DNA replication

By

Dr. Ahmed Okasha

Semiconservative

replication1-Separation of DNA strand occur

2-The new strand synthesized that

are complementary to parent

strand

3-The new DNA contain 2 strands

one from parents and the other is

new strand

Basic requirement for DNA

replication

1- Substrate

4 nucleotide of DNA (ATP, GTP,

CTP, TTP) must present

2- Template (parent strand)

Parent strand are required to

add complementary nucleotide

on the new strand

3- RNA primer

Present in the first new strand

and its function is to receive

the first new nucleotide that

added on the new strand

4- Enzyme :

DNA polymerase , Pol I, II , III

Step of replication in

prokaryotic (E. coli)

Origin of replication1-Replication start at

particular DNA sequence

called origin (ori)

2- Dna A &B& C protein bind

to strand produce

separation of 2 strand

2- Formation of replication fork

Replication fork represent the

unwinding the DNA strand by the

effect of helicase (Dna B) made

broken in hydrogen bond lead to

separation in helix

This need 2 ATP for each base pair

broken

Helicaes helped by 2 enzyme

TopoisomerasePrevent super coil (super twist)

that if occur lead to prevent

replication

Topoisomerase II

(Gyrase)

Topoisomerase I

1- Cut in 2 strand

2- Need ATP

3- Make negative

supercoil by making

break in phosphodiester

bond

1- Cut in one strand

2- Don’t need ATP

3- Prevent positive

supercoil

Single stranded binding

protein (SSBP)

Bind to single stranded DNA

prevent it from reunion

Formation of new complementary

strand

Always new DNA strand grow from

5 to 3

direction

strand Leading A-

This strand grow from 5 to 3

indirection of origin of replication

This grow continuously and made by

Pol III

B) Okazaki fragment (lagging strand)

This strand grow from 5 to 3 direction in

opposite of origin of replication

This grow discontinuously and made by

Pol III

The gap (space) filled by DNA Pol I and

the space is ligated By DNA ligase

enzyme

Leading strand

Lagging strand

Synthesis of DNA include

following main steps

RNA primer

1- RNA primer are required to

receive new nucleotide that

added on new strand

2- Primase enzyme synthesize

RNA primer

Pol I remove primer after termination of

replication

There are 1 primer on leading

strand and many primer in lagging

strand

Prof reading effect of DNA Pol III

Prof reading means ability to

remove any wrong nucleotide

added during synthesis as pairing

A=G must be A=T

DNA Pol IIHas role in repair of DNA

NB. Replication of DNA has high

fidelity means any error during

synthesis is corrected

Role in replication Comment

Broke hydrogen bond separation of helixDNA helicase (dan

B)

Bind to single stranded DNA prevent it

from reunionSSBP

synthesize RNA primerPrimase

Synthesis of leading strand

Okazaki fragmentDNA Pol III

Remove primer

Fill the gap in lagging strandDNA Pol I

Prevent super coil (super twist)Gyrase

(topoisomerase II)

Join the 2 end of DNA need ATPDNA ligase

Eukaryotic DNA replication

What are telomeres and telomerase?

A telomere is a repeating DNA

sequence (TTAGGG) at the end of the

body's chromosomes. . Telomeres

function by preventing chromosomes

from fusion of their ends.

Telomerase, is an enzyme

made of protein and RNAsubunits that elongates

chromosomes by adding

TTAGGG sequences to

the end chromosomes.

Because these somatic cells has

little telomerase, they age.

The result of aging cells an

aging body.

If telomerase is activated in a

cell, the cell will continue to grow

and divide. lead to cancer.

Telomerase has reverse

transcriptase activity

DNA transcription RNA

RNA reverse transcriptase (RT) DNA

telomerase Telomer

Eukaryotic DNA polymerase

Pol α synthesis of RNA primer and

lagging strand

Pol β DNA repair

Pol γ synthesis of mitochondrial DNA

Pol δ synthesis of leading strand

Pol ε removal of RNA primer

Difference between Eukaryotic

and prokaryotic DNA

replication

Prokaryotic DNA

replication

Eukaryotic DNA

replication

One

One

Primase

DNA Pol III

DNA Pol II

Pol III

Absent

Multiple

Multiple

DNA Pol α

DNA Pol α

DNA Pol β

Pol δ

Present

1.Origin of

replication

2.Replication fork

3.Primer synthesis

4.Lagging strand

synthesis

5.DNA repair

6.Leading strand

7.Telomerase