Melting temperature and annealing
In PCR, short single-stranded DNA primers must bind to the template at the annealing step. The melting temperature (Tm) is the temperature at which half of the primer molecules are bound to their target. The annealing temperature (Ta) is usually set a few degrees below the Tm of the lower primer: too low and primers bind to the wrong places, too high and they barely bind at all.
Formulas used here
Wallace rule, for short oligos of up to about 14 bases:
Tm = 2 × (A + T) + 4 × (G + C)
Salt-adjusted formula, for longer primers:
Tm = 81.5 + 16.6 × log10[Na⁺] + 0.41 × %GC − 600 ÷ length
with [Na⁺] in mol/L. G–C pairs have three hydrogen bonds and stronger stacking than A–T pairs, so GC-rich primers melt at a higher temperature. Higher salt stabilizes the duplex and also raises Tm.
Example
A 20-base forward primer with 10 G or C (50% GC) and a 22-base reverse primer with 11 G or C, in 50 mM salt:
- forward Tm ≈ 50.4 °C, reverse Tm ≈ 53.1 °C;
- suggested starting Ta ≈ 50.4 − 5 ≈ 45 °C.
Important caveat
These simple formulas ignore the actual sequence order, primer concentration and magnesium. Nearest-neighbor methods, used by polymerase manufacturers’ online tools, usually give Tm values several degrees higher for typical primers, and some high-fidelity polymerases need their own Ta rules. Use this calculator for a first estimate, then follow your polymerase’s instructions.
Primer design rules of thumb
- Length of 18 to 25 bases.
- GC content of 40% to 60%.
- The two primers’ Tm within 5 °C of each other.
- A G or C at the 3′ end (a “GC clamp”), but no more than 3 G or C in the last 5 bases.
- Avoid runs of 4 or more identical bases, hairpins and complementarity between primers, which cause primer dimers.
Optimizing the annealing temperature
- Run a gradient PCR across about 8 °C around the estimated Ta and pick the temperature that gives a single, strong band.
- Raise Ta if you see extra bands or smears; lower it if there is no product.
- Touchdown PCR starts above the Tm and lowers Ta by 0.5 to 1 °C per cycle, which favors specific binding early on.