The Meselson-Stahl experiment

The historical background of the Meselson steel experiment

• In 1869 the doctor Miescher first discovered deoxyribonucleic acid - DNA for short - in cells. In 1943 Avery was able to prove that DNA is the carrier of genetic information. In 1953 their structure was demonstrated by Watson and Crick.
• After it was proven that DNA contains the genetic information in encrypted form and what its structure is, it was still unclear how it is distributed between the two daughter cells during cell division. With each cell division, the genetic information must be passed on in full and must not be halved.
• In 1958, Meselson and Stahl published their experiment demonstrating that DNA is semi-conservatively doubled or replicated. They did research on bacteria.

The DNS replication mechanism simply explained

1. Before each cell division, the DNA is doubled, i.e. replicated, in order to then be completely distributed to both daughter cells again. So there is no loss of genetic information.
2. The DNA consists of two strands, both of which contain the genetic information, simply "mirrored" in each case. During replication, this double thread is opened and a new one is built on each strand.


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3. This mechanism is called "semiconservative replication", since at the end of each DNA molecule produced consists of an old and a newly synthesized strand. These are then distributed to the daughter cells.

The test sequence of the Meselson-Stahl experiment

• The experiment is based on the fact that new DNA strands are built up from materials from the cell during replication. One material is nitrogen. It is absorbed by the cells from the environment.
• Meselson and Stahl grew bacteria on a nutrient medium that contained heavier nitrogen than the normally occurring. The bacterial cells take up this heavy nitrogen and use it to each produce a new strand of DNA for their cell division.
• The newly synthesized DNA then consists of a strand with normal "light" nitrogen and a strand with the "heavy" nitrogen, since a new one is built up on each old strand. The newly created and the old bacterial cells all have two DNA strands of different weights.
• After the first cell division, after about 20 minutes, the bacteria are removed from the nutrient medium and centrifuged. With this technique, a material is separated according to the severity of the material. The bacterial DNA accumulates in a single place, since all cells - both the mother and daughter cells - have the same two strands of DNA.
• The DNA collects in a place where heavier substances than DNA, which is only made up of normal light nitrogen, collect. This explains that heavy nitrogen was also used in the newly synthesized DNA.
• In the next step of the Meselson-Stahl experiment, bacteria are again grown on a nutrient medium with heavy nitrogen, but this time waiting for them to divide twice. If the DNA of these bacteria is centrifuged, a DNA band collects at the level of the DNA, the bacteria, from the first step of the experiment. Some of the bacteria have DNA again, which is made up of one strand with heavy nitrogen and one strand with light nitrogen. A second DNA band, however, collects deeper because it is made up of two strands with heavy nitrogen.
• In the last step of the experiment, the following happened: First, a new "heavy" strand is synthesized on each strand of the light DNA. The two half-light, half-heavy DNA molecules are distributed to the daughter cells of the first generation. During the next cell division, heavy DNA is built up on each strand, so that in the end a semi-heavy, half light DNA molecule a DNA molecule with two pure heavy strands and again a mixed one are.

The Meselson-Stahl experiment simply explains that DNA replicates semi-conservatively, rather than discontinuously or conservatively.