题干

关于DNA分子的结构与复制的叙述中,正确的是(    )

①含有m个腺嘌呤的DNA分子复制n次需要游离的腺嘌呤脱氧核苷酸为(2n﹣1)×m个

②某DNA片段有300个碱基对,其中1条链上A+T比例为35%,则第3次复制该DNA片段时,需要780个胞嘧啶脱氧核苷酸

③细胞内全部DNA被32P标记后在不含32P的环境中进行连续有丝分裂,第2次分裂的每个子细胞染色体均有一半有标记

④DNA一条链被32P标记后,复制n次,子代DNA中有标记的占

A:①②

B:②③

C:③④

D:②④

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A

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    I always felt sorry for the people in wheelchairs. Some people, old and weak, cannot get around by themselves. Others seem perfectly healthy, dressed in business suits. But whenever I saw someone in a wheelchair, I only saw a disability,not a person.

    Then I fainted(昏倒) at Euro Disney due to low blood pressure. This was the first time I had ever fainted, and my parents said that I must rest for a while after first aid. I agreed to take it easy, but as I stepped toward the door, I saw my dad pushing a wheelchair in my direction! Feeling the colour burn my cheeks, I asked him to wheel that thing right back to where he found it.

    I could not believe this was happening to me. Wheelchairs were fine for other people but not for me. As my father wheeled me out into the main street, people immediately began to treat me differently.

    Little kids ran in front of me, forcing my father to stop the wheelchair suddenly. Bitterness set in as I was thrown back and forth.

    "Stupid kids! They have perfectly good legs. Why can't they watch where they are going?"I thought. People stared down at me,pity in their eyes. Then they would look away, maybe because they thought the sooner they forgot me the better.

    "I'm just like you!" I wanted to scream. "The only difference is that you've got legs,and I have wheels."

    People in wheelchairs are not stupid. They see every look and hear each word. Looking out at the faces, I finally understood: I was once just like them. I treated people in wheelchairs exactly the way they did not want to be treated. I realized it is some of us with two healthy legs who are truly disabled.