Semiconservative replication


Semiconservative replication

Semiconservative replication describes the method by which DNA is replicated in all known cells.This method of replication was one of three proposed modelscite book |author=Anthony J. F. Griffiths et al. |title=An Introduction to genetic analysis |publisher=W.H. Freeman |location=San Francisco |year=1999 |chapter = 8. The Structure and Replication of DNA |chapterurl = http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.1505 |isbn=0-7167-3520-2] [Meselson, M. and Stahl, F.W. (1958). "The Replication of DNA in Escherichia coli". PNAS 44: 671-82. PMID 16590258.] of DNA replication:
* 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.
* Semiconservative replication would produce two copies that each contained one of the original strands and one entirely new strand.

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 guessing 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 verified 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.

Testing the semi-conservative theory

The semi-conservative theory can be proven making use of the fact that DNA is made up of nitrogen bases. Nitrogen has an isotope N15 (N14 is the most common isotope) called heavy nitrogen. The experiment that proves the semi-conservative theory makes use of this isotope and runs as follows:

1.- Bacterial (E coli) DNA is placed in a media containing heavy nitrogen, which binds to the DNA, making it identifiable. 2.- This DNA is then placed in a media with the presence of N14 and left to replicate only once. The new bases will contain nitrogen 14 while the originals will contain N15 3.- The DNA is placed in test tubes containing caesium chloride (heavy compound) and centrifuged at 40,000 revolutions per hour. 4.- The caesium chloride molecules sink to the bottom of the test tubes creating a density gradient. The DNA molecules will position at their corresponding level of density (taking into account that N15 is more dense than N14)5.- These test tubes are observed under ultraviolet rays. DNA appears as a fine layer in the test tubes at different heights according to their density.

* According to the semi-conservative theory, after one replication of DNA, we should obtain 2 hybrid (part N14 part N15) molecules from each original strand of DNA. This would appear as a single line in the test tube. This result would be the same for the dispersive theory.
* On the other hand, according to the conservative theory, we should obtain one original DNA strand and a completely new one i.e. two fine lines in the test tube placed separately one from the other. Up to this point, either the semi-conservative or the dispersive theories could be truthful, as experimental evidence proved that only one line appeared after one replication. In order to conclude between those two, DNA had to be left to replicate again, still in a media containing N14.
* In the dispersive theory, after 2 divisions we should obtain a single line, but further up in the test tube, as the DNA molecules become less dense as N14 becomes more abundant in the molecule
* According to the semi-conservative theory, 2 hybrid molecules and 2 fully N14 molecules should be produced, so two fine lines at different heights in the test tubes should be observed. Experimental evidence proved that two lines were observed therefore offering compelling evidence for the semi-conservative theory.

ee also

*Molecular structure of Nucleic Acids

References


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • semiconservative replication — semiconservative replication. См. полуконсервативная репликация. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • semiconservative replication — The system of replication of DNA found in all cells in which each daughter cell receives one old strand of DNA and one strand newly synthesized at the preceding S phase. The existence of semiconservative replication was demonstrated by the… …   Dictionary of molecular biology

  • semiconservative replication — the mode in which DNA is replicated, each daughter molecule having one newly synthesized strand and one strand from the parent molecule …   Medical dictionary

  • Replication — may refer to:;Science * Self replication, an organism making a copy of itself or replicating oneself * Self replicating machines * DNA replication or DNA Synthesis, the process of copying a double stranded DNA molecule * Semiconservative… …   Wikipedia

  • Replication — A turning back, repetition, duplication, reproduction. * * * 1. The execution of an experiment or study more than once so as to confirm the original findings, increase precision, and obtain a closer estimate of sampling error. 2. Autoreproduction …   Medical dictionary

  • semiconservative — adjective Date: 1957 relating to or being genetic replication in which a double stranded molecule of nucleic acid separates into two single strands each of which serves as a template for the formation of a complementary strand that together with… …   New Collegiate Dictionary

  • semiconservative — adjective Describing the process of DNA replication in which the two strands of the molecule separate, each acting as a template for the formation of a complementary new strand …   Wiktionary

  • semiconservative — The process of replicating DNA in which the two strands remain intact, separate, and are copied and one parental strand goes to each daughter cell. * * * semi·con·ser·va·tive kən sər vət iv adj relating to or being genetic replication in which a… …   Medical dictionary

  • DNA replication — DNA replication. The double helix is unwound and each strand acts as a template for the next strand. Bases are matched to synthesize the new partner strands. DNA replication is a biological process that occurs in all living organisms and copies… …   Wikipedia

  • полуконсервативная репликация — semiconservative replication полуконсервативная репликация. Способ репликации двухцепочечной молекулы ДНК, при котором исходная молекула разделяется на две цепи (с образованием репликативной вилки <replication fork>), каждая из которых… …   Молекулярная биология и генетика. Толковый словарь.


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.