Mixing up helicase, topoisomerase and ligase
- The idea
- Predict
- See it
- Why
- Check
The idea
“Helicase unwinds DNA, so it also undoes the twisting. Ligase takes out the primers.”
These three enzymes work on the same stretch of DNA, and their jobs sound alike. It's easy to give one its neighbor's job.
The short version
The mistake: Helicase unwinds DNA, so it also undoes the twisting. Ligase takes out the primers.
What's actually true: Helicase splits base pairs at the fork. Topoisomerase releases twist ahead of it. Ligase seals breaks behind it.
- Topoisomerase releases twist by cutting and rejoining the backbone. It never separates base pairs.
- DNA polymerase can only add to an existing 3' end, so primase lays an RNA primer first.
- The lagging strand needs a new primer every so often, so it's built in short Okazaki fragments.
- Polymerase later swaps each primer for DNA, and ligase seals the break left behind.
On the exam: Ask where each one works. Helicase is at the fork, topoisomerase is ahead of it, and ligase is behind.