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（翻译）载体是一个复制子，其基因可保留在寄主细胞内，载体包括质粒和寄主于细菌的噬菌体。质粒载体应含有一些单一的普通限制性内切酶的切点，还要有一段抗生素抗性的决定子，以便于选择转化子。切断载体所用的酶与切断染色体DNA的内切酶相同，得到的DNA片段与线状的载体在有DNA连接酶存在的情况下混合培养，产生共价连接的DNA分子，结果，混合物中可得到多种多样的DNA分子。这包括二聚体、三聚体和多聚体DNA片段以及重新环化的质粒。其中有些质粒会插入一段目标DNA片段，这样就得到了杂合的质粒，即重组质粒。
Notes:
噬菌体 phage 限制性内切酶 restriction endonuclease
抗生素抗性 antibiotic resistance 决定子 determinant
共价的 covalently 二聚体 dimers
三聚体 trimers 多聚体 multimers
Wide-ranging research on tooth decay has recently produced some surprising findings. one indicates that cheddar may actually inhibit the tooth-decay process. It seems to have decay-slowing effect on human teeth if it is eaten immediately after sugar. Why cheese should have such an effect is unknown. It is speculated that the food might interfere with the acid that decays teeth or with bacteria that produce the acid. If so, it would be the first common food found to have this useful property. The other suprprising research finding was that heavily sweetened cereals proved about equally potent in causing decay whether they contained eight percent sugar or almost eight times that much.
1. According to the passage, how many of the test results were unexpected?
A. One B. Two C. Three D. Eight
2. According to the passage, what effect does cheddar cheese seem to have?
A. It interferes with the function of teeth.
B. It makes sugar taste sweeter.
C. It decreases the rate at which teeth decay.
D. It helps in the digestion of food.
3. It can be inferred from the passage that the research on the relationship between cheese and tooth.
A. has been discredited
B. will be slowed considerably
C. has been found to be conclusive
D. will be continued
4. Researchers discovered that sweetened cereals were.
A. important nutritionally
B. all surprisingly heavy in sugar
C. more expensive than cheese
D. all equally harmful to teeth
