问题 问答题 简答题

什么是突变?突变方式有几类?

答案

参考答案:

从分子水平上看,突变就是DNA分子上发生碱基改变,突变的原因可能来自遗传过程中复制的自发性改变,称为自发突变。也可能通过一些物理或化学手段的处理使DNA发生突变,这种人工手段引起的DNA突变,称为诱变。根据DNA分子的改变,可将突变分为4种方式。

(1)点突变:同型碱基的置换,如嘌呤代替另一嘌呤,叫转换;异型碱基的置换,如嘌呤变嘧啶或嘧啶变嘌呤,则叫颠换。

(2)缺失:一个碱基或一段核苷酸链从DNA大分子上消失。

(3)插入:一个原来没有的碱基或一段原来没有的核苷酸插入到DNA大分子中间。缺失和插入可引起移码突变,影响三联体密码的阅读方式。

(4)倒位:DNA链内部重组,使其呈段的方向反置,如从5’→3’整段颠倒为3’→5’,或大片段的链在DNA分子内迁移。

阅读理解

     One stormy night many years ago, an elderly man and his wife entered the hall of a small hotel in

Philadelphia. Trying to get out of the rain, they came the front desk hoping to get some shelter for the

night.

     "Could you possibly give us a room here?" the husband asked.

     The clerk, a friendly man with a winning smile, looked at the couple and explained that there were

three conventions in town. "All of our rooms are taken," the clerk said. "But I can't send a nice couple

like you out into the rain at one o'clock in the morning. Would you perhaps be willing to sleep in my

room? It's not exactly a suite, but it will be good enough to make you folks comfortable for the night."

     When the couple declined, the young man pressed on. "Don't worry about me; I'll make out just fine."

the clerk told them.So the couple agreed.

     As he paid his bill the next morning, the elderly man said to the clerk, "You are the kind of manager

who should be the boss of the best hotel in the United States. Maybe someday I'll build one for you."

The clerk looked at them and smiled. The three of them had a good laugh. As they drove away, the

elderly couple agreed that the helpful clerk was indeed exceptional, as finding people who are both

friendly and helpful isn't easy.

     Two years passed. The clerk had almost forgotten the incident when he received a letter from the old

man. It recalled that stormy night and enclosed a roundtrip ticket to New York, asking the young man to

pay them a visit.

     The old man met him in New York, and led him to the corner of Fifth Avenue and 34th street. He

then pointed to a great new building there, a pale reddish stone, with turrets and watchtowers thrusting

up to the sky. "That," said the older man, "is the hotel I have just built for you to manage." "You must be

joking, " the young man said. "I can assure you I am not," said the older man, a sly smile playing around

his mouth.

     The older man's name was William Waldorf Astor, and that magnificent structure was the original

WaldorfAstoria Hotel. The young clerk who became its first manager was George C. Boldt. This young

clerk never foresaw the turn of events that would lead him to become the manager of one of the world's

most glamorous hotels.

1. The purpose of the following author writing this story is to________.

A. give people a good laugh

B. cover some facts

C. promote the business of WaldorfAstoria Hotel

D. deliver a lesson

2. Which of the following statements about the story is NOT true ?

A. The story took place at about one a. m..

B. The old couple was too poor to afford a luxurious room.

C. The clerk was willing to help those in need.

D. The clerk received an unexpected invitation from the old man.

3. The underlined word "conventions" in Para 3.can be replaced by________.

A. rooms    

B. suites

C. meetings    

D. hotels

4. Which of the following proverbs suits the story the best?

A. Every little thing helps.

B. Make hay while the sun shines.

C. Man proposes; God disposes (处理, 决定).

D. One good turn deserves another.

单项选择题