For DNA replication to happen
For DNA replication to happen, helicase first unwinds the parental DNA double helix and form a replication bubble consists of 2 bi-directional replication fork. To avoid repairing of the separated DNA, single-strand binding protein binds to each single-strand DNA and holds them apart while they act as a template. Topoisomerase will bind to the DNA strands outside of the replication fork to prevent supercoiling during replication.
Next, primase binds to the template strands to synthesize RNA primer by adding RNA nucleotides. DNA polymerase III then elongates new DNA strand by adding free nucleotides that have complementary base pairings to the growing end of RNA primer. This is called the leading strands. The leading strands are synthesized continuously towards the replication fork using only one RNA primer while the other template is the lagging strand. The lagging strand is synthesized discontinuously, formed in short segments called the Okazaki fragments. The growth of lagging strand is directed away from the replication fork.
DNA polymerase I replaces the RNA primer from the Okazaki fragments with DNA nucleotides in both strands. DNA ligase then joins the DNA fragments by forming phosphodiester bond in between. Hence, two identical copies of the original DNA are formed.