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    【题目】用32P标记玉米体细胞（含20条染色体）的DNA分子双链，再将这些细胞转入不含32P的培养基中培养，让其分裂n次，若一个细胞中的染色体总条数和被32P标记的染色体条数分别是40条和2条，则该细胞至少是处于第几次分裂的分裂期

    A．第一次 B．第二次 C．第三次 D．第四次

    【答案】C

    【解析】有丝分裂后期染色体数目加倍，所以该细胞处于分裂后期。根据DNA的半保留复制，在第一次有丝分裂后期，被32P标记的染色体条数为40条。第二次有丝分裂后期，被32P标记的染色体条数为20条，未被标记的染色体也有20条。第二次有丝分裂后，20条带标记的染色体随机进入到2个子细胞中，所以第三次有丝分裂后期，被32P标记的染色体条数在0到20条之间，故选C。

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