Okazaki Fragments
DNA polymerases replicate DNA by moving along a template strand and synthesising a new complementary strand
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DNA polymerases add new nucleotides by joining the 5’-end of a new nucleotide to the 3’-end of the existing chain
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Hence DNA is synthesised in a 5’ → 3’ direction, as the 3’-terminal is the end that is being extended
Because double stranded DNA is antiparallel, this means that the extension of the two new strands will occur in opposite directions
DNA Directionality
DNA polymerase cannot initiate replication, it can only add new nucleotides to an existing strand
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For DNA replication to occur, an RNA primer must first be synthesised to provide an attachment point for DNA polymerase
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DNA polymerase then adds nucleotides to the 3’ end of a primer, extending the new chain in a 5’ → 3’ direction
Leading versus Lagging Strands
Because double-stranded DNA is antiparallel, DNA polymerase must move in opposite directions on the two strands
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On the leading strand, DNA polymerase is moving towards the replication fork and so can copy continuously
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On the lagging strand, DNA polymerase is moving away from the replication fork, meaning copying is discontinuous
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As DNA polymerase is moving away from helicase, it must constantly return to copy newly separated stretches of DNA
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This means the lagging strand is copied as a series of short fragments (Okazaki fragments), each preceded by a primer
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The primers are replaced with DNA bases and the fragments joined together by a combination of DNA pol I and DNA ligase
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Okazaki Fragments