In April 1953, James Watson and Francis Crick published a report in the scientific journal Nature of their proposed structure of DNA – the famous double helix, earning them the Nobel Prize in Physiology or Medicine in 1962. They also suggested that DNA replicated by semi-conservative replication, but how could this theory be confirmed experimentally?
All known living organisms and also many viruses carry DNA – this is the molecule that contains the genetic instructions involved in growth, reproduction and the general functioning of an organism. Most cells contain their own molecule of DNA, so when a cell divides for growth, or to replace damaged cells, the DNA must replicate also.
In the 1950s there were three postulated methods of how DNA replicated:
- In conservative replication, the two original strands would remain intact, and a new copy would be produced, consisting of two new strands.
- In semi-conservative replication, each of the two original strands would act as a template for a new strand, resulting in two copies that consist of an original strand and a new strand.
- In dispersive replication, two copies would be produced that contained a mixture of sections from the original strands and from the new strands.
In 1958, Matthew Meselson and Franklin Stahl conducted what has been named ‘the most beautiful experiment in biology’. Since nitrogen is a major constituent of DNA, they took advantage of the two isotopes of nitrogen: the abundant 14N and the rarer, heavier 15N. Isotopes are atoms of the same element that have different numbers of neutrons, but are essentially the same in any other way (see An-atom-y for information on atomic structure).
Under laboratory conditions, E. coli bacteria were grown for several generations on a medium with 15N. As a result the bacteria would use 15N to replicate DNA. The DNA was then extracted and centrifuged (spun around in a test tube at high speeds). Centrifugation causes heavier things to settle out further down the test tube and lighter things to settle out further up the test tube. The 15N DNA settled out at a distinct point up the test tube.
The 15N bacteria were then grown on a medium with 14N for one generation. The resultant DNA settled at an intermediate point up the test tube, between where 15N DNA and 14N DNA was expected to settle. This ruled out conservative replication because:
- With conservative replication, there would be the original 15N DNA molecule and the new 14N DNA molecule, so the DNA would have settled out at two points.
- With semi-conservative replication, each copy would have one original 15N strand and one new 14N strand, so the DNA would have weighed in between 15N DNA and 14N DNA.
- With dispersive replication, each copy would have a mixture of 15N and 14N DNA, so the DNA would have weighed in between 15N DNA and 14N DNA.
The bacteria were then grown on a medium with 14N for another generation. The resultant DNA settled out at two points: one at the same intermediate point as before, and a new one where 14N DNA was expected to settle. This ruled out dispersive replication because:
- With dispersive replication, each copy would have a mixture of 14N and 15N DNA, so the DNA would have weighed slightly less than before, but it should still have settled out at one point.
- With semi-conservative replication, one copy would contain two 14N strands and one copy would contain one 14N strand and one 15N strand. The DNA would have settled out at two points, which the results agree with.
Visually, each of the three methods of replication would result in DNA looking like this (blue is the original 15N DNA and red is the new 14N DNA):
And the results would have looked like this:
The diagram shows how the original 15N DNA settles out at a distinct point up the test tube (the red dashed line) and 14N DNA settles out at a point slightly further above (the yellow dashed line). After the first generation, each copy had one strand of 15N DNA and one strand had 14N DNA, so it settled out in between.
Today we have a very detailed understanding of DNA replicates, allowing us to be able to grow specialised cells that cannot be replaced naturally, and to control cell division, leading us one step closer to treating cancer.