Trace each double digest piece back
- The idea
- Predict
- See it
- Why
- Check
The idea
“Both short single digest pieces start from the same end, so I just subtract.”
Two short pieces look like distances from one starting point. Nothing says which end each one is from.
The short version
The mistake: Both short single digest pieces start from the same end, so I just subtract.
What's actually true: Each double digest piece is an end piece already seen in a single digest, or a new piece that lies between the two sites.
- A restriction enzyme cuts at a specific recognition sequence. One cut in linear DNA gives two pieces, the site's distance from each end.
- A single digest can't say which end each short piece is from. Test each arrangement against the double digest.
- An end piece with no other cut in it shows up unchanged. The piece matching neither single digest lies between the sites.
On the exam: Match each double digest piece to a single digest. The odd one out sits between the sites.