PCR Cycling Conditions Reference
A standard PCR program repeats three temperature steps — denaturation, annealing and extension — for 25–35 cycles, bracketed by an initial denaturation and a final extension. This reference gives generic starting temperatures and times for a 3-step protocol and explains how to set the annealing temperature and extension time for your reaction.
Standard 3-step PCR program
| Step | Temperature | Time | Notes |
|---|---|---|---|
| Initial denaturation | 94–98 °C | 30 s – 3 min | Once, before cycling; fully melts template (higher/shorter for high-fidelity enzymes) |
| Denaturation | 94–98 °C | 10–30 s | Per cycle; melt the new product |
| Annealing | 50–65 °C | 15–30 s | ~3–5 °C below the primers' Tm |
| Extension | 68–72 °C | by amplicon length | 72 °C for Taq; ~68–72 °C for proofreading enzymes |
| Final extension | 72 °C | 5–10 min | Once, after cycling; completes products / adds A-overhangs |
| Hold | 4–10 °C | ∞ | Storage until retrieval |
| Cycles | — | 25–35 | More cycles for low-template; too many increases artefacts |
These are general starting points across polymerase families, not vendor-specific settings. Always follow your polymerase's datasheet for exact temperatures, times and buffer.
The three steps
Each cycle has three steps. Denaturation at 94–98 °C melts the double-stranded template (and, after cycle one, the new product) into single strands. Annealing at 50–65 °C lets the primers bind their complementary sites. Extension at 68–72 °C is where the polymerase synthesises the new strand. These three steps repeat 25–35×, bracketed by a one-off initial denaturation before the first cycle and a one-off final extension after the last.
Setting the annealing temperature
Set the annealing temperature about 3–5 °C below the lower primer Tm. If it is too high, the primers do not bind and you get no product; if it is too low, they bind non-specifically and you get extra, non-specific bands. When you are unsure of the optimum, run a gradient PCR across a temperature range, or use a touchdown program (below) to converge on the correct product automatically.
Setting the extension time
Scale the extension time to the amplicon length and the polymerase family. Standard Taq is comparatively slow; proofreading / high-fidelity enzymes are faster, so a long template that needs minutes with Taq may need far less time with a high-fidelity enzyme. Use these rules of thumb as a starting point and confirm against your enzyme's datasheet.
| Polymerase family | Extension rate |
|---|---|
| Standard Taq | ~60 s per kb |
| Proofreading / high-fidelity | ~15–30 s per kb (faster; check your enzyme) |
Two-step and touchdown PCR
When the primers' annealing Tm is high — roughly ≥ 68 °C — the annealing and extension steps can be combined into a single step at 72 °C. This two-step PCR simplifies the program because the primers anneal efficiently at the extension temperature. Touchdown PCR instead starts the annealing temperature high and steps it down over the first cycles: the early high-stringency cycles favour the correct product so it dominates before any non-specific products can accumulate.
Frequently asked questions
- What annealing temperature should I use?
- As a starting point, set the annealing temperature about 3–5 °C below the lower of your two primers' melting temperatures (Tm). Too high and the primers will not bind, giving no product; too low and they bind non-specifically, giving extra bands. A gradient PCR or touchdown program helps find the best temperature.
- How long should the extension step be?
- Scale the extension time to the amplicon length and the polymerase family. As a rule of thumb, standard Taq extends at about 60 s per kb, while proofreading / high-fidelity enzymes are faster at roughly 15–30 s per kb — always check your enzyme's datasheet.
- How many cycles should I run?
- Most reactions use 25–35 cycles. Use more cycles when starting from very little template, but too many cycles increases non-specific products and artefacts as reagents are depleted.
- What is touchdown PCR?
- Touchdown PCR starts the annealing temperature a few degrees above the expected optimum and steps it down over the first several cycles. The early high-stringency cycles favour the correct product, which then dominates as the temperature drops — a simple way to improve specificity without optimising a single annealing temperature.
- Why are there separate initial denaturation and final extension steps?
- The initial denaturation runs once before cycling to fully melt the template (and, for hot-start enzymes, to activate the polymerase). The final extension runs once after cycling to let the polymerase finish any incomplete products and, for Taq, add the 3' A-overhangs used in TA cloning.
See also
Related tools and references
Use these related pages when this table raises a practical calculation or workflow question.