The GATE Grind

GATE 2020 BT – Question 37

Recombinant DNA Technology and Other Tools in Biotechnology · Molecular tools: PCR, DNA and RNA labelling, Sanger and next generation sequencing, Southern and northern blotting, in-situ hybridization, DNA fingerprinting · 2 marks · Multiple choice

The DNA below is to be amplified by PCR:

5′ GCTAAGATCTGAATTTTCC…TTGGGCAATAATGTAGCGC 3′

3′ CGATTCTAGACTTAAAAGG…AACCCGTTATTACATCGCG 5′

Which primer pair can be used?

  1. 5′ GGAAATTCAGATCTTAGT 3′ and 5′ TGGGCAATAATGTAGCGC 3′
  2. 5′ GCTAAGATCTGAATTTTC 3′ and 5′ GCGCTACATTATTGCCCA 3′
  3. 5′ CGGAAATTCAGATCTTAG 3′ and 5′ GCGCTACATTATTGCCCA 3′
  4. 5′ GCTAAGATCTGAATTTTC 3′ and 5′ TGGGCAATAATGTAGCGC 3′

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Show answer and explanation

Correct answer: (B) 5′ GCTAAGATCTGAATTTTC 3′ and 5′ GCGCTACATTATTGCCCA 3′

Explanation

DNA polymerase extends from each primer’s 3′ end, so the two primers must face inward on opposite strands. The forward primer matches the left end of the upper strand and anneals to the lower strand. The reverse primer is the reverse complement of the upper strand’s right end, GCGCTACATTATTGCCCA when written 5′→3′. These orientations and sequences identify B.

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