Student

1. AP Biology 2007-2008
DNA Replication
Ch.12.2

2. AP Biology
DNA Replication
 Purpose: cells need to make a copy of
DNA before dividing so each daughter
cell has a complete copy of genetic
information
 3 proposed Models of Replication

3. AP Biology
Meselson and Stahl Experiment

4. AP Biology
Semi-Conservative Model
 Replication of DNA
 base pairing allows
each strand to serve
as a template for a
new strand
 new strand is 1/2
parent template &
1/2 new DNA

5. AP Biology
Anti-parallel strands
 Nucleotides in DNA
backbone are bonded from
phosphate to sugar
between 3 & 5 carbons
 DNA molecule has
“direction”
 complementary strand runs
in opposite direction
THIS WILL CAUSE A
PROBLEM FOR
REPLICATION
3
5
5
3

6. AP Biology
Bonding in DNA
….strong or weak bonds?
How do the bonds fit the mechanism for copying DNA?
3
5 3
5
covalent
phosphodiester
bonds
hydrogen
bonds

7. AP Biology
DNA Replication
 Large team of enzymes coordinates replication

8. AP Biology
Replication: 1st step
 Unwind DNA
 helicase enzyme
 unwinds part of DNA helix
 stabilized by single-stranded binding proteins
 PREVENTS DNA MOLECULE FROM CLOSING!
 DNA gyrase
 Enzyme that prevents tangling upstream from the replication
fork
single-stranded binding proteins replication fork
helicase gyrase

9. AP Biology
Replication: 2nd step
 RNA Primase
 Adds small section of RNA (RNA primer) to the
3’ end of template DNA
 Why must this be done?
 DNA polymerase 3 (enzyme that builds new DNA
strand) can only add nucleotides to existing strands
of DNA

10. AP Biology
DNA
Polymerase III
Replication: 3rd step
 Build daughter DNA
strand
 add new
complementary bases
 With the help of the
enzyme DNA
polymerase III

11. AP Biology
Replication: 4th step
 Replacement of RNA primer by DNA
 Done by DNA polymerase I

12. AP Biology
Limits of DNA polymerase III
 can only build onto 3 end of
an existing DNA strand
Leading & Lagging strands
5
5
5
5
3
3
3
5
3
5
3 3
Leading strand
Lagging strand
ligase
Okazaki
Leading strand
 continuous synthesis
Lagging strand
 Okazaki fragments
 joined by ligase
 “spot welder” enzyme
DNA polymerase III


3
5
growing
replication fork

13. AP Biology
DNA polymerase III
RNA primer is added
 built by primase
 serves as starter sequence for DNA polymerase III
HOWEVER short segments called Okazaki fragments
are made because it can only go in a 5 3 direction
DNA replication on the lagging strand
5
5
5
3
3
3
5
3
5
3 5 3
growing
replication fork
primase
RNA

14. AP Biology
NEXT DNA polymerase I
 removes sections of RNA
primer and replaces with
DNA nucleotides
STRANDS ARE GLUED
TOGETHER BY DNA LIGASE
Replacing RNA primers with DNA
5
5
5
5
3
3
3
3
growing
replication fork
DNA polymerase I
RNA
ligase

15. AP Biology
Replication fork
3’
5’
3’
5’
5’
3’
3’ 5’
helicase
direction of replication
SSB = single-stranded binding proteins
primase
DNA
polymerase III
DNA
polymerase III
DNA
polymerase I
ligase
Okazaki
fragments
leading strand
lagging strand
SSB