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2007-2008
DNA Replication
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Double helix structure of DNA
“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Watson & Crick
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Directionality of DNA
You need to number the carbons! it matters!
OH
CH2
O
4
5
3 2
1
PO4
N base
ribose
nucleotide
This isIMPORTANT!!
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DNA replicates 5’ to 3’
OH
O
3
PO4
base
CH2
O
base
OPO
C
O–O
CH2
1
2
4
5
1
2
3
3
4
5
5
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Opposite DNA strands are antiparallel One strands runs 5’ to
3’ while the other is 3’ to 5’
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Bonding in DNA
….strong or weak bonds?How do the bonds fit the mechanism for copying DNA?
3
5 3
5
covalentbonds
hydrogenbonds
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Base pairing in DNA
Purines adenine (A) guanine (G)
Pyrimidines thymine (T) cytosine (C)
Pairing A : T
2 bonds C : G
3 bonds
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Copying DNA
Replication of DNA base pairing allows each
strand to serve as a template for a new strand
DNA replication is semi-conservative = new strand is ½ parent strand and ½ new DNA
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DNA Replication
Large team of enzymes coordinates replication
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DNA Replication
1. Unwind DNA helicase enzyme
unwinds part of DNA helix stabilized by single-stranded binding proteins
single-stranded binding proteins replication fork
helicase
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DNAPolymerase III
DNA Replication
2. Build daughter DNA strand add new
complementary bases DNA polymerase
enzyme
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Bases can only add nucleotides to 3 end of a growing DNA strand therefore new strand only grows 53 direction
DNA Replication
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Limits of DNA polymerase enzyme can only build onto 3 end of an
existing DNA strand
Leading & Lagging strands
5
5
5
5
3
3
3
53
53 3
Leading strand
Lagging strand
Okazaki fragments
Okazaki
Leading strand continuous synthesis
Lagging strand Okazaki fragments
DNA polymerase
3
5
growing replication fork
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3’
5’
3’
5’
5’
3’
3’ 5’helicase
direction of replication
DNA polymerase
Okazaki fragments
leading strand
lagging strand
SSB
DNA polymerase
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DNA polymerases
What we need to know is that a group of enzymes known as DNA polymerases catalyze the reaction that joins nucleotides together with covalent bonds