问题 问答题 简答题

大肠杆菌作为基因工程受体菌的优缺点是什么?

答案

参考答案:

优点:大肠杆菌培养方便、操作简单、成本低廉,基础生物学、分子遗传学等方面的背景知识清楚,对其基因表达调控的分子机理也比较了解,而且历经二十年的基因工程实践,大肠杆菌已发展成为一种安全的基因工程实验体系,有多种适用的寄主菌株和载体系列,大肠杆菌易于进行工业化批量生产。

缺点:在通常情况下,细菌的RNA聚合酶不能识别真核的启动子;许多真核基因的蛋白质产物需要经过翻译后的加工修饰,如正确的折叠和组装,而细菌中通常并没有这样的修饰机制,从而可能导致真核基因在大肠杆菌中的翻译产物无法产生有活性的蛋白;外源真核基因所表达的蛋白质分子往往能够被细菌的蛋白酶识别,并被当做“异已分子”降解掉。

阅读理解

阅读理解

     I had a summer camp with my classmates last year.

     Early in the morning, we gathered at the bus station. After saying goodbye to our parents, we got on the bus. ①It

took us more than two hours to arrive at the campground. 

     We got off the bus cheerfully, laughing and shouting; we jumped and ran all over the place. It was the first time for

us to be away from the parents. Some of us started to feel homesick(想家). However, when the night party and dances

began, the homesick was gone.

     The next day,②everybody rushed to the boating class, hoping to be at the head of the others. At first, my friends

and I worked hard but the boat wouldn't listen to us and kept going round and round. Then the teacher taught us how

to work together. After many tries, we did much better.

     Swimming class was my favorite. It was about the hottest time of a day and the best time to stay in the cool water.

The swimming teacher was a funny man, and during the class he often made us laugh happily.

     During the week I learned a lot of new things and made many new friends. I also learned how to take care of myself.

1. 将第①处改为带有spend的同义句 。

    ___________________________________

2. 将第②句翻译为汉语。

    ___________________________________

3. Why swimming class was the writer's favorite?

A. Because it was good to stay in the cool water when the weather was hot.

B. Because the swimming teacher was interesting.

C. Because the writer felt quite relaxed during the class.

D. All of the above.

4. Which statement is NOT true?

A. The children were happy to be away from Dad and Mum.

B. Their homesickness lasted for long.

C. Rowing boats is a kind of teamwork.

D. They camped for a week.

5. Which is the best title for the passage?

A. I Learnt a lot.

B. Boating and Swimming.

C. On the Campground.

D. My Summer Camp.

单项选择题 A2型题