Smeared Bands on Gel (PCR (Polymerase Chain Reaction))

Question

Accepted Answer

PCR products appear as continuous smears rather than discrete bands on gel, indicating template degradation, excessive cycling, or nonspecific amplification throughout a size range. Common causes: 1) Too much template added (high concentration causes polymerase inhibition, carryover inhibitors, or inefficient denaturation) 2) Template degraded or contains exonuclease (sheared or enzymatically digested DNA) 3) Too many cycles, excessive extension time, or excessive annealing time (increases nonspecific amplification) 4) Annealing temperature too low or thermal cycler ramp speed too slow (spurious priming) 5) Impure reagents: primers, dNTPs, or water contaminated Solutions: 1) Reduce template concentration, reduce cycle number, and/or increase denaturation time/temperature 2) Use fresh, intact template DNA; verify quality by gel electrophoresis 3) Use 20–35 cycles; use 1 min/kb extension time; use 30 sec annealing time 4) Set annealing temperature 5°C below primer Tm; optimize with thermal gradient; increase cycler to maximum ramp rate 5) Use high-quality dNTPs, desalted or highly purified primers, and fresh nuclease-free water