问题 单项选择题 B型题

磺胺类的作用机制是()

A.影响二氢叶酸合成酶

B.影响二氢叶酸还原酶

C.与核蛋白体30s亚基结合,阻止氨基酰tR-NA进入30s亚基A位,抑制蛋白质的合成

D.与核蛋白体50s亚基结合,使肽链的形成和延伸受阻,抑制蛋白质的合成

E.与青霉素结合蛋白(PBPs)结合,阻碍粘肽合成,导致细菌细胞壁的缺损

答案

参考答案:A

解析:四环素为广谱抑菌性抗生素,作用机制主要为与细菌核糖体30s亚单位在A位上特异性结合,阻止氨基酰tRNA在该位置上的联结,从而抑制肽链的延长和细菌蛋白质合成,此外,四环素尚可使细菌胞膜通透性发生改变,使胞内核苷酸和其他重要成分外漏,而抑制DNA复制;大环内酯类抗生素能与细菌核糖体50s亚单位的23s核糖体RNA(23srRNA)的特殊靶位及某种核糖体的蛋白质结合;甲氧苄啶为其他合成类抗菌药,抑制二氢叶酸还原酶而产生作用,其抗菌谱与磺胺甲恶唑相似;磺胺类药则抑制二氢叶酸合成酶,从而阻断叶酸代谢,抑制细菌核酸蛋白合成。

阅读理解

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Last month ,studenrs from 103 universities in 88 coontries took part in international computer programming contest. The Battle of the Brain took place in Harbin, China. Three---person terms from each school had hours to solve eleven real world problems.

Jerry Cain coached the team from Standford University in Palo Alto,California. He says the problems involved, among other things, paperweights, robots, castles and lakes.

JERRY CAIN: “One of the programming problems was trying to figure out how to break an chocolate bar into a certain number of pieces of a certain number of sizes and to do it as quickly as possible. And that’s probably the simplest of all them.”

The students first listed the problems in order of difficulty and then they wrote the needed software systems. They designed ways to test their solutions. And they wrote the needed software systems , Even the winning team from Shanghai Jiaotong University in China was not able to solved all the problems within the given time limit. Stanford’s team solved five problems and finished in fourteenth place. Standford was one of twenty---one America universities that took part in the conest this year. The conest began in 1970 at Texa A and M University. The first final competition was held in 1977 at the Association for Computer Machinery Computer Science Conference. It developed and grew as more and more schools took part in local and area contests.

Contest spokenman Doug Heintzman says the world champions receive prizes and scholarships. They are also guaranteed an offer of employment or practice period with IBM. “We’re had past world champions that IBM has given a post to in our Zurich research Iaboratory and they are now working on some of the Ieading edge materials in science and physics. So this competition is an opportunity to be recognized and to be recruited by some of the top  technology and firms around the world “

68. It can be inferred that__________.

A .the contest is increasingly unpopular with the universities

B..the contest this year was not easy for the participants

C. most of the American universities took part in this contest

D. whether to win or not has nothing to do with the participants’ future

69. Which of the following is true about contest?

A. Every team consisted of five members.

B. The participants from Shanghai Jiaotong University in China performed best.

C. Except American, there are sixty---seven countries taking part.

D. Finally Stanford’s team came fourth in the contest

70. The underline word in the text can be best replaced by_________.

A. promoted    B. fired   C. given a salary increase  D.employ

71. The purpose of this passage is to__________.

A.provide some information on how to enter for the contest

B.call on university students to take part in the contest

C.introduce the international computer programming contest. held this year

D.inform the readers of the result of the contest

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