MECHANISM OF DNA REPLICATION

Different stages are involved in DNA replication . Those are 

1- Initiation of replication

Replication of DNA in E-coli always begins at a definite site called replication origin .

Prokaryotes like bacteria and viruses have only one replication origin .

In e-coil replication is due to multienzyme complex known as replisome .

Eukaryotes have more than one replication origin .These are arranged in a linear manner called replisome.

2- Formation of primary strand 

A new strand is synthesized opposite the parallel strand act as templates strand.

Primase initiates the production of short primary strand of RNA the primary formed towards the 3' end present strand .

In this process some other enzymes are involved . These are collectively known as primasomes .

The the primer strands elongates.

Once the initiation of DNA synthesis is completed . The RNA primer is removed by enzyme DNA polymerase .

2 - Elongation of new strand 

After the formation of primer strand , DNA replication occurs in 5'- 3' direction .That means deoxy ribonucleotides are added to 3' OH of nee strand with help of enzyme DNA polymerase 3 .

As the DNA replication occurs synthesis of a daughter strand occurs continuously on the upper strand .It is known as leading strand .

In another daughter strand short pieces DNA is synthesized .These fragments are known as O'kazaki  fragments .

This strand is discontinue as and known as lagging strand .

Dis contineous pieces of laggind strands are later joined by enzyme ligase to form contineous daughter strand .

The okazaki fragments are 1000- 2000 nucleotides long .

3- proof reading  and DNA repair

Some times a wrong base is introduced during replication . Its frequency is one in ten thousand .

This error is corrected by DNA polymerase enzyme with its error correcting activity known as proof reading .

It recognises the error removed the wrong nucleotide and allows the addition of proper base and then moves forward .