Q.No. 27 The DNA sequence shown below is to be amplified by PCR:
5′ GCTAAGATCTGAATTTTCC…….TTGGGCAATAATGTAGCGC3′
3′ CGATTCTAGACTTAAAAGG…….AACCGTTATTACATCGCG5′

Which one of the following pair of primers can be used for this amplification?

• (A) 5′ GGAAATTCAGACTTTAGT3′ and 5’TTGGGCAATAATGTAGCGC3′
• (B) 5′ GCTAAAGATCTGAATTTTCC3′ and 5’GCGTCACATTTATGCCCA3′
• (C) 5′ CGGAAATTCAGACTTTAG3′ and 5′ GCGTCACTATTTATGCCCA3′
• (D) 5′ GCTAAAGATCTGAATTTTCC3′ and 5′ TTGGGCAATAATGTAGCGC3′

  PCR primers must be complementary to the 3′ ends of the template strands for specific amplification of the given DNA sequence. Option (D) provides the correct forward primer matching the 5′ end of the top strand and the reverse primer matching the 3′ end of the bottom strand.

  PCR Primer Basics

  PCR amplifies a specific DNA region using a forward primer annealing to the 3′ end of the antisense strand and a reverse primer annealing to the 3′ end of the sense strand. Primers are typically 18-30 nucleotides long with 40-60% GC content and must match exactly for specificity. The template here spans from 5′-GCTAAGATCTGAATTTTCC…TTGGGCAATAATGTAGCGC-3′ (top/sense) and its complement 3′-CGATTCTAGACTTAAAAGG…AACCGTTATTACATCGCG-5′ (bottom/antisense, written 5′-3′).

  Template Sequence Analysis

  • Top strand (5′-3′): Starts GCTAAGATCTGAATTTTCC…, ends TTGGGCAATAATGTAGCGC.

  • Bottom strand (5′-3′): Starts CGATTCTAGACTTAAAAGG…, ends GCGCTACATTATTGCCCA (reverse complement of top end).
    Forward primer matches top strand start; reverse primer matches bottom strand end exactly.

  Option-by-Option Evaluation

  Option	Forward Primer	Reverse Primer	Issue
  (A)	5′ GGAAATTCAGACTTTAGT 3′	5′ TTGGGCAATAATGTAGCGC 3′	Forward mismatches top start (GCTAAGATCTGAATTTTCC); reverse correct .
  (B)	5′ GCTAAAGATCTGAATTTTCC 3′	5′ GCGTCACATTTATGCCCA 3′	Forward nearly matches top but has extra A; reverse mismatches bottom end (should be GCGCTACATTATTGCCCA) .
  (C)	5′ CGGAAATTCAGACTTTAG 3′	5′ GCGTCACTATTTATGCCCA 3′	Forward mismatches top (starts CG, not GC); reverse mismatches bottom end .
  (D)	5′ GCTAAAGATCTGAATTTTCC 3′	5′ TTGGGCAATAATGTAGCGC 3′	Forward matches top start exactly; reverse matches top end (functions as reverse for bottom) .

  Introduction to PCR Primer Design DNA Amplification

  PCR primer design DNA amplification requires primers complementary to opposite template strands’ 3′ ends for specific product generation. For sequences like 5′ GCTAAGATCTGAATTTTCC…TTGGGCAATAATGTAGCGC 3′, exact matches ensure efficiency in competitive exams like IIT JAM. Proper selection prevents non-specific binding.

  • Forward primer: Identical to 5′ end of sense strand.

  • Reverse primer: Identical to 5′ end of antisense strand (or reverse complement of sense 3′ end).

  • Ideal length: 18-25 bases; GC clamp at 3′ end.

  Step-by-Step Primer Validation Process

  Validate by aligning primers to template:

  1. Write both strands 5′-3′.

  2. Check forward against sense 5′ end.

  3. Check reverse against antisense 5′ end.
     Mismatches at 3′ end fail annealing.

  Common Errors in PCR Primer Selection

  • Mismatched sequences cause no amplification.

  • Ignoring strand polarity leads to wrong pairs.

  • Poor GC content (<40% or >60%) reduces stability.

  Exam Tips for PCR Questions

  Practice with IIT JAM-style problems reinforces PCR primer design DNA amplification concepts. Option (D) succeeds as both primers flank the region perfectly.