问题 问答题 简答题

自从发现RNA拟酶(ribozyme)以来,许多人相信最初的遗传物质是RNA,而不是DNA。然而现代一切细胞都以DNA,而不是以RNA为遗传物质。你对此作何解释?

答案

参考答案:

所谓RNA拟酶是指起催化作用的RNA,它具有酶的主要特征:专一性强、加快反应速度、反应前后酶分子保持不变。根据从遗传信息到蛋白质合成的过程中RNA多方面作用的分析,RNA很可能是生物体系中最早出现的大分子种类,RNA的催化功能必然出现在DNA成为遗传信息载体之前,即在原始生命中,遗传信息载体是RNA而不是DNA,只不过由于进化的结果,DNA的稳定性和空间结构方面的优势取代RNA成为主要的遗传物质,而蛋白质则以其侧链的多样性和灵活性取代RNA成为主要催化剂。

阅读理解

阅读理解

    In 1969 I was a young girl working in Toronto. I was from a small town in Ontario and had just

moved to the big city about three months earlier. It was one week before Christmas and I was going

home on the last streetcar of the evening.

   I remember looking at my paycheque(工资单) and asking myself how it was going to endure. At

that time in my life I hardly had enough money to exist on. Bread for supper and oatmeal for breakfast

was the only way I kept hunger at bay.

   I got off the streetcar and started walking up the street, when I suddenly noticed a man was running

after me. I stopped, turned around, seeing that he was black. I snapped, "What do you want?" The man

answered in a soft voice, "I am returning your wallet. You left it on the streetcar seat."

   Because of that man's thoughtfulness, I was allowed the comfort of going home by bus for the summer

holidays. Otherwise, I would have stayed alone in my small room with a hog plate.

   I have never been so ashamed of myself, and that poor, kind man had to walk, who knows how far,

home.

   I recognized immediately that this was wrong of me and it changed my ways forever.

1. What can we learn from the second paragraph?   

A. The writer had no job when the story happened. 

B. The writer were leaving the big city for her hometown. 

C. The writer had little money when the story happened. 

D. The writer's parents were very poor when she was young.

2. What does the underlined word "snapped" most probably mean?

A. To speak in a soft voice.

B. To shout angrily. 

C. To scream for help.

D. To whisper.

3. Why was the black man running after the writer?

A. To rob her.

B. To scare her. 

C. To make friends with her.

D. To return her wallet.

4. What is the writer trying to tell her readers? 

A. Never judge people by their appearance. 

B. She had been struggling to her success. 

C. She had changed a lot since that night. 

D. Being kind to strangers will pay back.

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