Genetic modification can be used to produce crops with beneficial characteristics. Describe the roles of restriction enzymes and DNA ligase in the genetic modification of an organism.

Cambridge International IGCSE Biology (0610) — 21.3 Genetic modification · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Restriction enzymes cut DNA at specific recognition sites, allowing the desired gene to be isolated.
The cut DNA has sticky ends, which are short single‑stranded overhangs.
The same enzymes are used to cut the plasmid, giving complementary sticky ends.
DNA ligase then joins the complementary sticky ends of the gene and plasmid, sealing the phosphodiester bonds to form a recombinant plasmid.

Examiner tips

  • Use the term ‘restriction enzymes’ and ‘DNA ligase’ explicitly. Show the sequence of steps: cut → sticky ends → complementary ends → ligation. Mention ‘recombinant DNA’ to demonstrate understanding of the final product.
  • common_mistakes
  • :
  • Confusing restriction enzymes with DNA polymerase. Forgetting that ligase seals the phosphodiester bonds. Using vague terms like ‘cutting’ without specifying sticky ends.

Mark scheme (4 marks)

  1. Restriction enzymes cut DNA at specific sequences to isolate the desired gene
  2. Restriction enzymes produce sticky ends on the isolated gene (and on the cut plasmid)
  3. The same restriction enzymes are used to cut the plasmid, producing complementary sticky ends
  4. DNA ligase joins / seals the complementary sticky ends of the gene and plasmid together to form recombinant DNA / a recombinant plasmid

Key terms in this question

restriction enzymes · DNA ligase

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