八年级下册英语周报第19期，2020~2021。答案

One possible versionSeveral minutes later, the ice broke suddenly and Harry's feet sank into the water. It was noep, but his legs got wet to the knees. Harry was so upset that the accident would delay hisschedule. He had to find some firewood to build a fire to dry his clothes. He pulled off his glovestook out his matches, and lighted the fire. Seeing the burning fire, he had thought he was safe. Butsuddenly, without warming, a heavy mass of snow dropped down. The flame of hope was put outUnluckily, his dog ran away and the fear of death came over Harry. He had no choice but torun to save himself. However, he did not feel his feet when they hit the ground. Having falleneveral times, he decided to rest a while. As he lay in the snow, he noticed he was not shaking. Hecould not feel his nose or fingers or feet. A feeling of horror came into his mind. It was when herealized he was going to die that he heard a bark and the noise his friends made

1.D【解析】基因am和tet在同一个DNA分子上,不是一对等位基因,A项错误;质粒指细菌细胞中或酵母菌细胞中能自我复制的小型环状的DNA,动、植物病毒的DNA不属于质粒,B项错误;限制酶的作用部位是DNA分子中特定的两个核苷酸之间的磷酸二酯键,C项错误;重组质粒4中,氨苄青霉素抗性基因完好,四环素抗性基因被破坏,因此将重组质粒4导人大肠杆菌,该菌不能在含四环素的培养基上生长,D项正确。

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