Restriction enzymes: the molecular scissors sheet3-mark answer frameWhat are restriction endonucleases? Explain how EcoRI produces sticky ends and how these help to make recombinant DNA.
How to lay out the answer. Follow the frame, then write it in your own words.
- 1 · Define in one line. Restriction endonucleases are nucleases that cut DNA at specific recognition sequences within the molecule.
- 2 · Give the recognition site. EcoRI, from E. coli strain RY13, recognises the palindrome 5′ GAATTC 3′ / 3′ CTTAAG 5′.
- 3 · Explain the cut. It cuts each strand between G and A, a little away from the centre, leaving single-stranded AATT overhangs: sticky ends.
- 4 · Draw the cut. [Diagram: double strand GAATTC / CTTAAG, cut arrows, two fragments with AATT sticky ends]
- 5 · Explain joining. Vector and foreign DNA cut with the same enzyme have matching sticky ends; they pair by hydrogen bonds and DNA ligase seals them.
- 6 · Close in one line. So restriction enzymes and DNA ligase together make recombinant DNA.
Scoring tip: Write the palindrome with 5′ and 3′ ends on both strands, and mark the cut positions with small arrows.
