DNA REPLICATION

DNA REPLICATION: 

DNA REPLICATION

The Discovery: 

The Discovery Watson &Crick discovered DNA and proposed the mechanism

Definition: 

Definition The process by which DNA can make an exact copy of itself

The Need: 

The Need The DNA is situated in the nucleus, organized into chromosomes. Every cell must contain the genetic information and the DNA is therefore duplicated before a cell divides ( replication ).

Time of the process: 

Time of the process It takes place in ‘S’ phase of interphase between two mitotic cycles

Slide 6: 

Relication takes place in S phase

Before cell division replication : 

Before cell division replication

It is Semi-conservative: 

It is Semi-conservative The two new double helices formed would have one old and one new strand. Hence replication is a semi conservative process.

REQUIREMENT : 

REQUIREMENT DNA replication requires – a template DNA – it is a previously existing DNA on which the new DNA is to be synthesized a primer RNA DNA unwinding protein (Helicase ) Super helix relaxing protein Various enzymes (endonuclease, ligase, DNA polymerase & RNA polymerase ( primase etc)

A template DNA: 

A template DNA It is a previously existing DNA on which the new DNA is to be synthesized

Initiation :: 

Initiation : The DNA of chromosomes consists of many replicating units or replicons or Okazaki fragments According to Okazaki etal (1968) only one strand is continuously replicated, and the other strand replicates in a discontinous manner, in short segments which are known as Okazaki fragments. Replication takes place in 5’ 3’ direction

Initiation :: 

Initiation : Replication in each replicon is initiated only after a break in one of the two parental strands. The break or nick takes place at a specific point. This is made by an incision enzyme endonuclease

Unwinding of helix / the strand : 

Unwinding of helix / the strand The DNA double helix has to unwind in order to separate the two strands for replication. The unwinding of DNA double helix is brought about by DNA unwinding proteins/ helicase / rep – protein .

Unwinding of helix / the strand: 

Unwinding of helix / the strand The unwinding may result in the formation of super coils which are removed by the action of another enzyme gyrase .

Formation of Replication fork: 

Formation of Replication fork The unwinding of DNA molecule into two strands result in the formation of Y shaped structure called replication fork . The exposed single strands are stabilized by a protein called single strand binding protein(SSB) protein or helix de-stabilizing protein.

The SSB protein holds the two separate strands open to form the replication fork : 

The SSB protein holds the two separate strands open to form the replication fork

Formation of primer RNA :: 

Formation of primer RNA : A new strand is synthesized opposite the parental or template strand. The true replicase DNA polymerase – III is incapable of initiating DNA synthesis, i.e. it is unable to deposit the first nucleotide in a daughter strand. Another enzyme primase or RNA polymerase synthesizes a short primer strand of RNA

Elongation of Chain :: 

Elongation of Chain : Synthesis of the new DNA strand occurs by addition of DNA nucleotides to the 3’ OH group of RNA primer. This takes place in 5’ 3’ direction catalysed by DNA polymerase III. The newly synthesized DNA chains have primer RNA attached to their 5’ ends

Removal of RNA primer :: 

Removal of RNA primer : The RNA primer is hydrolysed by DNA polymerase I. The resulting gap is filled by DNA nucleotides again by the catalytic activity of DNA polymarease I. The newly made DNA is joined by DNA ligase to high molecular wt. DNA

Slide 22: 

A Summary

REPAIR: 

REPAIR

Slide 24: 

Proof Reading

Bidirectional Replication: 

Bidirectional Replication

Slide 26: 

END