Semiconservative replication

A summary of the three postulated methods of DNA synthesis

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^{[1] [2]} for DNA replication:

• Semiconservative replication would produce two copies that each contained one of the original strands and one new strand.
• Conservative replication would leave the two original template DNA strands together in a double helix and would produce a copy composed of two new strands containing all of the new DNA base pairs.
• Dispersive replication would produce two copies of the DNA, both containing distinct regions of DNA composed of either both original strands or both new strands.

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^[2][3] 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.

Rate and accuracy

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.^[4] 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.^[5] Thus semiconservative DNA replication is both rapid and accurate.

See also

• Molecular Structure of Nucleic Acids

References

1. Anthony J. F. Griffiths; et al. (1999). "8. The Structure and Replication of DNA". An Introduction to genetic analysis. San Francisco: W.H. Freeman. ISBN 0-7167-3520-2.
2. Meselson, M.; Stahl, F.W. (1958). "The Replication of DNA in Escherichia coli". PNAS 44: 671–82. doi:10.1073/pnas.44.7.671. PMC 528642. PMID 16590258.
3. Meselson M, Stahl FW. (2007 ) Demonstration of the semiconservative mode of DNA duplication. In “Phage and the Origins of Molecular Biology” (editors Cairns J, Stent GS, Watson JD). Cold Spring Harbor Laboratory Press ISBN 9780879698003
4. McCarthy D, Minner C, Bernstein H, Bernstein C (October 1976). "DNA elongation rates and growing point distributions of wild-type phage T4 and a DNA-delay amber mutant". J. Mol. Biol. 106 (4): 963–81. doi:10.1016/0022-2836(76)90346-6. PMID 789903.
5. Drake JW, Charlesworth B, Charlesworth D, Crow JF (April 1998). "Rates of spontaneous mutation". Genetics 148 (4): 1667–86. PMC 1460098. PMID 9560386.