2007-2008 DNA Replication Double helix structure of DNA “It has not escaped our notice that the...

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