DNA Replication

DNA to DNA

DNA Replication

Process by which DNA makes a copy of itself

DNA has to be copied before a cell divides

DNA is copied during “synthesis phase” of interphase

Cell cycle

What do we know about DNA?

DNA is a double-stranded

helix

Two strands of DNA are held

together by Hydrogen bonds

Adenine pairs with Thymine

(A-T or T-A), with 2 hydrogen

bonds

Cytosine pairs with Guanine

(C-G or G-C),with 3 hydrogen

bonds

What do we Know about DNA?

DNA molecule has “Direction”

General Concepts of DNA Replication

1. Semi-conservative

2. Starts at the ‘origin’

3. Replication is bidirectional

4. Synthesis always in the 5-3’ direction

5. RNA primers required

6. Replication obeys “complementary

base pairing rule”

1. DNA Replication is Semiconservative

Three possible replication patterns

Semi-conservative

Old OldNew New

Conservative

Both oldBoth new

Dispersive

Old & New Old & New

Meselson and Stahl experiment [1958] demonstrates semiconservative replication is

correct model

The key to their experiment was the use 15N (a heavy

isotope of Nitrogen)

Cells broken open

to extract DNA

E. coli grown in the presence

of 15N (a heavy isotope of

Nitrogen) for many generations

E. coli placed in medium containing

only 14N (a light isotope of Nitrogen)

• Cells get heavy-labeled DNA

Sampled

at:

0 min

1

2

3

40

min

20

min

Suspended DNA in cesium chloride (CsCl) solution.

4

15N medium

CsCl density gradient

centrifugation

5

Result

DNA replication is Semi-conservative

Each new DNA molecule

contains one old strand

&

one new strand

2. Replication “Starts” at the

“Origin”

Special sequence of nucleotides at which replication

begins

At origin replication different enzymes are attached

Prokaryotes have

single origin of

replication

Eukaryotes have

hundreds of origin

of replication

3. Replication is “Bidirectional”

ReplicationFork

Parental DNA Molecule

3’

5’

3’

5’

4. Synthesis of new DNA always

occurs in 5’-3’ direction

DNA PolymeraseNucleotide

5’

5’ 3’5' 3'

3' 5'

5. RNA primer is required

6. Replication Obeys

“Complementary Base Pairing Rule”

Replication based on principle of base pairing is crucial to the high accuracy of the genetic

information transfer

Energy for Replication

energy

ATPGTPTTPCTP

Energy of ReplicationWhere does energy for bonding usually come from?

ADPAMPGMPTMPCMP

modified nucleotide

energy

We comewith our own

energy!

And weleave behind anucleotide!

Youremember

ATP!Are there other ways

to get energyout of it?

Are thereother energynucleotides?

You bet!

Enzymes/Proteins for DNA Replication

Protein RoleDNA helicase Separates 2 DNA strands

Single-strand

binding proteins

Keep the two strands separated

DNA

polymerase

Synthesis of new DNA strand

RNA primase Synthesizes RNA primers

DNA ligase Joins the ends of DNA segments

Topoisomerase Unwind DNA, release stress

DNA ReplicationProcess

Three steps of DNA Replication

1. Initiation

2. Elongation

3. Termination

1. Initiation

Replication begins at a location on double helix called “Ori”

Helicase unwinds (denature) DNA by breaking Hydrogen bonds

Single-stranded DNA-binding proteins (SSBP) attach to separated DNA

strands

2. Elongation1. RNA Primase makes a short RNA primers

2. DNA polymerase (III) adds nucleotides to RNA primer

3. RNA primer is removed by RNaseH remove primer

4. DNA polymerase fills the gaps

5. DNA ligase seals gaps in DNA

3. TerminationTermination of DNA replication occurs

when DNA Polymerase reaches to the end of strands

Prokaryotes

Eukaryotes

DNA Replication Dilemmas

1. Condensed and twisted structure

of DNA

2. DNA Replication is Discontinous

on one strand

3. End problem of DNA replication

DNA Replication Dilemmas

1. Condensed and twisted

structure of DNA

DNA Replication Dilemmas………….

Semi-discontinuous replication

Anti parallel strands replicated simultaneously

Leading strand synthesis continuously in 5’– 3’

Lagging strand synthesis in fragments in 5’-3’

DNA Replication Dilemmas

2. DNA

Replication is

Discontinuous on

one strand

DNA Replication Dilemmas

DNA lagging strand

story

End problem of DNA replication

Telomerase can maintain lost

telomeric DNA

3. End problem of DNA

replication

Fig. 3.15 Synthesis of telomeric DNA by telomerase

THANK YOU