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AP Biology 2007-2008
DNA ReplicationCh.12.2
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
AP Biology
Meselson and Stahl Experiment
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
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 directionTHIS WILL CAUSE A
PROBLEM FOR REPLICATION
3
5
5
3
AP Biology
Bonding in DNA
….strong or weak bonds?How do the bonds fit the mechanism for copying DNA?
3
5 3
5
covalentphosphodiester
bonds
hydrogenbonds
AP Biology
DNA Replication Large team of enzymes coordinates replication
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
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
AP Biology
DNAPolymerase III
Replication: 3rd step Build daughter DNA
strand add new
complementary bases With the help of the
enzyme DNA polymerase III
AP Biology
Replication: 4th step Replacement of RNA primer by DNA Done by DNA polymerase I
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
53
53 3
Leading strand
Lagging strand
Okazaki fragments
ligase
Okazaki
Leading strand continuous synthesis
Lagging strand Okazaki fragments joined by ligase
“spot welder” enzyme
DNA polymerase III
3
5
growing replication fork
AP Biology
DNA polymerase III
RNA primer is added built by primase serves as starter sequence for DNA polymerase IIIHOWEVER 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 53 5 3
growing replication fork
primase
RNA
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
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
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