问题 问答题 简答题

试述紫外线长期照射个体后,对个体DNA的损伤和修复?

答案

参考答案:

(1)光修复:当嘧啶二聚体形成后,光复活酶先与DNA链上的胸腺嘧啶二聚体结合形成复合物,这种复合物以某种方式吸收可见光,并利用光能切断胸腺嘧啶二聚体之间的C-C键,使其变为单体,然后光复活酶就从DNA链上解离下来。

(2)切除修复:切除修复是一种多步骤的酶反映,包括切-补-切-封四个步骤。首先修复内切酶能够识别胸腺嘧啶二聚体所引起的DNA双螺旋结构的变形,并在二聚体的前头的糖—磷酸的骨架上做一切口,然后由DNA聚合酶I在3’-OH末端聚合形成一条新的DNA链,并在同时置换出大约20个核苷酸的DNA片段,被置换出来的片段由DNA聚合酶I的5’到3’的外切核酸酶活性切除,然后由连接酶封口,完成修复过程

(3)重组修复:当DNA复制到胸腺嘧啶二聚体时,大约暂停5秒,然后在二聚体后面又以一种未知的机制其始DNA的复制,再合成的子链上有许多大的缺口,这时受伤的链的互补链的相应片段转移至缺口处,产生一条完整的DNA链,以便作为下一轮复制的模板。受损伤链上的缺口由DNA聚合酶I和连接酶补齐

(4)sos修复:是一种旁路的修复系统。其修复原则是允许新生DNA链越过嘧啶二聚体而生长,其代价是不管其碱基配对是否正确。SOS修复系统是一种应急措施,当DNA受到严重损伤时才出现。

填空题
填空题

When I was growing up, I was embarrassed to be seen with my father. He (1) severely crippled and very short, and when we would walk together, his hand on my arm (2) balance, people would stare. I would inwardly be afraid (3) squirm at the unwanted attention. If he ever noticed or was bothered, he never let (4) .
It was difficult to coordinate (5) steps - his halting, mine impatient - and because of that, we didn’t say much as we went along. But as we started out, he always said, "You (6) the pace. I will try to adjust to you. "
Our usual walk was to or (7) the subway, which was how he got to (8) . He went to work sick, and (9) nasty weather. He almost never missed a day, and would (10) it to the office even if (11) could not: a matter of pride!
He never talked about himself as an object of pity, (12) did he show any envy of the more fortunate or able. What he looked for in others was a "good heart", and if he found one, the owner was good enough for him.
Now that I am older, I believe that is a proper standard (13) which to judge people, even though I still don’t know precisely (14) a "good heart" is. But I know the times I don’t have one myself.
He has (15) gone many years now, but I think (16) him often. I wonder if he sensed my reluctance to be seen with him during our (17) . If he did, I am (18) I never told him how sorry I was, how unworthy I was, how I regretted it. I think of him when I complain about trifles, when I am (19) of another’s good fortune, when I don’t have a "good heart".
At such times I put my hand on his arm to regain my (20) , and say, "You set the pace, I will try to adjust to you.\