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The property of DNA double helix which led Watson and Crick to hypothesise semi-conservative mode of DNA replication is that the two strands of DNA are antiparallel and complementary to each other in terms of their base sequences. This arrangement suggests that DNA replication is semiconservative. During replication, the two strands unwind and each strand acts as a template for the synthesis of a new strand. At the end of replication, DNA of parental types and recombinant types are formed. You must be knowing complementary pairing,Under normal circumstances, the nitrogen-containing bases adenine (A) and thymine (T) pair together, and cytosine (C) and guanine (G) pair together.

For more details refer to ncert 12th book and refer to molecular basis of inheritance chapter.

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