Question

Explain how the synthesis of the lagging strand occurs in DNA replication.

Answer 1

*Core’ factors for synthesis and maturation of lagging strands
The protein factors required for synthesis of lagging strands include Pol α-primase, Pol δ, PCNA, RFC, RPA, Fen1 (5’ to 3’ exonuclease or MF1, maturation factor 1), RNase H, and DNA ligase 1.
Among them, the two nucleases Fen1 and RNase H were shown to have roles in the removal of primer RNA of Okazaki fragments.

*Lagging Strand Elongation:-

Unlike leading strands, lagging strands are synthesized as discrete short DNA fragments, termed ‘Okazaki fragments’ which are later joined to form continuous duplex DNA. Synthesis of an Okazaki fragment begins with a primer RNA-DNA made by polymerase (Pol) α-primase. The synthesis of RNA portion (~ 10 to 15 ribonucleotides) and subsequent extension of short (~20 to 30 nucleotides, nt) DNA are coupled. The recognition of a primer RNA-DNA by the Replication-Factor C (RFC) complex leads to dissociation of Pol α-primase and loading of proliferating cell nuclear antigen (PCNA), resulting in recruitment of Pol δ to the primer-template junction, a process called ‘polymerase switching.’ Then the primer RNA-DNA is elongated by Pol δ. When Pol δ encounters a downstream Okazaki fragment, it displaces the 5’ end region of the Okazaki fragment, generating a single-stranded (ss) nucleic acid flap. The flaps formed can be efficiently processed by the combined action of Flap endonuclease 1 (Fen1) and Dna2 to eventually create nicks. The nicks are finally sealed by DNA ligase 1 to complete Okazaki fragment processing.