If you run a PCR product on a 1% agarose gel with a ladder and observe a single clear band at 900 bp, what does this indicate?

• A. A single product of approximately 900 base pairs; indicates specific amplification.
• B. Multiple bands across the lane indicate multiple products.
• C. A smear indicates degraded DNA.
• D. No bands indicate successful amplification.

Answer: (D)

Interpreting PCR products on an agarose gel hinges on size and band sharpness. A single, clear band at the expected size of 900 base pairs means the PCR produced one main fragment of that length, indicating specific amplification of the target sequence. The ladder provides a size reference, so you can confidently match the band to ~900 bp. If there were multiple bands, that would suggest non-specific amplification producing fragments of different sizes. A smear indicates degraded DNA or suboptimal reaction conditions leading to a range of fragment sizes. No bands would point to no amplification or a failed reaction, due to issues like inhibitors, poor template quality, or PCR setup problems.