Replication and Models of Replication

Replication

It is a process in which DNA copies itself to produce identical daughter molecules of DNA.

Models of Replication

Three models of replication had been proposed. They are 1. Conservative replication 2.Dispersive replication and 3.Semi conservative replication.

1. Conservative replication

According to this model, the parental DNA is conserved to one daughter cell and the newly synthesized DNA to another daughter cell.

2. Dispersive Replication

According to this model, the parental DNA is unequally distributed (randomly) to the daughter cells.

3. Semiconservative Replication

Semiconseravative model of replication showing the daughter DNA having one parental strand and one daughter strand.

This model was established to be the correct mode of replication by the experiment carried out by Messlson and Stahl in 1957. E. coli cells were grown in a medium containing 15NH₄Cl, for many generations such that the E.coli cells have density labelled 15N atoms in their DNA. The cells were then grown in a medium containing unlabelled NH₄Cl. DNA was harvested from the daughter cells after one generation and the density of the DNA was analyzed using Cesium chloride density gradient centrifugation. If replication happens to occur by conservative model, two bands are corresponding to heavy DNA and other unlabelled DNA should be got. But if replication is semi-conservative, daughter DNAs should possess one labelled strand and one unlabelled strand, so they will form only one band with intermediate density. The results show that they form bands with intermediate density alone in the first generation, which confirms the semiconservative model of replication.