PCR PCR Reagents & Consumables Management 1 — Questions and Answers
Question 1: What is the primary reason PCR-grade water (nuclease-free water) must be used instead of regular distilled water in PCR reactions?
- To eliminate DNase and RNase that would degrade template and primers (Correct answer)
- To increase ionic strength of the reaction
- To improve thermostability of Taq polymerase
- To lower the melting temperature of the amplicon
Correct answer: To eliminate DNase and RNase that would degrade template and primers
PCR-grade water is treated to remove nucleases (DNase/RNase) that would degrade the DNA template or RNA in RT-PCR reactions, preventing false negatives.
Question 2: Which buffer component in a standard PCR master mix is critical for stabilizing Taq DNA polymerase activity and must be verified when troubleshooting failed reactions?
- Magnesium chloride (MgCl₂) concentration (Correct answer)
- Potassium chloride concentration
- Glycerol percentage
- BSA concentration
Correct answer: Magnesium chloride (MgCl₂) concentration
MgCl₂ acts as a cofactor for Taq polymerase and affects primer annealing; its concentration must be optimized (typically 1.5–2.5 mM) as too little or too much inhibits amplification.
Question 3: When storing a prepared PCR master mix containing dNTPs and primers at -20°C, what is the recommended maximum number of freeze-thaw cycles before reagent degradation becomes a concern?
- No more than 5–10 freeze-thaw cycles (Correct answer)
- Up to 50 freeze-thaw cycles
- Freeze-thaw cycles have no effect on PCR reagents
- No more than 2 freeze-thaw cycles
Correct answer: No more than 5–10 freeze-thaw cycles
Repeated freeze-thaw cycling degrades dNTPs and primers through hydrolysis and deamination, with performance typically declining noticeably after 5–10 cycles.
Question 4: Which class of PCR consumables must be certified as 'low-retention' or 'thin-walled' to ensure accurate thermal transfer and prevent sample loss during reaction setup?
- PCR tubes and 96-well plates (Correct answer)
- Pipette tips only
- Microcentrifuge tubes for storage
- Gel electrophoresis combs
Correct answer: PCR tubes and 96-well plates
Thin-walled, low-retention PCR tubes and plates ensure efficient heat transfer between the thermocycler block and the reaction mix and minimize sample adhesion to the plastic walls.
Question 5: What is the purpose of including a 'hot-start' enzyme component in a PCR master mix?
- To prevent non-specific amplification and primer-dimer formation at room temperature during setup (Correct answer)
- To increase the extension speed of the polymerase
- To eliminate the need for a denaturation step
- To allow PCR to run at a lower annealing temperature
Correct answer: To prevent non-specific amplification and primer-dimer formation at room temperature during setup
Hot-start enzymes are inactive until activated by heat, preventing the polymerase from extending non-specifically annealed primers during room-temperature reaction setup.
Question 6: Which reagent additive can be included in a PCR master mix to reduce secondary structure in GC-rich templates and improve amplification efficiency?
- DMSO (dimethyl sulfoxide) at 5–10% (Correct answer)
- Ethanol at 70%
- SDS at 0.1%
- EDTA at 5 mM
Correct answer: DMSO (dimethyl sulfoxide) at 5–10%
DMSO lowers the melting temperature of GC-rich DNA regions by interfering with hydrogen bonding, reducing secondary structures and improving polymerase access to the template.
PCR PCR Reagents & Consumables Management 1
What is the primary reason PCR-grade water (nuclease-free water) must be used instead of regular distilled water in PCR reactions?