DNA Replication
What is DNA replication?
When does it happen?
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DNA replication is the process by which
the DNA molecule duplicates itself to
create identical copies during the S phase
prior to cell division – either mitosis or
meiosis.
Semi-Conservative Replication

DNA replication is also called semi-conservative
replication
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One DNA molecule is used as a template to produce two
new, identical molecules.
Thus, each new DNA molecule consists of one “parent”
strand and one newly synthesized strand.
Steps in DNA Synthesis

Anneal-the pairing of complementary strands
of DNA through hydrogen bonding.

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DNA replication requires a number of different
steps, each associated with a different enzyme.
The rules of complementary base pairing (A-T,
G-C) helps explain how DNA is replicated prior to
cell division.
Ensures that DNA REPLICATION occurs rapidly
3. SSB
- Destabilizes helix
- Facilitates unwinding by blocking
H-bonding
- Stability of replication fork
2. Helicase
- Unwinds DNA
1. Gyrase at replication
fork by breaking
- Relieves
pressure in coil the H bonds
caused by
unwinding
- Initiates
unwinding of
DNA
4. RNA Primer,
Primase
- Initiates
complementary chain
Leading
Strand
- Uses
3’ 5’
templat
e as a
guide
growing
5&7 DNA Polymerase
- Constructs growing
complementary chain
Okazaki Fragment
(5’  3’)
- Short fragment of DNA that
is the result of the synthesis
of the lagging strand
Laggin
g
Strand
- Uses
5’3’
templat
e as a
guide
6. DNA Ligase
- joins the gaps
in the Okazaki
Enzyme/Protein
Effect
DNA gyrase
Relieves any tension from the unwinding of the double
helix
Single-stranded binding proteins (SSB)
Keep separated strand of DNA apart by blocking
hydrogen bonding.
helicase
Breaks the hydrogen bonds between the base pairs
primase
Synthesizes RNA primers that will be used by DNA
polymerase as a starting point to build the new
complementary strands
DNA polymerase III
Cannot initiate a new complementary strand by itself
Requires an initial starting 3’ end to start elongation.
Synthesize DNA in the 5’ to 3’ direction. Add free
deoxyribonucletides triphosphates to a 3’ end of the
elongation strand.
Uses the energy from breaking the bonds b/w first and
second phosphate to drive the condensation reaction that
adds the complementary nucleotide.
Act as quality control checker by proofreading the new
strands of DNA
DNA polymerase I
Removes RNA primers from the leading strand and
replaces them with the appropriate DNA nucleotides to
one end of the growing complementary strand of
nucleotides and builds the DNA in a 5’ to 3’ direction
exonuclease
Remove sections of DNA that are incorrectly paired to the
complementary strand. This must be done quickly to
avoid mistakes in the replication of DNA.
ligase
Joins the gaps in the Okazaki fragments by binding the
backbones (phosphate to sugar) of the fragments

A leading strand:
replication proceeds
continuously along it
toward the replication
fork.
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A lagging strand:
Composed of short segments
of DNA, known as Okazaki
fragments, is built
discontinuously away from the
replication fork.
The process occurs in separate
short segments because DNA
polymerase I can only add
nucleotides to the 3’ end of a
growing DNA strand.
The overall direction of growth
of the lagging strand must be
from its 3’ end toward its 5’
end, which is opposite to the
direction of nucleotide addition
by DNA polymerases.
DNA REP-overview.MOV
DNA Replication.MOV