PCR Primer Design & Optimization 2 — Questions and Answers
Question 1: What is the recommended primer length range for standard PCR applications?
- 5–10 nucleotides
- 15–30 nucleotides (Correct answer)
- 35–50 nucleotides
- 55–70 nucleotides
Correct answer: 15–30 nucleotides
Primers of 15–30 nucleotides are long enough to be specific while short enough to anneal efficiently under standard PCR conditions.
Question 2: Which in silico tool is most commonly used to check primer specificity against a genome database?
- ClustalW
- BLAST (Basic Local Alignment Search Tool) (Correct answer)
- Phred/Phrap
- GATK HaplotypeCaller
Correct answer: BLAST (Basic Local Alignment Search Tool)
BLAST aligns the primer sequence against a reference database to identify potential off-target binding sites that could reduce specificity.
Question 3: A hairpin structure in a primer is problematic primarily because it:
- Increases the primer's Tm
- Sequesters the 3' end, preventing template binding and extension (Correct answer)
- Causes the primer to degrade at room temperature
- Increases the rate of nucleotide misincorporation
Correct answer: Sequesters the 3' end, preventing template binding and extension
Intramolecular hairpin loops fold the primer on itself, sequestering the 3' end and preventing it from annealing to and being extended on the template.
Question 4: When designing primers for a region with high GC content, which modification can improve amplification efficiency?
- Adding DMSO (dimethyl sulfoxide) to the PCR reaction (Correct answer)
- Increasing the extension time and adding a GC-rich enhancer
- Reducing the primer concentration to 50 nM
- Using a lower-fidelity polymerase
Correct answer: Adding DMSO (dimethyl sulfoxide) to the PCR reaction
DMSO destabilizes GC-rich secondary structures in the template, improving denaturation and primer access in high-GC regions.
Question 5: Primer concentration in a standard PCR reaction is typically in which range?
- 0.01–0.05 µM
- 0.1–0.5 µM (Correct answer)
- 1–5 µM
- 10–50 µM
Correct answer: 0.1–0.5 µM
Primer concentrations of 0.1–0.5 µM (100–500 nM) provide sufficient primers for amplification without promoting non-specific products or primer-dimer formation.
Question 6: Which characteristic of a primer pair is essential for multiplexed PCR to avoid inter-primer interference?
- Both primers should have identical sequences
- All primers must have compatible Tm values and no cross-complementarity (Correct answer)
- All primers should be the same length
- Each primer pair should target overlapping amplicons
Correct answer: All primers must have compatible Tm values and no cross-complementarity
In multiplex PCR, all primers must share compatible Tm values and show no cross-complementarity to prevent primer-dimers and ensure each target is amplified.
Question 7: To amplify a cDNA template produced by reverse transcription, what additional consideration must primer design account for?
- Primers must span an exon-exon junction to ensure genomic DNA contamination is detectable (Correct answer)
- Primers must be located within intron sequences
- Primer Tm must be reduced by 15°C to accommodate RNA
- Primers must target the poly-A tail exclusively
Correct answer: Primers must span an exon-exon junction to ensure genomic DNA contamination is detectable
Spanning an exon-exon junction ensures that genomic DNA (which retains the intron) produces a larger or absent band, distinguishing cDNA-derived amplicons.
PCR Primer Design & Optimization 2
What is the recommended primer length range for standard PCR applications?