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2007-2008
DNA Replication
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
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!!
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
Opposite DNA strands are antiparallel One strands runs 5’ to
3’ while the other is 3’ to 5’
Bonding in DNA
….strong or weak bonds?How do the bonds fit the mechanism for copying DNA?
3
5 3
5
covalentbonds
hydrogenbonds
Base pairing in DNA
Purines adenine (A) guanine (G)
Pyrimidines thymine (T) cytosine (C)
Pairing A : T
2 bonds C : G
3 bonds
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
DNA Replication
Large team of enzymes coordinates replication
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
DNAPolymerase III
DNA Replication
2. Build daughter DNA strand add new
complementary bases DNA polymerase
enzyme
Bases can only add nucleotides to 3 end of a growing DNA strand therefore new strand only grows 53 direction
DNA Replication
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
3’
5’
3’
5’
5’
3’
3’ 5’helicase
direction of replication
DNA polymerase
Okazaki fragments
leading strand
lagging strand
SSB
DNA polymerase
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