DNA replication by moazzam

Replication

of DNABy MOAZZAM ALI SHAHID

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DNA Replication By Moazzam Ali

Shahid

Nucleic acids are required for

the storage and expression

of genetic information.

There are two types of

nucleic acids

• DNA

• RNA

Nucleic Acid

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Louis Pasteur (1822-

1895)

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The DNA double helix has two strands twisted together.

(In the rest of this unit we will look at the structure of one strand.)

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N.Base

phosphate

deoxyribose

The backbone of the molecule is alternating

a sugar, parts The teeth are nitrogenous.

phosphate

deoxyribose

bases


phosphate

deoxyribose

bases

Remember, DNA has two strands that fit together something like a zipper.

The teeth are the nitrogenous bases but why do they stick together?


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When making hydrogen bonds, cytosine always pairs up with guanine,

And adenine always pairs up with thymine.

(Adenine and thymine are shown here.)

C

C

C

C

N

N

O

O

C


One deoxyribose together with its phosphate and base

make a nucleotide.

C C

C

OPhosphate

O

O -P O

O

O

O -P O

O

O

O -P O

O

O

Nitrogenous

base

Deoxyribose

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Each cell has about 2 m of DNA.

The average human has 75 trillion cells.

The average human has enough DNA to go from the earth to the sun more than 400 times.

DNA has a diameter of only 0.000000002 m.

The earth is 150 billion m

or 93 million miles from

the sun.


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DNA Replication(

[email protected]) 19

Three Models of DNA Replication

Conservative model

Parent molecule remains the same

Completely new copy of the double helix is made

Semiconservative model

Parent strands separate and serve as templates for new

strand synthesis

Hybrid molecules are made

Dispersive model

New strands contain a mixture of old molecules and

newly synthesized molecules

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Semiconservative Model:

1. Watson and Crick showed: the two strands of the parental moleculeseparate, and each functions as a templatefor synthesis of a new complementary strand.

.Parental DNA

DNA Template

New DNA

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DNA Replication(

[email protected]) 24

1. DNA replication occurs on template in 5’ - 3’

direction.

2. Requires RNA primer for de novo synthesis: made by

primase.

3. Asymmetric: Continuous on leading strand

Discontinuous on lagging strand

4. Okazaki Fragments on lagging strand.

5. Replication is semi-conservative.

6. Specific origins of replication. Sequence dependent.

Usually bidirectional.

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STEPS INVOLVED IN DNA REPLICATION

1. Identification of the origins of replication (ori)

2. Unwinding (denaturation) of dsDNA to provide a

ssDNA template

3. Formation of replication fork

4. Initiation of DNA synthesis and elongation

5. Formation of replication bubbles with ligation of the

newly synthesized DNA segment

6. Reconstitution of chromatin structure

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Enzymes in DNA Replication

•Helicases

•SSB

•DNA Primase

•DNA Polymerases

•DNA Ligase

•Topoisomerases

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Polymerase Functions Proof Reading

Pol alpha Contain Primase

-

Initiate DNA -

Pol Beta Repair +

Pol gama Replicates mDNA

+

Pol sigma Elongate leading and lagging strand

+

Pol eta Repair +

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DNA Polymerase in Eukaryotes

origins of DNA replication (every ~150 kb)

replication bubble

daughter chromosomes

fusion of bubbles

bidirectional replication

Origins of DNA replication on mammalian chromosomes

5’

3’

3’

5’

5’

3’

3’

5’

3’

5’

5’

3’1/20/2015 29


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3’

5’ 3’

RNA primer

newly synthesized DNA

5’

5’

DNA polymerase

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Discontinuous synthesis of DNA

3’

5’

5’ 3’

3’ 5’

Because DNA is always synthesized in a 5’ to 3’ direction,

synthesis of one of the strands...

5’

3’

...has to be discontinuous.

This is the lagging strand.

5’

3’

3’

5’

5’

3’

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3’

5’

5’ 3’

3’ 5’

5’

3’

3’

5’

5’

3’

leading strand (synthesized continuously)

lagging strand (synthesized discontinuously)

Each replication fork has a leading and a lagging strand

• The leading and lagging strand arrows show the direction

of DNA chain elongation in a 5’ to 3’ direction

• The small DNA pieces on the lagging strand are called

Okazaki fragments (100-1000 bases in length)

replication fork replication fork

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RNA primer

5’

3’

3’

5’

3’

5’

direction of leading strand synthesis

direction of lagging strand synthesis

replication fork

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5’

3’

3’

5’

3’

5’

Strand separation at the replication fork causes positive

supercoiling of the downstream double helix

• DNA gyrase is a topoisomerase II, which

breaks and reseals the DNA to introduce negative

supercoils ahead of the fork

• Fluoroquinolone antibiotics target DNA gyrases in many

gram-negative bacteria: ciprofloxacin and levofloxacin (Levaquin)

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5’

3’ 5’

3’

Movement of the replication fork

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Movement of the replication fork

RNA primer

Okazaki fragment

RNA primer

5’

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newly synthesized DNA

(Okazaki fragment)5’

3’

5’3’

DNA ligase seals the gap by catalyzing the formation

of a 3’, 5’-phosphodiester bond in an ATP-dependent reaction

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1. Why is replication necessary?

2. When does replication occur?

3. Describe how replication works.

4. Use the complementary rule tocreate the complementary strand:

A---?

G---?

C---?

T---?

A---?

G---?

A---?

G---?

C---?

A---?

G---?

T---?

Replication Quiz

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1. Why is replication necessary?So both new cells will have the correct DNA

2. When does replication occur?During interphase (S phase).

3. Describe how replication works.Enzymes unzip DNA and complementary

nucleotides join each original strand.

4. Use the complementary rule to create the complementary strand:

A---T

G---C

C---G

T---A

A---T

G---C

A---T

G---C

C---G

A---T

G---C

T---A

Replication Quiz

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Components of the replication apparatus

dnaA binds to origin DNA sequence

Primasome

dnaB helicase (unwinds DNA at origin)

dnaC binds dnaB

dnaG primase (synthesizes RNA primer)

DNA gyrase introduces negative supercoils ahead

of the replication fork

Rep protein helicase (unwinds DNA at fork)

SSB binds to single-stranded DNA

DNA pol III primary replicating polymerase

DNA pol I removes primer and fills gap

DNA ligase seals gap by forming 3’, 5’-phosphodiester

bond

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Date post:	18-Jul-2015
Category:	Health & Medicine
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DNA replication by moazzam

Health & Medicine