问题 单项选择题 B型题

成釉细胞最早分泌的釉基质蛋白是()

A.釉原蛋白

B.釉蛋白

C.鞘蛋白

D.内源性金属蛋白酶

E.丝蛋白酶

答案

参考答案:B

解析:釉基质主要由多种蛋白质和酶组成,其基本特征为:釉原蛋白是釉基质的主要成分,约占90%,是分子量25kD的疏水性蛋白;釉蛋白是成釉细胞最早分泌的釉基质蛋白,与釉原蛋白比较具有更强的酸性和亲水性;鞘蛋白是一组由同一条DNA编码的糖蛋白,主要均匀分布于新形成的釉基质中。釉基质中还存在一些蛋白水解酶,主要为内源性金属蛋白酶和丝蛋白酶,前者主要存在于釉质分泌期,后者主要存在于釉质发育期,可分解和清除釉质蛋白质,有利于釉质的矿化。

阅读理解

阅读理解。

      "It's over! Thank goodness!"

      School was over and I was tired. I sat at the front of the school bus.

      Janie, the driver, always tries to break the uncomfortable atmosphere (气氛) by talking. I try to listen

politely, but usually I'm too busy thinking about my day. On this day, however, her talk was worth listening

to.

      "My father's ill," she said to no one in particular (特别地). I could see worry in her eyes. I had never

seen her like this before. She always meets students with a smile.

      With a sudden chance of interest, I asked, "What's wrong with him?"

      With her eyes wet and her voice unusual, she answered, "Heart trouble." Her eyes lowered as she

continued, "I've already lost my mum, so I don't think I can stand losing him." I couldn't answer. My heart

ached for her.

      I sat on the seat thinking of the great pain my own mother was thrown into when her father died.

      I saw how hard it was, and still is, for her. I wouldn't want anyone to go through that.

      Suddenly I realized Janie wasn't only a bus driver, which was just her job. She had a whole world of

family and cares too.

      I shouldn't have been so selfish. I paid no attention to Janie because she was a bus driver. I had judged

her by her job and brushed her off as unimportant.

      I shouldn't have been so selfish and self-centered.

      Understanding people is an art.

1. When the students get on the school bus, Janie usually _____. [ ]

A. listens to music

B. talks about her own worry

C. sits on her seat without words

D. meets them with a smile

2. After she learned Janie's story, the writer thought of _____. [ ]

A. her father's death

B. her mother's pain

C. the ache of her own heart

D. the pain of Janie's parents

3. The writer felt herself selfish because she had _____. [ ]

A. thought of Janie as nothing but a driver

B. made only a few friends in the school

C. hardly thought of herself

D. never listened to others

4. In this passage the writer tries to tell us that _____. [ ]

A. losing parents makes people sad and helpless

B. understanding the people around us takes time

C. we should learn to understand the people around us

D. it's not right to judge the people around us by their clothes

单项选择题