DNA Replication
Synthesis Phase (S phase)


S phase in interphase of the cell cycle.
Nucleus of eukaryotes
S
phase
DNA replication takes
place in the S phase.
G1
interphase
Mitosis
-prophase
-metaphase
-anaphase
-telophase
G2
DNA Nucleotide
Phosphate
Group
O
O=P-O
O
5
CH2
O
N
C1
C4
Sugar
(deoxyribose)
C3
C2
Nitrogenous base
(A, G, C, or T)
Figure 11.7 Base Pairing in DNA Is Complementary
DNA Replication
DNA Replication
•
•
DNA must replicate during each cell
division
THREE alternative models for DNA
replication were hypothesized:
•
•
•
Semiconservative replication
Conservative replication
Dispersive replication

Meselson and Stahl’s
experiment proved
replication of DNA to
be semiconservative
- See Practical
- See pages 206-207

Figure 11.8
Watson & Crick predicted
that each DNA strand could
serve as a template for the
replication of a new strand
Q: What is the mode of
replication?
The Mechanism of DNA Replication
•
•
•
•
•
DNA replication is catalyzed by DNA
polymerase
DNA polymerase needs an RNA primer
DNA polymerase adds nucleotides to the 3’
end of the growing strand
Nucleotides are added by complementary
base pairing with the template strand
Figure 11.11
The Mechanism of DNA Replication


Many proteins assist in DNA replication
DNA helicases unwind the double helix, the
template strands are stabilized by other
proteins
Figure 11.16
The Mechanism of DNA Replication




Many proteins assist in DNA replication
DNA helicases
RNA primase catalyzes the synthesis of short
RNA primers, to which nucleotides are
added.
DNA polymerase III extends the strand in
the 5’-to-3’ direction
RNA primase
Figure 11.15
The Mechanism of DNA Replication




DNA synthesis on the leading strand is
continuous. It reads the template in the 3’-to 5’ direction (the same direction as helicase)
The lagging strand grows in the opposite
direction to the Replication Fork. DNA is
always made in the 5’-to-3’ direction.
Therefore, DNA synthesis on the lagging
strand is discontinuous
DNA is created as short fragments (Okazaki
fragments) that are subsequently ligated
together
Figure 11.17
The Mechanism of DNA Replication






Many proteins assist in DNA replication
DNA helicases
RNA primase
DNA polymerase III
DNA polymerase I degrades the RNA primer
and replaces it with DNA
DNA ligase joins the DNA fragments into a
continuous daughter strand
Figure 11.18
Figure 11.8 a
DNA Replication
Animations



http://www.bioteach.ubc.ca/TeachingR
esources/MolecularBiology/DNAReplicati
on.swf
http://bcs.whfreeman.com/thelifewire/c
ontent/chp11/1102002.html
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