DNA replication is the process of creating two identical DNA replicas from one original strand, occurring during the S phase of interphase. The process is semiconservative, involving parental and newly synthesized strands, and requires various enzymes such as DNA polymerase, helicase, and ligase. Replication begins at origins of replication and includes initiation, elongation with leading and lagging strands, and termination when replication forks meet.

1. DNA REPLICATION
Dr Rama Kant Upadhyay

2. Definition
 DNA replication is the process of producing two identical replicas of DNA from one
DNA molecule.
 DNA replication occurs during the S Phase of the Interphase of cell cycle.
Parent DNA
Daughter DNA

3. DNA Replication is Semiconservative
Messelson & Stahl Experiment
Newly synthesized DNA contains one old strand and one new strand of DNA.
Parental strand
Parental strand
New Strand
used E. coli bacteria to study DNA replication

4.  Cultured E.coli in a medium constituting 15NH4Cl over many generations.
 Due to this, 15N was integrated into the bacterial DNA.
 Later replace the medium with normal 14NH4Cl.
 After 20 minutes the sample was subjected to CsCl-based centrifugation and observed
that bacterial DNA showed an intermediate density (15 N14N)
 In the next cycle (after 40 minutes), the sample contained DNA with both intermediate
(15N14N) and light densities (14N 14N)in the same proportion.

5. ENZYMES ROLES
DNA POLYMERASE POLYMERIZATION
HELICASE UNWINDING OF DNA HELIX ( DENATURATION)
SSB (SNGLE STRAND BINDING PROTEIN) STABILIZES SINGLE STRANDED DNA
TOPOISOMERASE/ GYRASE REDUCE SUPERCOILING IN DNA
PRIMASE SYNTHESIS OF PRIMEERS
DNA LIGASE BINDING OF DNA STRANDS NICK
Enzymes involved in DNA Replication

6. Steps
Initiation
Elongation
Termination
Replication begins at specific sites on the DNA called origins
of replication (Ori)
Polymerase enzymes synthesize the complementary
sequence in each of the strands as they separate
the lagging strand is the strand of daughter DNA that is
synthesized discontinuously
Termination of replication occurs in different ways in
different organisms

7. Initiation
• DNA Helicase enzyme cleavage on both strands of DNA at the origin of the replication
site.
• Breaking of H- bond between nitrogenous base.
• Topoisomerase reduces supercoiling.
• Formation of two ss DNA.
• Single-stranded binding protein (SSB) stabilizes both strands of DNA.

8. REPLICATION FORK

10. Elongation
• Primase enzyme synthesizes primers.
• DNA polymerase added dNTPs (as per complementary base pairing) on 3’ OH end of
primers.
• Direction- 5’ 3’
• Leading strand synthesis (Continuous)
• Lagging strand synthesis (Discontinuous) – Okazaki synthesis

11. POLYMERIZATION

12. Ligation
Joining of two nucleotides
3’OH and 5 ‘ Phosphate group

13. On the leading strand, DNA synthesis occurs continuously.
On the lagging strand, DNA synthesis restarts many times as
the helix unwinds, resulting in many short fragments called
“Okazaki fragments.”
DNA ligase joins the Okazaki fragments together into a single
DNA molecule

14. • The lagging strand is synthesized discontinuously by DNA polymerase in sections
called Okazaki fragments.
• These fragments are later connected by DNA ligase to form a complete
complementary strand.
These are short DNA nucleotide sequences are discontinuously synthesized
discovered in the 1960s by the Japanese molecular biologists Reiji and Tsuneko Okazaki
Okazaki fragments

15. Termination
• Termination of DNA replication occurs when the
two forks meet and fuse.