Restriction Enzymes and How to Plan a Digest
6 min read · Updated June 8, 2026
Restriction enzymes are molecular scissors that cut DNA at specific sequences, and they remain a workhorse of cloning. This guide covers how they recognise their sites, the ends they leave, and how to plan a digest that gives you the fragments you want.
Recognition sites and cut positions
A restriction enzyme binds a short, usually palindromic recognition sequence — for example EcoRI recognises GAATTC — and cuts at a defined position within or near it. Some enzymes have degenerate recognition sequences described with IUPAC codes (for instance HinfI cuts at G^ANTC, where N is any base).
Sticky ends vs. blunt ends
- Sticky (cohesive) ends: a staggered cut leaves short single-stranded overhangs that base-pair with complementary overhangs, making directional ligation easier.
- Blunt ends: a straight cut leaves no overhang; blunt ligation is more flexible but less efficient and non-directional.
Planning a digest
- Scan your sequence for recognition sites so you know which enzymes cut, where, and how many times.
- Choose enzymes that cut your insert and vector compatibly — ideally leaving matching sticky ends.
- For a double digest, check the two enzymes share a compatible buffer and temperature.
- Predict the fragment sizes you expect to see on a gel before you run it.
Common pitfalls
Watch for enzymes that cut inside your insert (you'll lose or fragment it), star activity under suboptimal conditions, and methylation-sensitive sites that may not cut in DNA from certain hosts. Scanning the sequence first avoids most surprises.
Frequently asked questions
- What is the difference between sticky and blunt ends?
- Sticky ends have short single-stranded overhangs from a staggered cut, which base-pair to aid ligation. Blunt ends come from a straight cut with no overhang and ligate non-directionally.
- How do I know if an enzyme cuts my sequence?
- Scan the sequence for the enzyme's recognition site. A restriction site finder reports every match, its position and the cut site, including degenerate recognition sequences.