复制的延长指在DNA-pol催化下，dNTP以 dNMP的方式逐个加入引物或延长中的子链上，其化学本质是磷酸二酯键的不断生成。

形成3‘，5’-磷酸二酯键：

DNA聚合酶（DDDP）、引物酶（DDRP）、DNA连接酶、拓扑异构酶，端粒酶（RDDP）

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原核 —— 10^6

真核 —— 10^9

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滞后链上不连续性片段的连接

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