DNA Replication

BHOORO MAL KISHNANI

BSMT 3RD YEAR

SDLS LNH

Introduction

It is the process that can duplicate the DNA of a cell

Every cell (of eukaryotes or prokaryotes) has one or more DNA (or RNA) polymer molecules that need to duplicate in order the cell duplication to take place.

DNA Replication

In the eukaryotes DNA is formed in two strands, each composed of units called Nucleotides.

The two strands look like two chains that form the DNA Double Helix.

The DNA Replication Process is capable of opening the Double Helix and separating the two strands.

Then the two strands are copied

Result two new DNA molecules are created.

The next step is the cell division. After that a daughter cell is

created. In its nucleus lies a copy of the

parental DNA.

DNA Replication models

There are three possible models that describe the accurate creation of the daughter chains:

Semiconservative Replication Conservative Replication Dispersive Replication

Semiconservative Replication

DNA Replication would create two molecules.

Each of them would be a complex of an old (parental and a daughter strand).

Conservative Replication

The DNA Replication process would create a brand new DNA double helix made of two daughter strands while the parental chains would stay together.

Dispersive Replication

According to this model the Replication Process would create two DNA double-chains, each of them with parts of both parent and daughter molecules.

The correct model is Semiconservative DNA Replication was proved by the experiment of Meselson - Stahl.

Steps of DNA Replication Step 1

The first major step for the DNA Replication to take place is the breaking of hydrogen bonds between bases of the two antiparallel strands.

The unwounding of the two strands is the starting point.

The splitting happens in places of the chains which are rich in A-T.

That is because there are only two bonds between Adenine and Thymine (there are three hydrogen bonds between Cytosine and Guanine).

Helicase is the enzyme that splits the two

strands.

The initiation point where the splitting starts is called "origin of replication".

The structure that is created is known as "Replication Fork".

Step 2

One of the most important steps of DNA Replication is the binding of RNA Primase in the the initiation point of the 3'-5' parent chain.

RNA Primase can attract RNA nucleotides which bind to the DNA nucleotides of the 3'-5' strand due to the hydrogen bonds between the bases.

RNA nucleotides are the primers (starters) for the binding of DNA nucleotides

Step 3

The elongation process is different for the 5'-3' and 3'-5' template.

a)5'-3' Template: The 3'-5' proceeding daughter strand -that uses a 5'-3' template- is called leading strand because DNA Polymerase ä can "read" the template and continuously adds nucleotides (complementary to the nucleotides of the template, for example Adenine opposite to Thymine etc).

b)3'-5'Template: The 3'-5' template cannot be "read" by DNA Polymerase ä. The replication of this template is complicated and the new strand is called lagging strand.

In the lagging strand the RNA Primase adds more RNA Primers. DNA polymerase å reads the template and lengthens the bursts.

The gap between two RNA primers is called "Okazaki Fragments".

The RNA Primers are necessary for DNA Polymerase å to bind Nucleotides to the 3' end of them.

The daughter strand is elongated with the binding of more DNA nucleotides.

Step 4

In the lagging strand the DNA Pol I -exonuclease-reads the fragments and removes the RNA Primers.

The gaps are closed with the action of DNA Polymerase (adds complementary nucleotides to the gaps) and DNA Ligase (adds phosphate in the remaining gaps of the phosphate - sugar backbone).

Each new double helix is consisted of one old and one new chain.

we call

semiconservative replication.

Step 5

The last step of DNA Replication is the Termination.

This process happens when the DNA Polymerase reaches to an end of the strands.

When the RNA primer is removed, it is not possible for the DNA Polymerase to seal the gap (because there is no primer

So, the end of the parental strand where the last primer binds isn't replicated.

These ends of linear (chromosomal) DNA consists of noncoding DNA that contains repeat sequences and are called telomeres.

As a result, a part of the telomere is removed in every cycle of DNA Replication.

Step 6

The DNA Replication is not completed before a mechanism of repair fixes possible errors caused during the replication.

Enzymes like nucleases remove the wrong nucleotides and the DNA Polymerase fills the gaps.

Speed of DNA Replication

• The Genome of complex eukaryotes is huge and the process of DNA Replication should be incredibly fast.

• It is amazing that a Chromosome of 250 million pair of bases can be replicated in several hours.

• The speed of DNA replication for the humans is about 50 nucleotides per second per replication fork (low speed comparing to the speed of the bacterial DNA Replication).

But the human Genome can be copied only in a few hours because many replication forks take place at the some time (multiple initiation sites).

The speed of DNA replication in bacteria is much longer (about 1000 nucleotides per second) and that is a reason why during the process of bacterial replication the rate of errors is much higher.

Enzymes of DNA Replication• Helicase: Unwounds a portion of the DNA

Double Helix

RNA Primase: Attaches RNA primers to the replicating strands.

DNA Polymerase delta (ä): Binds to the 5' - 3' strand in order to bring nucleotides and create the daughter leading strand.

DNA Polymerase epsilon (å): Binds to the 3' - 5' strand in order to create discontinuous segments starting from different RNA primers.

• Exonuclease (DNA Polymerase I): Finds and removes the RNA Primers

DNA Ligase: Adds phosphate in the remaining gaps of the phosphate - sugar backbone

Nucleases: Remove wrong nucleotides from the daughter strand.