During DNA* replication, a double stranded DNA molecule separate, and each strand is used as a template for the synthesis of a new strand. This results in the formation of two identical copies of the original double stranded molecule. This is called semiconservative replication. This term captures the idea that each round of DNA replication produces hybrid molecules each of which contains one old strand and one newly synthesized strand.
The following illustration shows this process over two rounds of replication:
During each round of replication, the amount of DNA is doubled. The original strands remain intact and end up in different daughter strands.
The pattern of Semiconservative DNA replication was proposed in a 1953 paper by Watson and Crick. They did not call it semiconservative, but their description captures the idea that each of the two original strands are used as templates to make new double strands:
"…our model for deoxyribonucleic acid is, in effect, a pair of templates, each of which is complementary to the other. We imagine that prior to duplication the hydrogen bonds are broken, and the two chains unwind and separate. Each chain then acts as a template for the formation onto itself of a new companion chain, so that eventually we shall have two pairs of chains, where we only had one before. Moreover, the sequence of the pairs of bases will have been duplicated exactly."
Source Watson and Crick, 1953 (pdf)
Watson and Crick's paper proposed a mechanism, but provided no experimental evidence. The evidence for semiconservative replication was provided several years later by a 1958 paper by Matthew Meselson and Franklin W. Stahl (pdf).
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