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DNA Replication

Unit 4: Heredity

Topic B

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DNA Replicates before Cell Division

The DNA in cells needs to replicate before cell division (mitosis or meiosis)

The process of DNA replication takes place in two parts, at the same time:

  1. On the leading strand
  2. On the lagging strand

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The Leading and Lagging Strands

REMEMBER: the two strands of the DNA double helix run in opposite directions

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The leading strand runs in the 5’ to 3’ direction

  • DNA is read and synthesized from the 5’ to 3’ direction
  • The leading strand is the easy strand when it comes to DNA replication

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The lagging strand runs in the 3’ to 5’ direction

  • The lagging strand is more complicated when it comes to DNA replication
  • Replicating the lagging strand takes many more steps than the leading stand

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Steps of DNA Replication

  1. An enzyme called helicase unzips the DNA double helix
  2. REMEMBER: hydrogen bonds between nitrogenous bases are weak and are easily broken by helicase

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The place where the DNA splits into two strands is called the replication fork

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Before either strand can be replicated, the strands must be “prepped” by a new enzyme

  1. Primase is an RNA molecule that places small segments of nucleotides on each strand

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  • An enzyme called DNA polymerase latches onto the RNA segments that primase created
  • DNA polymerase begins attaching complementary nucleotides to each strand

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On the leading strand, DNA polymerase attaches nucleotides in a row, from 5’ to 3’ direction

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The lagging strand runs in the opposite direction and must be copied in segments called Okazaki fragments

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  1. Okazaki fragments are joined together by an enzyme called ligase

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Amoeba Sisters break down DNA Replication