问题 单项选择题

噬菌体侵染大肠杆菌的实验不能说明的是:()

A.DNA分子能自我复制

B.DNA分子是遗传物质

C.DNA分子能控制蛋白质的合成

D.DNA分子能产生可遗传的变异

答案

参考答案:D

解析:噬菌体在侵染大肠杆菌的过程中,只有DNA注入到大肠杆菌内,而蛋白质外壳留在外面。此时在噬菌体自身DNA的指导下,复制形成多的DNA,并且控制合成多个蛋白质外壳,并且组装形成多个子代噬菌体。由此可见,DNA分子能自我复制并能控制蛋白质的合成,证明了DNA分子是遗传物质,而该实验中没有发生变异,因此不能证明DNA分子能否产生可遗传的变异,故本题选D。

填空题
阅读理解

阅读理解。

                    Find Which Direction Is South

     Do you have a good sense of direction? If not, please take

with you a compass. But if you forget to take a compass, you

can still find your way.

     It's never a good idea to imagine that the family member

who was entrusted (委托) with the job of map-reading actually

knows where the family is. You can tell by the slightly

confused load on their faces that nothing on the ground seems

to match the map. Never mind. The sun is shining and it's still

morning.If you don't know the exact time, you can still find

out where south is, but you'll need to be patient.

    ①Find a straight stick and put it in the ground in a place

where you can mark its shadow.

    ②Try to position the stick as vertically(垂直)as you can.

You can check this by making a simple plumb line (铅锤线)

with a piece of string and weight. You haven't got any string?

OK, use a thread from your clothes with a button tied at the

end to act as a weight.

    ③Mark the end of the shadow cast by the stick.

    ④Wait approximately half an hour and mark the end of

the shadow again.

    ⑤Keep doing this until you have made several marks.

    ⑥The mark nearest the stick will represent the shortest

shadow, which is cast at midday, when the sun is highest in

the sky and pointing to the exact south.

    ⑦Pick a point in the distance along the line between the

shortest shadow and the stick.

    ⑧That point is south of where you are.

    ⑨Now you can turn the map, like you did before, and

find which way you should be travelling.

1. To find the direction, we ought to be patient probablybecause ____ .     

A. it is not easy to find a proper stick

B. it is not easy to position the stick

C. it takes hours to make the marks

D. it takes about half an hour to make the marks

2. The passage would probably be most helpful to ____. 

A. those who draw maps

B. those who get lost

C. those who make compasses

D. those who do experiments

3. Which of the following pictures best shows the way of finding the direction of south?

4. The author presents this passage by _____.

A. telling an interesting story

B. describing an activity in a lively way

C. testing an idea by reasoning

D. introducing a practical method