题目

把培养在含轻氮（14N）环境中的细菌，转移到含重氮（15N）环境中培养相当于复制一次的时间，然后放回原环境相当于连续复制两次的时间后，细菌DNA组成分析表明 A．3/4轻氮型、1/4中间型 B．1/4轻氮型、3/4中间型 C．1/2中间型、1/2重氮型 D．1/2轻氮型、1/2中间型 答案：A 解析: 将14N的细菌放于15N环境中复制一次，每个模板DNA可产生2个子DNA分子均为14N、15N的DNA即中间型DNA，而这两个中间型DNA放回原环境（14N）中再复制两次后各产生4个DNA，共8个子DNA，这8个子DNA中有2个含15N，其余均为“只含14N”，即中间型DNA应为2/8，轻氮型DNA应为6/8。阅读下面短文，用括号内所给单词的正确形式填空。A husband and wife were driving down a country road on their way to visit some friends. As they came to a muddy (泥泞的) part of the road, their car【1】(break) down.They tried to get the car out by themselves, but it didn’t work. Then they saw a farmer【2】 (come) down the road. He was driving some oxen (牛). He stopped when he saw the couple was in trouble. He said he would pull the car out of the mud for $20 (130 yuan). They agreed and minutes 【3】(late) the car was free.The farmer turned to the husband and said, “You know, you’re the【4】(ten) car I’ve helped out of the mud today.”The husband looked around at the large land and asked the farmer, “When do you have time to plow (犁) all your【5】(field) ?At night?”“No,” the farmer answered, “I put the water in the hole at night!”