1. DNA Replication
Replication is a polymerization reaction and
can be divided into stages of
initiation,
elongation and
termination.

2. ….. complicated process
A supply of energy is required to unwind the helix
The single strands resulting from the unwinding tend to
form intrastrand base pairs
A single enzyme can catalyze only a limited number of physical and
chemical reactions and many reactions are needed in replication.
Several safeguards have evolved that are designed both to prevent
replication errors and to eliminate the rare errors that do occur.
Both circularity and the enormous size of the DNA molecules impose
geometric constraints on the replicative system and
how this fit into the system has to be understood.

3. Models for the replication of DNA

4. Meselson- Stahl Experiment

5. Semidiscontinuous replication

6. …..this model has a paradox,
• cellular DNA replication is semi conservative
• both daughter strands are extended simultaneously
• the strands of the parental duplex are antiparallel
• DNA polymerases extend DNA only in the 5’ → 3’ direction.

8. Mechanism of DNA replication
Initiation

9. Elongation
Polymerase
Poly (nucleotide)n-3’-OH + dNTP ⇔ Poly (nucleotide)n+1-3’-OH + PPi

10. Termination
In a unidirectionally replicating molecule, replication terminates at the origin.
In bidirectionally replicating molecule, it may be of two types:
5. there might be definite termination sequence.
7. two growing points collide and termination occurs where ever the
collision point happens to be.

11. Fidelity of DNA replication
Transition mutations
exchange of one purine, or pyrimidine, for another purine, or pyrimidine
Transversion mutations
exchange of a purine for a pyrimidine or visa versa.

12. 1. Light-dependent repair
DNA photolyase
uses light energy to cleave the covalent cross-link

13. 2. Excision repair
Nucleotide excision repair
Base excision repair

14. Nucleotide excision repair

15. Mismatch repair

16. Post-replication repair
Error prone repair system