若1个35S标记的大肠杆菌被1个32P标记的噬菌体侵染，裂解后释放的噬菌体（　）。

2023-08-08

A.一定有35S，可能有32P
B.只有35S
C.一定有32P，可能有35S
D.只有32P

参考答案：A

噬菌体在侵染细菌时，只将自身的DNA注射到细菌菌体中，蛋白质外壳留在细菌体外。由于DNA是半保留复制的，因此亲代噬菌体被32P标记的那两条DNA链存在于某两个子代噬菌体中，而细菌中没有32P标记，因此子代噬菌体中始终只有两个子代DNA分子有32P标记。由于子代噬菌体的蛋白质是以细菌体内的原料合成的，而大肠杆菌被35S标记，因此合成的噬菌体的蛋白质外壳均有”S标记，因此子代噬菌体一定有35C，可能有32P。

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