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PCR (Polymerase Chain Reaction) severe

Incorrect PCR Product Size

Symptom
Gel electrophoresis shows PCR product band(s) at unexpected molecular weight, either larger or smaller than the predicted amplicon size.
Common Causes
  1. 1 Incorrect annealing temperature causing non-specific primer binding
  2. 2 Mispriming due to primers having complementary regions within template DNA
  3. 3 Suboptimal Mg²⁺ concentration affecting polymerase processivity and specificity
Solutions
  1. 1 Recalculate primer Tm values using NEB Tm calculator and adjust annealing temperature accordingly
  2. 2 Verify primers have no additional complementary regions within template DNA using BLAST or primer design software
  3. 3 Optimize Mg²⁺ concentration by testing increments of 0.2–1 mM
  4. 4 Repeat reactions using fresh reagent solutions to eliminate nuclease contamination
Related Video (3)
YouTube (Curated Tutorials) ★ 85
Primer Design: Important Considerations and Tips for Good Primer Design
"Directly addresses primer design considerations and tips, which is critical for understanding how incorrect primer design leads to non-specific binding and wrong PCR product sizes."
Addgene ★ 78
Polymerase Chain Reaction (PCR) Protocol
"Comprehensive PCR protocol walkthrough that covers the complete process including annealing temperature optimization, essential for diagnosing and preventing the incorrect product size failure."
Bilibili (China-Accessible Mirrors) ★ 72
First-person PCR and gel electrophoresis demonstration
"Hands-on demonstration combining PCR workflow with gel electrophoresis visualization, allowing direct observation of how PCR conditions affect final product size and band appearance."
Source: neb.com ↗
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