DNA replication is the process where a DNA molecule makes an identical copy of itself. There are three proposed models of replication: conservative, dispersive, and semi-conservative. The widely accepted semi-conservative model involves unwinding the DNA double helix, attaching primers to each strand, and synthesizing new complementary strands in the 5' to 3' direction along each template strand. This results in two double-stranded DNA molecules each with one original strand and one new strand.

1. DNA Replication
Instructor: Halid Juma

2. Meaning of DNA replication
• Is the process by which the exact copy of DNA
molecule is produced by the old DNA
molecule and thus the genetic information it
contains is duplicated.
• Assignment: Study on models of DNA
replication and state with reasons which one
is acceptable.
Conservative replication model
Dispersive replication model
Semi-conservative replication model

3. Mechanism of DNA replication
• According to the semi-conservative model
which is widely accepted hypothesis of DNA
replication, DNA replication takes place
through the following stages:
• 1st stage: Unwinding or unzipping of the two
intertwined anti-parallel double strands of the
DNA.
DNA replication starts with the
unwinding or unzipping of the two
intertwined anti-parallel double strands
of the DNA by the enzyme DNA helicase.

4. Cont….
This enzyme untwists the helices at locations
called replication origins and breaks the weak
hydrogen bonds that hold the two strands of
DNA together.
The separation of the two single strands of
DNA creates a “Y” shape called a “replication
fork”.
Each of the two separated strands will now act
as a template to which a complimentary set of
nucleotides would attach (base pairing) for
making the new strands of DNA.

5. Cont…..
Because the two strands of the DNA are
always anti-parallel, then one of the strands is
oriented in the 3’ to 5’ direction (towards the
replication fork), this is the leading strand
whereas the other strand is oriented in the 5’
to 3’ direction (away from the replication
fork), this is the lagging strand. Unfortunately,
DNA polymerase III can work only in a 5’ to 3’
direction.

6. Cont….
• 2nd stage: Replication of each of the parental
strands (one being a new strand and another
old).
• As a result of their different orientations (anti-
parallelism), the two DNA strands are
replicated differently as follows:
• A: Replication of the leading strand.
Formation of a short piece of RNA called a
primer (produced by RNA polymerase enzyme
called primase).

7. Cont….
Then, a primer comes along and binds to the
end of the leading strand. This is necessary
because the enzyme DNA polymerase III
cannot initiate the synthesis of new DNA
strands without a primer. The primer acts as a
starting point for new DNA-strand synthesis.

8. Cont….
An enzyme DNA polymerase III then binds to
the leading strand and then ‘walk’ along it,
adding new complimentary nucleotide bases
(A, C, G and T) to the strand of DNA in the 5’
to 3’ direction (i.e. the template DNA strand
must be read in the 3’ to 5’ direction). This
process is straight-forward for the leading
strand, and DNA synthesis can proceed in an
uninterrupted manner in the entire length of
the leading strand hence called continuous
replication.

9. Cont….
• B: Replication of the lagging strand
However, DNA synthesis cannot proceed
uninterrupted on the other template strand,
called the lagging strand because DNA
polymerase III must as well move in the 5’ to
3’ direction, on the lagging strand (i.e. the
enzyme must move away from the replication
fork).

10. Cont….
But if the enzyme moves away from the
replication fork, and the fork is uncovering
new DNA that needs to be replicated, then
DNA synthesis or replication on the lagging
strand is discontinuous, occurring in short
sections, and produces short fragments or
chunks (about 100-200 nucleotides in length)
of DNA called Okazaki fragments, named after
their discoverer who identified them. (Ogawa
and Okazaki 1980).

11. Cont….
Therefore, numerous RNA primers are made
by the primase enzyme and bind at various
points along the lagging strand and each of
the primer becomes the starting point for
each Okazaki fragment.

12. Cont….
While each individual segment is replicated
away from the replication fork, each
subsequent Okazaki fragment is replicated
more closely to the receding replication fork
than the fragment before. This type of
replication is called discontinuous replication
as the Okazaki fragments will need to be
joined up later.

13. Cont….
An illustration to show replication of the leading and lagging strands of DNA

14. Cont…..
• 3rd stage: Removal of the primers and filling
the gaps previously occupied by the primers
Once all of the bases are matched up (A-T and
C-G), an enzyme called DNA polymerase I
strips away the primers and it fills the gaps
where the primers were with complimentary
nucleotides.

15. Cont….
• 4th stage: Proofreading and correcting or
repairing in case of any mistake
The new strand is proofread by enzymes DNA
polymerase III and DNA polymerase I to make
sure there are no mistakes in the new DNA
sequence. They also exercise and repair any
incorrect base pairing

16. Cont…..
• 5th stage: Ligation of the Okazaki fragments
and zipping up of both pairs of the DNA
strands
Finally, an enzyme called DNA ligase joins or
stitches the sugar-phosphate backbones of
the Okazaki fragments to create a continuous
strand of DNA and it also seals up the
sequence of DNA into two continuous double
strands by catalyzing the construction of the
hydrogen bonds between the complementary
base pairs of the DNA strands.

17. Cont…..
The result of DNA replication is two DNA
molecules each consisting of one new and one
old chain of nucleotides (i.e. each DNA
molecule constructed, one old strand is
retained or conserved). This is why DNA
replication is described as semi-conservative.

18. Cont….