The elongation phase of replication consists of two seemingly similar operations that are mechanically quite distinct: Leading strand synthesis and Lagging synthesis. Several enzymes at the replication fork are important to the synthesis of both strands. “DNA helicases” unwind the parental DNA. “DNA topoisomerases” relieve the tropological stress included by the “Helicases”, and “SSBP” stabilizes the separated strands.
ReplicationFork
a) Leading strand synthesis:
It begins with the synthesis by “Primase” of a short (10 to 60 nucleotide) RNA primer at the replication origin. Deoxyribonucleotides are then added to this primer by DNA polymerase-III once begun, leading stand synthesis proceeds continuously, keeping pace with the replication fork.
b) Lagging strand synthesis:
It must be accomplished in short fragments (Okaazakki fragments) synthesized in the direction opposite to the fork movement. Each fragment must have its own RNA primer synthesized by “primase”, and positioning of the primers must be controlled and coordinated with fork movement. The regulatory apparatus for “lagging strand synthesis” is a traveling protein machine called a “ PRIMOSOME”, which consists of several different proteins including the Dna.B protein, Dna.C protein and Primase.
The primosome moves along the lagging strand template in the 5’à3’ direction, keeping pace with the replication fork. As it moves the primosome at intervals compels primase to synthesize a short (10 to 60) residues. RNA primer to which DNA is then added by DNA polymerase-III.
The direction of the synthetic reactions of “Primase” and “Polymerase-III” is opposite to the direction of primosome movement. When the new okazakii fragment is complete, the RNA primer is removed by DNA polymerase-I and is replaced with DNA by the same enzyme. The remaining nick is sealed by “DNA ligase”.