That's one of the properties of mitosis, that the DNA gets completely duplicated during the S phase, during DNA synthesis phase.

有丝分裂的一个特征,就是DNA在S期内完全复制,S期就是DNA合成期

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Synthesis always occurs in one direction and that makes sense to you now because you know there's a directionality and the chemistry is different going one way than the other and this DNA polymerase only works on the chemistry going in one direction.

NA复制过程是单向的,现在你们该明白原因了,因为DNA单链有方向性,DNA在两个方向上化学性质不同,DNA聚合酶,只在一个方向上起作用

耶鲁公开课 - 生物医学工程探索课程节选