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【简答题】
用 BamHI切割一重组 DNA 分子并从中分离纯化了两个 DNA片段，一400bp，另一 900bp。现在希望将这两个片段重新连接起来，得到如图 1 所示的结果。 于是将这两种片段混合起来，并在连接酶的存在下进行，分别在 30 分钟和 8 小时取样进行凝胶电泳分析，惊讶的是，连接产物并非是理想中的 1．3kb的重组分子，而是一种复杂的片段模式（图 A）。同时发现随着时间的延长，较小片段的浓度逐渐降低，大片段的浓度逐渐增加。如果用 BamHI来切割连接后的混合物，则起始的片段可以重新产生（图 A）。 从凝胶中分离纯化出 1．3kb 的片段，并取出一部分用 BamHI进行切割，以检查它的结构。正如预料的一样，出现了两个原始的带（图 B）。但是用 EcoRI切割另一部分样品，希望能够产生两个 300bp 和一个长度为 700bp 的核苷酸片段，然而，凝胶电泳的结果，惊奇（图 B）。 在原始连接混合物中为什么有如此多的带？
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