- A no-template control checks whether amplification can occur without added template.
- A positive control checks whether the reaction chemistry and cycling program can work at all.
- A reagent blank can help isolate water, buffer, or primer contamination.
- If only the no-template control amplifies, the problem is usually setup environment, reagent carryover, or primer-dimer interpretation.
Separate work areas
Keep pre-PCR setup, template handling, and post-PCR analysis physically or procedurally separated. The most common weak point is not a dramatic spill; it is repeated small exposure to amplified product, shared pipettes, shared racks, or opening tubes in the wrong place.
What to document
Record which water aliquot, primer batch, polymerase mix, template dilution, and bench area were used. If contamination appears later, these details make it possible to identify a pattern rather than repeating the same uncertain setup.
Practical review questions
- Did the negative control amplify at the same size as the expected product?
- Did the negative control amplify late, faintly, or inconsistently?
- Were primers recently opened near post-PCR products?
- Was the template dilution prepared fresh or reused from another experiment?
- Was the gel area handled before returning to setup?
How to respond
Do not change every variable at once. Start by replacing water and primers, then repeat with a clean setup area and fresh aliquots. If the same contamination pattern remains, review template dilution handling and post-PCR product movement.