bdA well-designed primer gives one sharp band on the gel and a clean Sanger read. A poor primer gives extra bands, no product or mixed peaks.
1. Choose the target region
- Start from genomic DNA, a plasmid or the gene of interest, and know the exact region to amplify (for example a gene, 16S rRNA or a plasmid insert).
- Use the sequence in FASTA format in the design tools.
2. Use a design tool
- NCBI Primer-BLAST (design plus specificity check): ncbi.nlm.nih.gov/tools/primer-blast
- Primer3 (flexible, many options): primer3.ut.ee
3. Recommended parameters
| Parameter | Recommended | Why |
|---|---|---|
| Primer length | 18–24 nt | Specific, yet binds efficiently |
| Melting temperature (Tm) | 55–65 °C, ideally 58–60 °C; pair within 2 °C | Stable annealing |
| GC content | 40–60% | Too high binds too strongly; too low too weakly |
| GC clamp | 1–2 G or C at the 3′ end | Firm start for the polymerase |
| Amplicon size | 100–1,000 bp; best 300–800 bp for sequencing | Clear on the gel, fully readable by Sanger |
| 3′ complementarity | None between or within primers | Prevents primer-dimers |
| Repeats | No runs of more than 4 identical bases | Prevents slipping and hairpins |
4. Check dimers and hairpins
Paste both primers into a tool such as IDT OligoAnalyzer. Aim for hairpin ΔG above −2 kcal/mol and self- or cross-dimer ΔG above −5 kcal/mol.
5. Check specificity
BLAST the primer sequences to make sure they do not bind elsewhere, especially in complex genomes or mixed samples.
6. Add tags only if needed
For cloning, add restriction sites or adaptors; for sequencing, universal sites such as M13 or T7. Make sure overhangs do not bind inside the amplicon.
PCR and gel tips
| Tip | Effect |
|---|---|
| Use good-quality template DNA | Degraded DNA gives weak or no bands |
| Start annealing at Tm − 3 °C, then optimise by gradient PCR | Specific, efficient amplification |
| Use a hot-start polymerase | Fewer non-specific products |
| Adjust Mg²⁺ | Too much: extra bands; too little: no band |
| Use 2% agarose for short amplicons | Sharper separation |
Troubleshooting
| Problem | Likely cause | Solution |
|---|---|---|
| Several bands | Non-specific binding, low annealing temperature | Raise annealing temperature or redesign primers |
| No band | Poor design or degraded DNA | Check Tm and sequence; use fresh DNA |
| Smear | Too much template or degraded product | Reduce template; optimise extension time |
| Mixed sequencing peaks | Mixed product or primer-dimers | Gel-purify the band; clean up the PCR |
Checklist
- Length 18–24 nt; Tm 58–60 °C; GC 40–60%; GC clamp at the 3′ end
- No strong dimers or hairpins
- Product 300–800 bp for sequencing
- Specificity checked by BLAST
- Sequencing primer = your forward or reverse PCR primer (or an internal primer)