A circle gives one piece per cut
- The idea
- Predict
- See it
- Why
- Check
The idea
“Every cut adds a piece, so a plasmid cut once should give 2 fragments.”
Most DNA is drawn as a straight line, and a line cut once makes 2 pieces. A plasmid is a closed loop, like a rubber band.
The short version
The mistake: Every cut adds a piece, so a plasmid cut once should give 2 fragments.
What's actually true: A circular molecule gives as many fragments as cuts. A linear one gives one more fragment than cuts.
- Restriction enzymes cut DNA at specific sequences called restriction sites. A gel sorts the fragments by size, with one band for each size.
- A line starts with 2 ends. Each cut adds one more piece, so n cuts give n + 1 fragments.
- A plasmid has no ends. The first cut opens it and each later cut adds a piece, so n cuts give n fragments. In a full digest, one full-length band means one cut.
- The fragment sizes add up to the full length. On a plasmid map, one fragment usually crosses the point where the numbering starts. That point is a label, not an end.
On the exam: Count the ends first. A circle has none, so pieces equal cuts. A line has two, so it gets one extra piece.