The entire process of DNA replication involves following steps.
(a) Recognition of the initiation point: First, DNA helix unwinds by the enzyme helicase which use the energy of ATP and replication of DNA begin at a specific point, called initiation point or origin where replication fork begins.
(b) Unwinding of DNA: The unwinding proteins bind to the nicked strand of the duplex and separate the two strands at DNA duplex. Topoisomerase helps in unwinding of DNA.
(c) Single stranded binding protein (SSBP): These proteins help in keeping DNA in single stranded position and also known as helix destabilising protein (HDP).
(d) RNA priming: The DNA directed RNA polymerase now synthesises the primer strands of RNA (RNA primer). The priming RNA strands are complementary to the two strands of DNA and are formed of 50 to 100 nucleotides.
(e) Formation of DNA on RNA primers: The new strands of DNA are formed in the 5′ → 3′ direction from the 3′→ 5′ template DNA by the addition of deoxyribonucleotides to the 3′ end of primer RNA. Addition of nucleotide is done by DNA polymerase III. The leading strand of DNA is synthesized continuously in 5′→3′ direction as one piece. The lagging strand of DNA is synthesized discontinuously in its opposite direction in short segments. These segments are called Okazaki fragments.
(f ) Excision of RNA primers: Once a small segment of an Okazaki fragment has been formed. The RNA primers are removed from the 5′ by the action of 5′→3′ exonuclease activity of DNA polymerase I.
(g) Joining of Okazaki fragments: The gaps left between Okazaki fragments are filled with complimentary deoxyribonucleotide residues by DNA polymerase I. Finally, the adjacent 5′ and 3′ ends are joined by DNA ligase.
