题干

蚕豆根尖细胞在含3H标记 的胞嘧啶脱氧核糖核苷酸培养基中完成一个细胞周期,然后在不含放射性标记的培养基中继续分裂至中期,其染色体的放射性标记分布情况是(   )

A:每条染色体的两条染色单体都被标记

B:每条染色体中都只有一个单体被标记

C:只有半数的染色体中有一条染色单体被标记

D:每条染色体的两个单体都不被标记

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答案(点此获取答案解析)

B

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    It was a cold winter. The wind blew all night and the snow was blinding. When morning came, my three children and I got up and made our way to the windows. As we looked out the window, we saw that the henhouse was gone. Our three hens had been blown away.

    I looked at the emptiness outside. Then I saw all three chickens sat around the edge of a white bucket. How was this violent wind not blowing them into the field beyond? I quickly pulled on long snow pants and heavy winter coat, wrapped a scarf and stuck my feet into very large boots .

    I shouted at the wind as it blew. I was alone, save for my children. They stared out the window into the vast white sea of snow, their eyes peeled for any sign of movement. Outside I heard the sound of my boots as I walked against the wind.

    The snow circling around me, I steadily made my way to the soft cluck-cluck-cluck sound my hens always made. When I reached them, I saw that their little feet were holding on to the edge of the bucket, heads bent forward and away from the wind. I gently lifted each hen and put it carefully into the warm inside. Then I began the freezing walk back to the small shed directly behind our house. One by one I laid my chickens on the cold floor, and they began to cluck softly.

    As I shut the shed doors, my eyes went directly to the window where my children were watching. They jumped up and down cheering, and so did I! I wasn't some dragon slayer (屠龙者) from a fairy tale. I was simply a mom, but the look on my children's faces told me that they thought I was a hero mom.