DNA replication is the process by which a cell makes an identical copy of its DNA. It involves unwinding the double helix at an origin of replication and using each parental strand as a template to synthesize new daughter strands. This results in two identical copies of the DNA molecule. Replication is semi-conservative, meaning each new DNA molecule contains one original parental strand and one newly synthesized strand. It is also bidirectional and semi-discontinuous. The leading strand is copied continuously while the lagging strand is copied discontinuously in fragments that are later joined.

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Fatima Parvez
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2. DNA Replication
 A process in which daughter DNAs are synthesized
using the parental DNAs as template.
 Transferring the genetic information to the
descendant generation.

3. •DNA replication is semi conservative
Each strand of template DNA is being copied.
•DNA replication is semi discontinuous
The leading strand copies continuously
The lagging strand copies in segments (Okazaki
fragments) which must be joined
•DNA replication is bidirectional
Bidirectional replication involves the formation of
replication forks, which move in opposite directions

5. DNA Replication
DNA replication includes:
•Initiation – replication begins at an origin of
replication
•Elongation – new strands of DNA are synthesized by
DNA polymerase
•Termination – replication is terminated differently in
prokaryotes and eukaryotes

6. Prokaryotic DNA Replication
•The chromosome of a prokaryote is a circular molecule
of DNA.
•Replication begins at one origin of replication and
proceeds in both directions around the chromosome.

7. Bidirectional replication of circular DNA molecules

8. Initiation
•The enzymes involved in DNA replication are closely
associated in one large complex called, the replisome.
•The replisome consists of
•the primosome - composed of primase and
helicase
• DNA polymerase III molecules
• DNA gyrase, topoisomerase, SSB, ligase, initiator
proteins
•The replication fork moves in opposite direction,
synthesizing both strands simultaneously.

9. DNA Replication in Bacteria
•To begin DNA replication, unwinding enzymes called
DNA helicases cause the two parent DNA strands to
unwind and separate from one another at the origin of
replication to form two "Y"-shaped replication forks.
•These replication forks are the actual site of DNA
copying.

10. DNA Replication in Bacteria
•Helix destabilizing proteins bind to the single-
stranded regions so the two strands do not rejoin.
•Enzymes called topoisimerases produce breaks in
the DNA and then rejoin them in order to relieve the
stress in the helical molecule during replication.

11. Model for the formation of a replication bubble at a replication origin in
E. coli and the initiation of the new DNA strand

13. Model for the “replication machine,” or replisome, the complex of key
replication proteins, with the DNA at the replication fork

14. •Primase binds to the first priming sequence on the leading strand
template and synthesizes a short RNA primer that is
complementary to the DNA template.
DNA Polymerase III uses the primer to initiate DNA synthesis by
adding deoxyribonucleotides to its 3’ end. The leading strand
requires only one priming event, because DNA synthesis is
continuous thereafter, in the 5’ 3’direction.
Elongation

15. DNA Replication in Bacteria
•As the strands continue to unwind in both directions
around the entire DNA molecule, new complementary
strands are produced by the hydrogen bonding of free
DNA nucleotides with those on each parent strand
•As the new nucleotides line up opposite each parent strand
by hydrogen bonding, enzymes called DNA polymerases
join the nucleotides by way of phosphodiester bonds.

16. DNA Replication by
Complementary Base Pairing

17. DNA Replication in Bacteria
• DNA polymerase enzymes are only able to join the
phosphate group at the 5' carbon of a new nucleotide
to the hydroxyl (OH) group of the 3' carbon of a
nucleotide already in the chain.
•As a result, DNA can only be synthesized in a 5' to 3'
direction while copying a parent strand running in a 3' to
5' direction.

18. DNA Replication in Bacteria
DNA polymerase enzymes cannot begin a new DNA chain from
scratch.
•It can only attach new nucleotides onto 3' OH group of a nucleotide
in a preexisting strand.
•To start the synthesis of the leading strand and each DNA fragment
of the lagging strand, an RNA polymerase complex called a
primosome or primase is required.
•The primase is capable of joining RNA nucleotides without requiring
a preexisting strand of nucleic acid - forms what is called an RNA
primer .

19. RNA primer

20. DNA Replication in Bacteria
•After a few nucleotides are added, primase is replaced by
DNA polymerase.
•DNA polymerase can now add nucleotides to the 3' end of
the short RNA primer.

21. DNA Replication in Bacteria
•The two strands are antiparallel –
•one parent strand - the one running 3' to 5' is called the
leading strand can be copied directly down its entire
length .
•the other parent strand - the one running 5' to 3' is
called the lagging strand must be copied
discontinuously in short fragments called as Okazaki
fragments.
•Okazaki fragments are of around 100-1000
nucleotides each as the DNA unwinds.

22. •For the lagging strand, DNA synthesis is discontinuous
and requires a series of RNA primers. DNA is
synthesised at the 3’ end of each primer, generating an
okazaki fragment that grows untill it meets the adjacent
fragments. The RNA primer is then removed by the 5’ to
3’ exonuclease activity of DNA polymerase I and
replaced with DNA by the polymerase activity of the
same enzyme.

23. DNA ligase links together adjacent Okazaki fragments with
covalent, phosphodiester bonds.
It joins the 5’ phosphate of one DNA molecule to the 3’ OH of
another, using energy in the form of NAD (prokaryotes) or ATP
(eukaryotes). It prefers substrates that are double-stranded, with
only one strand needing ligation, and lacking gaps.