Smeared Bands on Gel

Symptom

PCR products appear as continuous smears rather than discrete bands on gel, indicating template degradation, excessive cycling, or nonspecific amplification throughout a size range.

1. 1 Too much template added (high concentration causes polymerase inhibition, carryover inhibitors, or inefficient denaturation)
2. 2 Template degraded or contains exonuclease (sheared or enzymatically digested DNA)
3. 3 Too many cycles, excessive extension time, or excessive annealing time (increases nonspecific amplification)
4. 4 Annealing temperature too low or thermal cycler ramp speed too slow (spurious priming)
5. 5 Impure reagents: primers, dNTPs, or water contaminated

1. 1 Reduce template concentration, reduce cycle number, and/or increase denaturation time/temperature
2. 2 Use fresh, intact template DNA; verify quality by gel electrophoresis
3. 3 Use 20–35 cycles; use 1 min/kb extension time; use 30 sec annealing time
4. 4 Set annealing temperature 5°C below primer Tm; optimize with thermal gradient; increase cycler to maximum ramp rate
5. 5 Use high-quality dNTPs, desalted or highly purified primers, and fresh nuclease-free water

Related Video (3)

Addgene ★ 78

Polymerase Chain Reaction (PCR) Protocol

"Addgene's step-by-step PCR protocol demonstrates correct template amounts and cycling, directly addressing the too-much-template cause of smearing."

Bilibili (China-Accessible Mirrors) ★ 74

First-person PCR and gel electrophoresis demonstration

"First-person PCR plus gel electrophoresis shows both setup and band visualization, useful for connecting protocol errors to smear symptoms."

Bilibili (China-Accessible Mirrors) ★ 70

PCR protocol fundamentals—hands-on operation guide

"Structured hands-on PCR protocol training reinforces correct reaction setup and cycling to prevent non-specific amplification and smearing."