letter, which I am unable to answer fully for the reasons given!

Dear David,

I’m a middle school student. Now I am preparing for a writing competition. Could you please check the mistakes in my article about using the Internet and

Give me some advice?                           No name                                                                               David writes: I’m very sorry to say that I think it would be very unfair for me to help you win a competition--- unfair to the other competitors!

But I’ll say that your article is very interesting and I wish you good luck.

Dear David,

I’m now learning English by myself. Can you introduce some useful websites where I can make friends with foreigners? Please help me.

Moonlight

David writes: Well, dear moonlight, I think you made a good choice by using the Net in your self-studies. Two very good websites I’d suggest to you are:

www.britishcouncil.org.cn and www.britishcouncil.org.cn/learn.

By the way, for those readers interested in the teaching of English, there is an Internet discussion group called ELTCSChi-L (English Language Teaching Contacts Scheme China List).

And also, a healthy warning to all my readers: If over-used, the Internet can be a WOMBAT—A Waste of Money, Brains and Time!

小题1:David is probably a ________.

A.language expert

B.computer expert

C.college student

D.club chairman小题2: What is the reason why the first letter is unable to be fully answered?

A.David is unable to think out good advice.

B.David thinks it is expected to play fair in the competition.

C.David is going to talk on another topic-Internet.

D.David finds no mistakes in that article.小题3:“www.britishcouncil.org.cn” is a (an)________.

A.English newspaper for students and language teachers.

B.website where one can join a discussion group.

C.newspaper column for answering readers’ questions .

D.website where one can make foreign friends.

名词解释