问题 选择题

多聚酶链式反应(PCR)是一种体外迅速扩增DNA片段的技术。PCR过程一般经历下述三十多次循环:95℃下使模板DNA变性.解链→55℃下复性(引物与DNA模板链结合)→72℃下引物链延伸(形成新的脱氧核苷酸链)。下列有关PCR过程的叙述中不正确的是:  [ ]

A.变性过程中破坏了DNA分子内碱基对间的氢键,也可利用解旋酶实现

B.复性过程中引物与DNA模板链的结合是依靠碱基互补配对原则完成

C.延伸过程中需要DNA聚合酶、ATP、四种核糖核苷酸

D.PCR与细胞内DNA复制相比所需要酶的最适温度较高

答案

答案:C

阅读理解

阅读理解。

     Trees are useful to man in three important ways.

     The first important way is that they provide (提供) man with wood and other products. Trees provide

not only man with food, but also many animals with food .Without trees, many animals could not live on

earth. It's not easy for man to live on earth ,either.

     The second important way is that trees give us shade (阴凉).On a hot summer day, people are

eager(渴望) to have a rest under the tree after they have a long walk .You can imagine how important the

shade of a tree is to man and animals.

     The third important way is that trees help to prevent droughts and floods. However, in many parts of

the world, man hasn't realized the third way. They cut trees down in large numbers. In the end they find

that they have lost the best friends they had.

根据短文内容,选择正确答案。

1. How many important ways are trees useful to man?

A. Two ways  

B. Three ways  

C. Four ways  

D. Five ways

2. Trees only provide man with food ,don't they?

A. Yes they are  

B. No, they don't  

C. Yes, they do  

D. No, they aren't

3. On a hot summer, what do trees give us?

A. They give us food on a hot summer day.

B. They dive us wood on a hot summer day.

C. They give us shade on a hot summer day.

D. They  give us leaves on a hot summer day.

4. What does the underlined word "they" means?

A. The men who cut down trees in many parts of the world.

B. The humans  

C. the men who enjoy the shade  

D. The men who enjoy the food

5. What's the best title for the passage?

A. What is a Tree                

B. How to plant trees  

C. The Importance (重要性) of Trees  

D. How to protect Trees

填空题