User:AMYCREYNOLDS/Semiconservative replication
Semi-Conservative Replication
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Semiconservative replication describes the mechanism of DNA replication in all known cells. It derives its name from the production of two copies of the original DNA molecule, each of which contains one original strand, and one newly-synthesized strand. [1][2]
The structure of DNA (as deciphered by James D. Watson and Francis Crick in 1953) suggested that each strand of the double helix would serve as a template for synthesis of a new strand. However, it was not known how newly synthesized strands combined with template strands to form two double helical DNA molecules. The semiconservative model of replication seemed most reasonable since it would allow each daughter strand to remain associated with its template strand.
The semiconservative model was anticipated by Nikolai Koltsov and is supported by the Meselson-Stahl experiment[2][3]
Semi-Conservative Replication
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Semiconservative replication describes the mechanism of DNA replication in all known cells. It derives its name from the production of two copies of the original DNA molecule, each of which contains one original strand, and one newly-synthesized strand. The structure of DNA (as deciphered by James D. Watson and Francis Crick in 1953) suggested that each strand of the double helix would serve as a template for synthesis of a new strand. However, it was not known how newly synthesized strands combined with template strands to form two double helical DNA molecules.
Multiple experiments were conducted to determine how DNA replicates. The semiconservative model was anticipated by Nikolai Koltsov and later supported by the Meselson-Stahl experiment[2][3]. The Meselson-Stahl experiment confirmed that DNA replicated semi-conservatively by conducting an experiment using two radioisotopes: nitrogen-15 (15N) and nitrogen-14 (14N). When 14N was added to the 15N-15N heavy DNA, a hybrid of 15N-14N was seen in the first generation. After the second generation, the hybrid remained, but light DNA (14N-14N) was seen as well. This indicated that DNA replicated semi-conservatively. This mode of DNA replication allowed for each daughter strand to remain associated with its template strand.[1]
Models of Replication
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Semiconservative replication derives its name from the fact that this mechanism of transcription was one of three models originally proposed[2][3] for DNA replication:
- Semiconservative replication would produce two copies that each contained one of the original strands and one new strand[2]. Semiconservative replication is beneficial to DNA repair. During replication, the new strand of DNA adjusts to the modifications made on the template strand [4].
- 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 [2].
- 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 [2].
Models of Replication
[edit]edited version
Semiconservative replication derives its name from the fact that this mechanism of transcription was one of three models originally proposed[2][3] for DNA replication:
- Semiconservative replication would produce two copies that each contained one of the original strands of DaN and one new strand[2]. Semiconservative replication is beneficial to DNA repair. During replication, the new strand of DNA adjusts to the modifications made on the template strand [4].
- 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 [2].
- 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 [2]. The strands of DNA were originally thought to be broken at every tenth base pair to add the new DNA template. Eventually, all new DNA would make up the double helix after many generations of replication[5]
Separation and Recombination of Double-Stranded DNA
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For semi-conservative replication to occur, the DNA double-helix needs to be separated so the new template strand can be bound to the complementary base pairs. Topoisomerase is the enzyme that aids in the unzipping and recombination of the double-helix. Specifically, topoisomerase prevents the double-helix from supercoiling, or becoming too tightly wound. Three topoisomerase enzymes are involved in this process: Type IA Topoisomerase, Type IB Topoisomerase, and Type II Topoisomerase[6]. Type I Topoisomerase unwinds double stranded DNA while Type II Topoisomerase breaks the hydrogen bonds linking the complementary base pairs of DNA[7]
Further Applications
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Semiconservative replication provides many advantages for DNA. It is not only fast and accurate[8], but it has been found to allow DNA to be repaired easily and is responsible for phenotypic diversity in a few prokaryotic species. The process of creating a newly synthesized strand from the template strand allows for the old strand to be methylated at a separate time from the new strand. This allows repair enzymes to proofread the new strand and correct any mutations or errors[9].
DNA could have the ability to activate or deactivate certain areas on the newly synthesized strand that allows the phenotype of the cell to be changed. This could be advantageous for the cell because DNA could activate a more favorable phenotype to aid in survival. Due to natural selection, the more favorable phenotype would persist throughout the species. This gives rise to the idea of inheritance, or why certain phenotypes are inherited over another[9].
- ^ Hanawalt, P. C. (2004-12-17). "Density matters: The semiconservative replication of DNA". Proceedings of the National Academy of Sciences. 101 (52): 17889–17894. doi:10.1073/pnas.0407539101. ISSN 0027-8424.
- ^ a b c d e f g h Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart WM (1999). "Chapter 8: The Structure and Replication of DNA". An Introduction to Genetic Analysis. San Francisco: W.H. Freeman. ISBN 978-0-7167-3520-5.
- ^ a b Meselson M, Stahl FW (July 1958). "The Replication of DNA in Escherichia". Proceedings of the National Academy of Sciences of the United States of America. 44 (7): 671–82. doi:10.1073/pnas.44.7.671. PMC 528642. PMID 16590258.
- ^ a b Norris V (June 2019). "Does the Semiconservative Nature of DNA Replication Facilitate Coherent Phenotypic Diversity?". Journal of Bacteriology. 201 (12). doi:10.1128/jb.00119-19. PMC 6531617. PMID 30936370.
- ^ Molecular biology of the gene. Watson, James D., 1928- (Seventh edition ed.). Boston. ISBN 978-0-321-76243-6. OCLC 824087979.
{{cite book}}:|edition=has extra text (help)CS1 maint: others (link) - ^ Brown, Terence A. (2002). Genome Replication. Wiley-Liss.
- ^ Molecular biology of the gene. Watson, James D., 1928- (Seventh edition ed.). Boston. ISBN 978-0-321-76243-6. OCLC 824087979.
{{cite book}}:|edition=has extra text (help)CS1 maint: others (link) - ^ McCarthy, David; Minner, Charles; Bernstein, Harris; Bernstein, Carol (1976-10). "DNA elongation rates and growing point distributions of wild-type phage T4 and a DNA-delay amber mutant". Journal of Molecular Biology. 106 (4): 963–981. doi:10.1016/0022-2836(76)90346-6. ISSN 0022-2836.
{{cite journal}}: Check date values in:|date=(help) - ^ a b Norris, Vic (2019-04-01). "Does the Semiconservative Nature of DNA Replication Facilitate Coherent Phenotypic Diversity?". Journal of Bacteriology. 201 (12). doi:10.1128/jb.00119-19. ISSN 0021-9193.