DNA ReplicationReplication is a polymerization reaction andcan be divided into stages ofinitiation,elongation andtermination.
….. complicated processA supply of energy is required to unwind the helixThe single strands resulting from the unwinding tend toform intrastrand base pairsA single enzyme can catalyze only a limited number of physical andchemical reactions and many reactions are needed in replication.Several safeguards have evolved that are designed both to preventreplication errors and to eliminate the rare errors that do occur.Both circularity and the enormous size of the DNA molecules imposegeometric constraints on the replicative system andhow this fit into the system has to be understood.
Models for the replication of DNA
Meselson- Stahl Experiment
…..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.
TerminationIn 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.
Fidelity of DNA replication Transition mutations exchange of one purine, or pyrimidine, for another purine, or pyrimidineTransversion mutations exchange of a purine for a pyrimidine or visa versa.
1. Light-dependent repair DNA photolyase uses light energy to cleave the covalent cross-link
2. Excision repair Nucleotide excision repair Base excision repair