2022英语周报八年级外研25答案

24.(12分除标注外,每空1分)(1)③⑤病毒RNA宿主细胞核糖体(2分)(2)DNA的一条链核糖核苷酸识别密码子并转运相应的氨基酸(2分)(3)丙氨酸的密码子A-U(4)体细胞中某基因发生改变,生殖细胞中不一定出现该基因(合理即可,2分)【解析】(1)该病毒遗传信息的传递方式为RNA复制和翻译。病毒RNA聚合酶合成的模板是病毒RNA,合成场所是宿主细胞核糖体。(2)tRNA在细胞中的合成是以DNA的一条链为模板转录形成的,转录的原料是四种核糖核苷酸tRNA的作用是识别密码子并转运相应的氨基酸。(3)翻译时,tRNA上一端携带丙氨酸,另一端的三个碱基称为反密码子,与mRNA上丙氨酸的密码子配对。图二为翻译过程,与DNA复制过程相比,特有的碱基配对方式是A_U。(4)DNA分子中碱基对的缺失、增添或替换而引起基因结构的改变叫做基因突变,基因突变不一定会引起子代生物性状发生改变,原因有:①体细胞中某基因发生改变,生殖细胞中不一定出现该基因;②DNA上某个碱基对发生改变,它不一定位于基因的外显子部位;③若为父方细胞质内的DNA上某个碱基对发生改变,则受精后一般不会传给子代;④若该亲代DNA上某个碱基对发生改变产生的是一个隐性基因,并将该隐性基因传给子代,而子代为杂合子,则隐性性状不会表现出来;⑤根据密码子的简并性,不同的密码子可以决定相同的氨基酸;⑥性状表现是遗传因素和环境因素共同作用的结果,在某些环境条件下,改变了的基因可能并不会在性状上表现出来。

第三节One possible versionParagraph 1As the cashier put the DVD into a plastic bag, / walkeder to the younger woman. "Is that your mom? "I askedhalfway expecting her to tell me it was none of my businessBut she rolled her eyes and said, "Yeah. "I smiled toshow her I wasn't criticizing.Cherish her, "I said. Andn I answered her curious expression by saying, "Whenshes gone, it's the little moments like this that'llcomeack to you. " It was true. I missed my mother still andremembered the moments when I was so impatient as tomake her life miserableParagraph 2The elderly woman moved with her deliberate slownessback to her daughter. Together they made their way towardsthe exit of the video store. They stood there for a momentside bywaiting for the rushing holiday current and foin it. Then the daughter glanced over andmomentarily regarded her mother. And slowly, almostreluctantly, she placed her arm with apparently unaccus.tomed affection around her mother's shoulders and gentlguided her back into the deluge.

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