DNA REPLICATION , FEATURE , STAGES IN DNA REPLICATION

What is DNA replication ....? 

DNA replication is the process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules. 

- Replication is an essential process because, whenever a cell divides, the two new     daughter cells must contain the same genetic information, or DNA, as the parent cell.

Feature and characteristics :- 

 1) DNA Replication Is Semiconservative.

Each DNA strand serves as a template for the synthesis of a new strand, producing two new DNA molecules, each with one new strand and one old strand.

2) This hypothesis is proven by Matthew Meselson and Franklin Stahl in 1957.

3) DNA synthesis proceeds in a 5′→3′ direction and is semi-discontinuous.DNA polymerase requires a single strand DNA to act as template.

4) DNA polymerase requires primer strand to provide the free hydroxyl group at the 3′ end to which the new nucleotide unit is added.

DNA replication can be explained in 3 stages based on reactions takes place and enzymes required during replication.

1) Initiation 

2) Elongation 

3) Termination 

1) Initiation :- 

-  Prokaryotic replication initiates at origin, called “oriC”. 

- oriC consists of 245 bp highly conserved DNA sequence rich in A=T bases.

- DnaA protein recognizes oriC sequence and opens DNA double helix at specific sites in origin.

- DnaB protein (helicase enzyme) unwinds DNA at oriC and creating replication forks.

- DnaG protein (primase), a DNA dependent RNA polymerase binds to and synthesizes the primer on each DNA strand.

- Single-stranded DNA–binding protein (SSB) bind to and stabilize the separated strands, and protect it from nucleases.

- DNA gyrase (DNA topoisomerase II) relieves the topological stress induced ahead of the fork by the unwinding reaction.

- Loading of DNA pol III on to the each ssDNA completes the initiation stage.

2) Elongation :- 

- DNA polymerase III is the prime prokaryotic DNA polymerase  and initiates the synthesis of new DNA strand by adding deoxynucleotide to the 3` end of the RNA primer.

- DNA polymerase III can synthesize a new chain only in the 5` to 3` direction.

- Both the DNA strands are synthesized simultaneously but in opposition direction, one is in the direction of moving replication fork called as leading strand and other in the opposite direction of the moving replication fork is called as lagging strand.

- Leading strand is synthesize in continuous manner and require only one primer whereas the lagging strand synthesizes in the discontinuous manner in fragments called as okazaki fragments and requires more than one primer.

- DNA pol III has polymerization rate of 25—1000 nucleotide/sec.

Termination:- 

- Termination sequence “ter” directs termination of replication.A specific protein “ter binding protein” binds these “ter” sequences and prevents the helicase from further unwinding of DNA and facilitates the termination of replication.

- Upon completion of lagging strand synthesis, DNA polymerase I removes the RNA primers and fills the gaps that are produced by removal of the primer leaving only a nick.

- These nick were sealed by DNA ligase thus joining all the Okazaki fragments into single DNA strands.Complimentary base selection and proofreading activity due to 5′→3′ exonuclease activity of DNA polymerase I & 3′→5′ exonuclease activity of DNA polymerase I  & III are combine responsible for accuracy in replication process, so that it has one net error for every 10⁶ to 10⁸ bases added.