DNA REPLICATION

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DNA REPLICATION
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Fig. 16-9-3
A
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C
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G
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C
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G
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G
(c) Daughter DNA molecules, each consisting of
one parental strand and one new strand
(b) Separation of strands
(a) Parent molecule
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DNA Replication is semi-conservative

• each replicated DNA molecule consists of one
  old and on new strand.

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Getting Started

• Replication begins at special sites called
  origins of replication, where the two DNA strands
  are separated, opening up a replication bubble
• A eukaryotic chromosome may have hundreds or even
  thousands of origins of replication
• Replication proceeds in both directions from each
  origin, until the entire molecule is copied

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Fig. 16-12a
Origin of replication
Parental (template) strand
Daughter (new) strand
Replication fork
Double-stranded DNA molecule
Replication bubble
0.5 µm
Two daughter DNA molecules
(a) Origins of replication in E. coli
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Fig. 16-12b
Origin of replication
Double-stranded DNA molecule
Parental (template) strand
Daughter (new) strand
0.25 µm
Replication fork
Bubble
Two daughter DNA molecules
(b) Origins of replication in eukaryotes
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1. Helicase breaks hydrogen bonds between
bases,unzips and unwinds the helix
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• Helicase
• Is an enzyme (a protein that speeds up chemical
  reactions)
• Is made during G1
• Begins to unwind the DNA at the ORIGIN OF
  REPLICATION (a specific nucleotide sequence)

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Helicase enzymes move in both directions from the
Point of Origin, forming a REPLICATION BUBBLE.
At either end of the replication bubble is a
REPLICATION FORK, a Y-shaped region where the
new strands of DNA are elongating.
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2. Single stranded binding proteins hold the DNA
strands apart

• Keeps the separated strands apart and stabilize
  the unwound DNA

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• RNA nucleotides bind with complementary base
  sequences under the direction of RNA primase.
  These RNA nucleotides act as a primer for DNA
  nucleotides.
• Primers are short segments of RNA, about 10
  nucleotides long
• Must have a primer because DNA polymerase can
  only add nucleotides to another nucleotide

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Primase
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4. DNA polymerase III adds DNA nucleoside
triphosphates to the RNA primer sequence in a 5
to 3 direction.

Two of the phosphates are stripped off in the
bonding
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• New nucleotides can only be added to the 3 end
  of a growing DNA chain
• So we say DNA grows 5 to 3

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Leading Strand

• DNA polymerase III can synthesize a complementary
  strand on one side of the template in the 5 to
  3 direction with no problem.

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What about the other strand??
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3
DNA Polymerase III Can only add to this side
AWAY from the replication fork
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Lagging Strand

• DNA polymerase III must work away from the
  replication fork.
• Makes a short strand of DNA, called an Okazaki
  fragment.
• As the bubble widens, it can make another short
  strand, and so on.

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(No Transcript)
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1. RNA primers are removed and replaced with DNA
   nucleotides by DNA Polymerase I.

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• Along the lagging strand the Okazaki fragments
  are joined by DNA Ligase to form a single DNA
  strand.

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1. Proofreading by DNA Polymerase III and I occurs,
   and replication is complete.

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The Animation
http//207.207.4.198/pub/flash/24/menu.swf
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Fig. 16-13
Add onto the 3 side (synthesized 5?3)
Primase
Single-strand binding proteins
3?
Topoisomerase
5?
3?
RNA primer
5?
5?
3?
Helicase
Animation DNA Replication
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Fig. 16-15
Overview
Origin of replication
Leading strand
Lagging strand
Primer
Leading strand
Lagging strand
Overall directions of replication
Origin of replication
3?
5?
RNA primer
5?
Sliding clamp
3?
5?
DNA poll III
Parental DNA
3?
5?
5?
3?
5?
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Fig. 16-16
Overview
Origin of replication
Lagging strand
Leading strand
Lagging strand
2
1
Leading strand
Overall directions of replication
5?
3?
3?
5?
Template strand
RNA primer
3?
5?
3?
1
5?
3?
Okazaki fragment
5?
3?
1
5?
5?
3?
3?
2
5?
1
5?
3?
3?
5?
1
2
5?
3?
3?
5?
1
2
Overall direction of replication
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Table 16-1
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Fig. 16-17
Overview
Origin of replication
Lagging strand
Leading strand
Leading strand
Lagging strand
Single-strand binding protein
Overall directions of replication
Helicase
Leading strand
DNA pol III
5?
3?
3?
Primer
Primase
5?
Parental DNA
3?
Lagging strand
DNA pol III
5?
DNA pol I
DNA ligase
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3?
5?
3
1
2
3?
5?
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Fig. 16-UN3
DNA pol III synthesizes leading strand
continuously
3?
5?
Parental DNA
DNA pol III starts DNA synthesis at 3? end of
primer, continues in 5? ? 3? direction
5?
3?
5?
Lagging strand synthesized in short Okazaki
fragments, later joined by DNA ligase
Primase synthesizes a short RNA
primer
3?
5?
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Fig. 16-UN5
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DNA replication is ...

• Accurate
• Only 11,000,000,000 nucleotides are incorrectly
  paired

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DNA replication is ...

• Extremely rapid
• In prokaryotes, up to 500 nucleotides are added
  per second
• 50 per second in eukaryotes

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Other Good Animations

• http//www.ncc.gmu.edu/dna/repanim.htm
• http//www.stolaf.edu/people/giannini/flashanimat/
  molgenetics/dna-rna2.swf
• http//www.wiley.com/college/pratt/0471393878/stud
  ent/animations/dna_replication/index.html