DNA Replication

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DNA replication is the process by which DNA is copied and occurs in the S phase of interphase (part of the cell cycle)

It involves unwinding and separating the double stranded molecule, before synthesising complementary strands to the separated templates

DNA replication is a semi-conservative process, because when a new double-stranded molecule is formed:

  • One strand will be from the original molecule
  • One strand will be newly synthesised


DNA Replication is a Semi-Conservative Process


Stages of DNA Replication


1.  Separation of DNA strands

  • Helicase unwinds and separates the double stranded DNA by breaking the hydrogen bonds between base pairs
  • DNA gyrase moves ahead of helicase and reduces the torsional strain created by the subsequent unwinding of the double helix


2.  Synthesis of an RNA primer

  • RNA primase synthesises a short RNA primer on each template strand to provide an attachment and initiation point for DNA polymerase


3.  Copying of the template strands 

  • DNA polymerase adds nucleotides to the 3' end of the polynucleotide chain, synthesising in a 5' - 3' direction
  • The nucleotides pair up opposite their complementary base partner
    • Adenine and thymine pair via two hydrogen bonds
    • Cytosine and guanine pair via three hydrogen bonds
  • DNA polymerase moves in opposite directions on the two anti-parallel strands 
    • Synthesis is continuous on the strand moving towards the replication fork (leading strand) 
    • Synthesis is discontinuous on the strand moving away from the replication fork (lagging strand) leading to the formation of Okazaki fragments


Okazaki Fragments occur on the lagging strand because DNA polymerase is moving in the opposite direction to helicase


4.  Removal and replacement of RNA primer

  • The lagging strand requires the addition of multiple RNA primers to generate Okazaki fragments - these must be removed and replaced
  • DNA polymerase removes the RNA primers and replaces them with DNA
  • DNA ligase joins the Okazaki fragments together to create a continuous strand


Overview of DNA Replication


Origins of Replication

  • Because eukaryotic genomes are (typically) much larger than prokaryotic genomes, DNA replication is initiated at many points simultaneously in order to limit the time required for DNA replication to occur
  • The specific sites at which DNA unwinding and initiation of replication occurs are called origins of replication and form replication bubbles
  • As replication bubbles expand in both directions, they eventually fuse together, two generate two separate semi-conservative double strands of DNA

Origins of Replication