DNA replication is an essential process occurring in all living cells. It is required to maintain genetic continuity and inheritance of the genome from parents to offspring. DNA gets duplicated in the replication process. It occurs in the S-phase of the cell cycle, which precedes mitosis. DNA duplication ensures that each daughter cell gets one equal set of DNA after the cell division.
DNA replication is an enzyme catalysed reaction. The two strands of DNA separate and each acts as a template for the synthesis of a complementary strand. DNA replication is a semiconservative process, i.e. each DNA molecule after replication contains one parental DNA and one newly formed strand. Meselson and Stahl provided the experimental proof for semiconservative nature of DNA replication.
DNA replication occurs in three steps:
- Initiation – Double strands of DNA unwind by DNA helicase and formation of replication fork at the origin. RNA primer formation by primase.
- Elongation – DNA polymerase polymerize nucleotides in 5’→3’ direction with complementary base pairing. One strand is replicated continuously, i.e. leading strand and the other is replicated discontinuously called a lagging strand.
- Termination – DNA replication occurs at many points simultaneously so replication forks terminate at many points. Discontinuously synthesized Okazaki fragments are joined by DNA ligase.
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