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Semiconservative replication describes the mechanism by which DNA is replicated in all known cells. This mechanism of replication was one of three models originally proposed  for DNA replication:
The deciphering of the structure of DNA by Watson and Crick in 1953 suggested that each strand of the double helix would serve as a template for synthesis of a new strand. However, there was no way of knowing how the newly synthesized strands might combine with the template strands to form two double helical DNA molecules. The semiconservative model seemed most reasonable since it would allow each daughter strand to remain associated with its template strand. The semiconservative model was supported by the Meselson-Stahl experiment and other even more revealing experiments that allowed for autoradiographic visualization of the distribution of old and new strands within replicated chromosomes.
Experimental evidence confirmed that two lines were observed, therefore offering compelling evidence for the semi-conservative theory.
The rate of semiconservative DNA replication in a living cell was first measured as the rate of phage T4 DNA strand elongation in phage-infected E. coli. During the period of exponential DNA increase at 37°C, the rate of strand elongation was 749 nucleotides per second. The mutation rate per base pair per round of replication during phage T4 DNA synthesis is 2.4 x 10−8. Thus semiconservative DNA replication is both rapid and accurate.