题干

小鼠杂交瘤细胞表达的单克隆抗体用于人体试验时易引起过敏反应,为了克服这个缺陷,可选择性扩增抗体的可变区基因(目的基因)后再重组表达。下列相关叙述正确的是 (  )

A:一定要根据目的基因编码产物的特性选择合适的受体细胞

B:用PCR 方法扩增目的基因时必须知道基因的全部序列

C:PCR 体系中一定要添加从受体细胞中提取的DNA 聚合酶

D:设计扩增目的基因的引物时不必考虑表达载体的序列

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A

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    I grew up in New Hampshire, a small town in South Canada, where in my father's words for the seasons were “Spring, Summer, Fairtime and Winter!”At that time, a week-long fair was held in the town every autumn. Thousands of people from other towns came to sell and buy things. It was the busiest time of the year.

    When “Fairtime” came, my grandma became the busiest person of the family. Grandma was a kind old lady. She was good at cooking. All her relatives liked the food she cooked. During “Fair time”, they would come to live in her house and have meals there. Grandma was always happy to look after them.

    Year after year, many people moved to big cities. There was no longer “Fairtime”. Grandma became very old and was gradually going blind. My parents and I moved to live with Grandma. We did our best to make her life comfortable. I was at high school then. What I often did at home was to help Grandma with the daily newspaper's crossword puzzle. However, she didn't look happy. She often sat in her room for hours, without saying a word.

    To attract(吸引) people to move back, the Town Hall decided to reopen the fair. One day, when I came back from school, I saw Grandma wearing her glasses, washing the dishes in the kitchen. With a big smile on her face, she looked much younger. She told me that her two nieces(侄女) would come. “They said the food I cooked was very delicious and they wanted to stay in my house again,” Grandma said happily. “They will stay here for one week and we can have a big party. That must be the busiest week I've had in years!”

    I suddenly realized that Grandma didn't want to be looked after. She wanted to be “useful” and helpful.

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