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Genetic Technology: Question 1

Syllabus 19.1

Multiple choice A2 1 mark

Which statement correctly describes the roles of restriction endonuclease and DNA ligase in genetic engineering?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Working through the options

Genetic engineering relies on two enzymes doing precisely opposite jobs, and it is easy to muddle them under exam pressure.

Restriction endonuclease (a restriction enzyme) recognises a specific, short base sequence in DNA and cuts (cleaves) the sugar-phosphate backbone at that sequence. Many restriction endonucleases make a staggered cut, leaving short, single-stranded, unpaired overhangs called sticky ends. Because the recognition sequence is usually palindromic, any DNA cut by the same restriction endonuclease has complementary sticky ends, whether it is the gene of interest or a plasmid vector.

DNA ligase does the opposite job: it joins DNA fragments together. Once two sticky ends have base-paired (annealed) with one another, DNA ligase catalyses the formation of phosphodiester bonds in the sugar-phosphate backbone on both strands, sealing the fragment permanently into the vector and producing a single molecule of recombinant DNA.

Checking each option:

  • A, reverses the two roles; incorrect.
  • B, correctly assigns cutting (at a specific recognition sequence, producing sticky ends) to restriction endonuclease and joining (via phosphodiester bonds) to DNA ligase. Correct.
  • C. Restriction endonucleases cut at specific recognition sequences, not randomly, and DNA ligase does not cut DNA at all; incorrect.
  • D, describes neither enzyme’s actual role; incorrect.

Final answers

  • B, restriction endonuclease cuts DNA at a specific sequence to produce sticky ends; DNA ligase joins fragments by forming phosphodiester bonds.