In order to increase their number, DNA has taking some process to duplicate them, or we knows those process called DNA Replication. Firstly, there is three models of DNA replication which popular in scientist community, those are:
1. Conservative replication. The parental double helix remain intact (is totally conserved) and some how direct synthesis of a ‘progeny’ double helix composed of two newly synthesized strands.
2. Dispersive replication. A replication in which segment of parental strands and progeny or nascent strands become interspersed through some kind of fragmentation, synthesis and rejoining process.
3. Semi conservative replication.
Each strand of DNA become template for formation of new strand, so in the end of replication process will produce two double helix DNA that each strand have one old strand and the new one.
Three postulate of DNA Replication
Meselson and Stahl doing their experiment using radioisotope 15N and 14N. Isotope nitrogen 15N heavier than nitrogen 14N. Meselson and Stahl grew E. Coli cells for many generation in a medium in which the heavu isotope of nitrogen , 15N, have been substituted for the normal, light isotope 14N. the purine and pyrimidine bases in DNA contain nitrogen; thus, the DNA of cells grown on medium containing 15N will have a greater density (weight per unit volume)than DNA of cells grown on medium containing 14N. since molecule of different densities can be separated by procedure called equilibrium density-gradient centrifugation, Meselson and Stahl were able to distinguish between the three possible modes of DNA replication by following the changes in the density of DNA of cells grown on 15N medium and transferred to medium with 14N for various periods of time (so-called density transfer experiment) (Gardner, 1991).
The density of most DNAs is about the same as the density of concentrated solution of heavy salts such as cesium chloride (CsCl). If DNA is present in such a gradient, it will move to the position where the density of the salt is equal to its own density.Thus if the mixture of E.coli DNA containing 15N (heavy DNA) and E.Coli containing 14N (light DNA) is subjected to CsCl Equilibrium density-gradient centrifugation, DNA molecules will separate into two ‘bands’, one containing heavy DNA and one containing light DNA.
Meselson and Stahl took cells that had been growing in medium containing 15N for several generation and thus contained heavy DNA, washed them to remove the 15N containing medium and transferred them to the medium containing 14N. After allowing the cells to grow in the presence of 14N for varying periods of time, the DNA have extracted and analyze in CsCl equilibrium-density gradient. The result of their experiment are only consistent with semiconservative replication.
All the DNA isolated from cells after one generation of growth in medium containing 14N had a density halfway between the density of heavy and light DNA. Its intermediate density is usually referred to ‘hybrid’ density. After two generation of growth in medium 14N, half of the DNA was of hybrid density nd half was light. These result are precisely those predicted by a Watson and Crick semiconservative mode of replication (Gardner, 1991).
Principles of Genetics (eight edition), Gardner and friend, 1991.