Smeared Bands Due to Excessive Thermal Cycling (PCR (Polymerase Chain Reaction))

Question

Accepted Answer

Gel shows smeared or diffuse bands rather than discrete sharp bands. The smearing pattern suggests heterogeneous product populations from excessive amplification or nonspecific priming. Common causes: 1) Too many cycles (>35 cycles) increasing opportunity for errors and nonspecific amplification 2) Extension time too long (>1 min/kb) allowing excessive nonspecific amplification 3) Annealing time too long (>30 sec) increasing spurious priming events 4) Annealing temperature too low promoting nonspecific primer binding 5) Thermal cycler ramping speed too slow allowing spurious annealing during temperature transitions 6) Primer Tm calculated incorrectly resulting in suboptimal annealing temperature Solutions: 1) Use 20–35 cycles; reduce cycles when template concentration is high 2) Use extension time of 1 min/kb (do not exceed unless necessary) 3) Use annealing time of 30 sec (not longer) 4) Set annealing temperature 5°C below lowest primer Tm; optimize using thermal gradient 5) Increase thermal cycler ramp rate to maximum speed 6) Recalculate primer Tm using oligocalc with default salt concentration and 0.2–1 µM primer concentration; use lowest Tm