Each replication step needs the one before it
- The idea
- Predict
- See it
- Why
- Check
The idea
“Priming, sealing and unwinding can happen in any order, as long as they all happen.”
A list of enzymes is easy to shuffle. Once the helix is open, though, each step works on something an earlier step made.
The short version
The mistake: Priming, sealing and unwinding can happen in any order, as long as they all happen.
What's actually true: Helicase opens the helix, primase lays a primer, DNA polymerase builds from it, the primer is swapped for DNA, then ligase seals.
- Helicase separates the strands. Each one becomes a template whose exposed bases pair with new nucleotides.
- DNA polymerase can only add to an existing end. So primase builds a short RNA primer, and polymerase extends it 5' to 3'.
- Last, the RNA primers are replaced with DNA, and ligase seals the nicks between pieces.
- Across a chromosome, these steps run at once in different places. At any one spot, the order holds.
On the exam: Ask what each enzyme needs first. Primase needs open strands, polymerase a primer, ligase a nick.