问题 问答题 简答题

简述果蝇性别决定的分子机制?

答案

参考答案:

X:A(X是指性染色体X的条数,A是常染色体的套数)的比值是决定果蝇性别的第一步,当该比值高时,雌性化基因sxl基因在卵受精后2h激活,使胚胎启动朝向雌性发育的途径;若sxl基因失去功能,则胚胎出现雄性表型。但是,sxl基因转录激活不久,其第二个启动子激活,因此雄性胚胎也能产生SxlmRNA,此时雌性中已形成Sxl蛋白,可以将该mRNA剪接成雌性胚胎的SxlmRNA,但是雄性中转录是后来进行的,新合成的该mRNA被其它蛋白处理成雄性胚胎的SxlmRNA。此外,tra基因在雌雄胚胎中的转录物也不同,在两者中均有非专一的该蛋白,但雌性中还有雌性专一的TramRNA,该专一的蛋白与tra-2基因协同产生雌性表性,并且tra-2基因在雌性专一的Tra蛋白质存在的条件下,作用于dsx基因,抑制雄性性状发育。

阅读理解

I remember my maths teacher Mr Young very well. He stood out because the kids made fun of him. He was missing one of his fingers. In school Maths was my worst subject. I don’t think there was ever a day I went to school that I was not afraid.

One day, I was told that if I got one more E on my report card, my parents would be called to school.

I tried really hard for weeks to learn Maths. I just couldn’t understand how to make different parts of numbers into whole things; my brain just couldn’t do it, no matter how hard I tried.

The day before report cards were to come out, I knew that Mr Young would give me an E, just like he always did.

After class ended, I went to Mr Young and told him what would happen if I got another E. He told me there was nothing he could do; it would be unfair to the other kids if he gave me a better grade than I had actually earned.

I looked at the teacher and said, “Mr Young, you know how all the kids make fun of you because you’re missing your finger?”

He looked at me, and said nothing.

“They shouldn’t do that to you because you can’t help not having a finger, Mr Young. Just like I can’t help not being able to learn numbers and matters like that,” I said.

Again, he said nothing as he looked down at his desk, and began grading papers.

The next day, when I got my report card, I looked at my grades: Geography: B- , English: D-  , History: C- , Gym: B+ , Art: C, Maths: D- .

That Math grade was the most favorite one I ever received in my whole life. I knew that someone in the world finally understood what it was like for me to be missing a finger inside my head.

小题1:Where was Mr Young different from others?

A.He had only nine fingers in all.

B.He played jokes with students.

C.He usually gave higher mark to students.

D.He taught Maths in an interesting way.小题2:What would happen to the writer if he got another E?

A.He would be asked to leave school.

B.He might be punished by his parents.

C.He would not be allowed to play any more.

D.He would be laughed at by other students.小题3:The writer got a Math D- at last because            .

A.he achieved a higher level after working hard

B.Mr Young was afraid of being made fun of again

C.Mr Young knew the trouble of being poor in mind

D.Mr Young thanked him for telling him the opinions of students小题4:The story tells us that              .

A.students should respect(尊敬) the disabled teachers

B.teachers should treat all the students fairly

C.students need to be cared for in their study,

D.teachers should have an understanding of the students

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