DNA Replication

  • DNA is copied semi-conservatively. This means that each old strand of DNA pairs with a strand made from new nucleotides.
  • Replication starts at a fixed point and is bidirectional (replicates in both directions). In Eukaryotic DNA, there are multiple replication forks. Eg. E.Coli:
  • The DNA duplex is opened up and nucleotides read 3′-5′ on the OLD strand.. Eg.
    OLD   3′-ACTACTACTACT-5′
    NEW   5′-TGATGATGATGA-3′
  • This means DNA Polymerases synthesise DNA in the 5′-3′ direction.
  • Replication starts from an existing primer. (A primer is a small oglionucleotide sequence that has been made by Primase.)
  • The addition of a nucleotide to the strand involves the removal of 2 phosphate groups from a deoxynucleotide triphosphate as only 1 phosphate is needed for the backbone. This means the addition units (the deoxynucleotide triphosphates) leave behind a 2 phosphate complex known as a pyrophosphate.

The deoxynucleotide is the nucleotide that attaches to the DNA chain below. The deoxynucleotide molecule can also be called a deoxynucleotide monophosphate, for obvious reasons.

I found the above image on the website of the Chemistry & Biochemistry department at the University of Texas at Austin, US. It shows the old strand (blue) unzipping and then new strands binding to this and forming.

Q: “I thought you said it only synthesised in the 5′-3′ direction! How can both new strands be formed especially when the one on the left seems to be going 3′ -5′?”

A: Easy. It still synthesises 5′-3′, it just synthesises in chunks. Hopefully the image below will explain:

As the double DNA strand unzips, the leading prime is free to synthesise a new chain directly in the 5′-3′ direction. That’s how the DNA polymerase works.

But as it can’t synthesise DNA in the 3′-5′ direction it instead synthesises short 5′-3′ fragments for the lagging phase – these are called okazaki fragments and are later joined by DNA ligase.