DNA REPLICATION
SBI4U
Ms. Manning
DNA Replication
Produces two identical copies of the
chromosome during S phase of interphase
 Catalyzed by many enzymes including:
DNA polymerase, helicase, ligase and
primase
 Replication takes place at a point where
the DNA double helix separates (called
the replication fork)

STEPS OF DNA REPLICATION
1. UNWINDING AND SEPARATION OF
THE PARENT STRAND – “Initiation”
-separation is initiated at one or more sites
called ORIGINS OF REPLICATION
-the structure of 2 unwound DNA strands is
referred to as a replication fork
Three enzymes work together to
unwind and stabilize the double helix:
A.
B.
C.
DNA HELICASE – breaks the hydrogen
bonds that connect the nitrogenous bases
which allows the double helix to unwind and
separate.
DNA GYRASE – an enzyme that relieves the
tension produced by unwinding of DNA – is
a type II topoisomerase.
SINGLE STRANDED BINDING PROTEINS
(SSBs) – work to keep separated strands of
DNA apart
gyrase
2. DNA SYNTHESIS (“Elongation”
Replication begins in 2 directions from the origins
as a region of DNA is unwound. Replication
proceeds towards the direction of the replication
fork on one strand, and away from the fork on the
other.
 In eukaryotes, more than one replication fork may
exist on a DNA molecule.
 A replication bubble forms when 2 replication
forks are in close proximity to each other

Fig 5.17 p.223
DNA Synthesis cont’d…
DNA POLYMERASE III takes free
nucleotides found within the cell and adds
them in the 5’  3’ direction to form the
new strand
 The parent strand is used as a template
i.e. If A is in the parent, then T is
inserted into the daughter

LEADING STRAND
The daughter strand that grows
continuously towards the replication fork
as the double helix unwinds.
 This occurs quickly
 The new strand grows from 5’  3’

LAGGING STRAND
The other daughter strand that cannot
grow towards the replication fork,
therefore it grows in the opposite
direction
 It is built in short segments (in the 5’  3’
direction) away from the replication fork.
 This is much slower than the leading
strand!

Details about the lagging strand:
The 3’ to 5’ parent strand is a problem for
DNA polymerase since it must synthesize in the
5’  3’ direction
 Short RNA primer sequences of 10-60 RNA
bases are bonded to regions of the lagging
strand with the purpose of initiating DNA
replication
 PRIMASE – an enzyme that binds the RNA
primers to the DNA

Lagging strand cont’d…

DNA polymerase builds short sequences of DNA off
of the RNA primers called OKAZAKI fragments.
This lagging strand…is…slow!

The lagging strand takes longer to
replicate than the leading strand
because it involves more steps and is
not continuous replication.
2b) LINKING OF NITROGENOUS
BASES
Nitrogen bases of nucleotides of opposite
strands (parent and daughter) form new H
bonds
 Once hydrogen bonds are formed, DNA
automatically twists into a double helix

3) Termination
DNA polymerase I excises the RNA primers
and replaces them with the appropriate
deoxyribonucleotides.
 DNA ligase joins the gaps in the Okazaki
fragments by the creation of a phosphodiester
bond.

4) QUALITY CONTROL –
“proofreading”



DNA POLYMERASE III & I – act
as a proof-reader by checking
the newly synthesized strand for
any incorrectly inserted bases.
If a mistake is found,
polymerase acts as an
EXONUCLEASE – cutting out the
mis-paired base and replacing
it with the correct nucleotide.
Errors missed by proofreading
can be corrected by one of
several repair mechanisms after
replication is complete.
What happens at the ends of
chromosomes?
Telomeres
Shrinking
telomeres and
aging