DNA Replication show 3

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 Replication - 4 Step Process
 Initiation – unwinding and unzipping.
 Elongation – addition of new nucleotides.
 Termination – The completion of addition.
 Proofreading and Correction – A second
polymerase re-reads the strand looking for errors
to fix.
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 INITIATION
• Many proteins assist in DNA replication
• DNA helicases unwind the double helix, the
template strands are stabilized by other proteins
The Mechanism of DNA Replication –
ELONGATION
• DNA replication is
catalyzed by DNA
polymerase
• DNA polymerase needs an
RNA primer --- WHY??
• DNA polymerase needs to
add nucleotides to the 3’
end of a growing strand of
DNA.
The Mechanism of DNA Replication –
ELONGATION
• Nucleotides are added by complementary
base pairing rules by reading the template
strand
• *** DNA Polymerase can ONLY read a DNA in
the 3’ to 5’ direction, because the polymerase
can ONLY build in the 5’ to 3’ direction ***
Figure 11.11
Antiparallel??
• Since the original DNA template strands run in
opposite directions, both sides of the DNA
cannot be created in the same way.
• NOTE:
• Leading Strand – follows the helicase protein.
• Lagging Strand – reads in the opposite
direction as the helicase protein.
Figure 11.16
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’-to3’ 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 –
Proofreading and Correction
•
•
•
•
•
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
Termination – DNA Recoils
Figure 11.8 a
DNA Replication
Animations
• http://www.bioteach.ubc.ca/TeachingResourc
es/MolecularBiology/DNAReplication.swf
• http://bcs.whfreeman.com/thelifewire/conten
t/chp11/1102002.html