Meselson–Stahl experiment

The Meselson–Stahl experiment was an experiment by Matthew Meselson and Franklin Stahl in 1958 which supported the hypothesis that DNA replication was semiconservative. Semiconservative replication means that when the double stranded DNA helix was replicated, each of the two double stranded DNA helices consisted of one strand coming from the original helix and one newly synthesized. It has been called "the most beautiful experiment in biology.^[1]"

Hypotheses

A summary of the three postulated methods of DNA synthesis

Three hypotheses had been previously proposed for the method of replication of DNA.

In the semiconservative hypothesis, proposed by Watson and Crick, the two strands of a DNA molecule separate during replication. Each strand then acts as a template for synthesis of a new strand.^[2]

The conservative hypothesis proposed that the entire DNA molecule acted as a template for synthesis of an entirely new one. According to this model, histone proteins bound to the DNA, distorting it in such a way as to expose both strands' bases for hydrogen bonding.^[3]

The dispersive hypothesis is exemplified by a model proposed by Max Delbrück, which attempts to solve the problem of unwinding the two strands of the double helix by a mechanism that breaks the DNA backbone every 10 nucleotides or so, untwists the molecule, and attaches the old strand to the end of the newly synthesized one. This would synthesize the DNA in short pieces alternating from one strand to the other.^[4]

Each of these three models makes a different prediction about the distribution of the "old" DNA in molecules formed after replication. In the conservative hypothesis, after replication, one molecule is the entirely conserved "old" molecule, and the other is all newly synthesized DNA. The semiconservative hypothesis predicts that each molecule after replication will contain one old and one new strand. The dispersive model predicts that each strand of each new molecule will contain a mixture of old and new DNA.^[5]

Experimental procedure and results

Nitrogen is a major constituent of DNA. ¹⁴N is by far the most abundant isotope of nitrogen, but DNA with the heavier (but non-radioactive) ¹⁵N isotope is also functional.

E. coli were grown for several generations in a medium with ¹⁵N. When DNA is extracted from these cells and centrifuged on a salt density gradient, the DNA separates out at the point at which its density equals that of the salt solution. The DNA of the cells grown in ¹⁵N medium had a higher density than cells grown in normal ¹⁴N medium. After that, E. coli cells with only ¹⁵N in their DNA were transferred to a ¹⁴N medium and were allowed to divide; the progress of cell division was monitored by measuring the optical density of the cell suspension.

DNA was extracted periodically and was compared to pure ¹⁴N DNA and ¹⁵N DNA. After one replication, the DNA was found to have close to the intermediate density. Since conservative replication would result in equal amounts of DNA of the higher and lower densities (but no DNA of an intermediate density), conservative replication was excluded. However, this result was consistent with both semiconservative and dispersive replication. Semiconservative replication would result in double-stranded DNA with one strand of ¹⁵N DNA, and one of ¹⁴N DNA, while dispersive replication would result in double-stranded DNA with both strands having mixtures of ¹⁵N and ¹⁴N DNA, either of which would have appeared as DNA of an intermediate density.

The authors continued to sample cells as replication continued. DNA from cells after two replications had been completed was found to consist of equal amounts of DNA with two different densities, one corresponding to the intermediate density of DNA of cells grown for only one division in ¹⁴N medium, the other corresponding to DNA from cells grown exclusively in ¹⁴N medium. This was inconsistent with dispersive replication, which would have resulted in a single density, lower than the intermediate density of the one-generation cells, but still higher than cells grown only in ¹⁴N DNA medium, as the original ¹⁵N DNA would have been split evenly among all DNA strands. The result was consistent with the semiconservative replication hypothesis.^[6]

Literature cited

1. ^ John Cairns to Horace F Judson, in The Eighth Day of Creation: Makers of the Revolution in Biology (1979). Touchstone Books, ISBN 0-671-22540-5. 2nd edition: Cold Spring Harbor Laboratory Press, 1996 paperback: ISBN 0-87969-478-5.
2. ^ WATSON JD, CRICK FH (1953). "The structure of DNA". Cold Spring Harb. Symp. Quant. Biol. 18: 123–31. PMID 13168976. 
3. ^ Bloch DP (December 1955). "A POSSIBLE MECHANISM FOR THE REPLICATION OF THE HELICAL STRUCTURE OF DESOXYRIBONUCLEIC ACID". Proc. Natl. Acad. Sci. U.S.A. 41 (12): 1058–64. doi:10.1073/pnas.41.12.1058. PMC 528197. PMID 16589796. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=528197. 
4. ^ Delbrück M (September 1954). "ON THE REPLICATION OF DESOXYRIBONUCLEIC ACID (DNA)". Proc. Natl. Acad. Sci. U.S.A. 40 (9): 783–8. doi:10.1073/pnas.40.9.783. PMC 534166. PMID 16589559. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=534166. 
5. ^ Delbrück, Max; Stent, Gunther S. (1957). "On the mechanism of DNA replication". In McElroy, William D.; Glass, Bentley. A Symposium on the Chemical Basis of Heredity. Johns Hopkins Pr.. pp. 699–736. 
6. ^ Meselson, M. and 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. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=528642. 

External links

• Matthew Meselson iBioMagazine talk: "The Semi-Conservative Replication of DNA"
• DNA From The Beginning A animation which explains the experiment.
• The Meselson–Stahl Experiment Another useful animation.
• Description of the Meselson-Stahl Experiment including original data from Visionlearning