A major requirement of genetic material is that it should be able to replicate, so that its' messages can be passed on from cell to cell, as an organism develops, and also from one generation to the next. It is also vital that during replication, identical copies of DNA are made so that the correct genetic messages are passed on.
DNA replication takes place during interphase of the cell cycle, so that by the time nuclear division starts, two identical copies of each DNA molecule are already present for distribution into daughter cells.
The replication of DNA is semi-conservative mechanism, which means one strand of the original molecule forms a strand of the two new molecules. The other strand of each new molecule is new. This can be seen clearly in the diagram below
This model suggests that the DNA double helix gradually "unzips" to expose the bases of each strand. New nucleotides align themselves in a complementary fashion against the bases of each parental strand. These nucleotides are joined by an enzyme (DNA polymerase) to make new polynucleotides. Therefore in the two new double helices, one strand is the original parental strand, whilst the other is newly made.
The following experiment carried out by Meselsohn and Stahl (1957) identified that the semi-conservative model was correct.
The bacterium E.coli was transferred from a growth medium containing normal nitrogen 14N, to a medium containing the heavy isotope of nitrogen 15N. They were grown on this medium for a sufficient number of generations so that all the nitrogenous bases in their DNA contained this heavy isotope. Light and heavy DNA can easily be distinguished by special centrifugation techniques where heavy DNA sediments to form a band lower down the centrifuge tube than light DNA. The bacteria that had been grown in 15N medium were then transferred back to 14N medium and grown for one generation. A sample of cells was taken, the DNA extracted, and centrifuged to see if it was light or heavy. It was found that neither type was present. Instead an intermediate band was formed. The cells were left to grow on 14N medium for a second generation, and the DNA sampled as before. This time, two bands were seen, light and intermediate.