DNA Melting Temperature Calculator: Tm for Primers

The Importance of DNA Melting Temperature (Tm)

In molecular biology and genetics, the Melting Temperature (Tm) is the temperature at which half of the DNA strands are in a random coil or single-stranded state. Knowing this exact value is fundamentally essential for designing effective primers in Polymerase Chain Reaction (PCR) and microarray experiments.

🧬 DNA Tm Calculator

Input oligonucleotide sequences to compute Tm via accurate thermodynamic algorithms.

Calculate Tm →

📐 The Mathematics of Tm Calculation

While basic estimation uses the Wallace Rule (Tm = 2°C(A+T) + 4°C(G+C)), true laboratory-grade calculation uses Nearest-Neighbor Thermodynamics. This considers not just the number of bases, but their specific sequence and stacking interactions, along with salt concentrations. Dive deeper into the details via Wikipedia's guide on Nucleic Acid Thermodynamics.

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1. Enter Sequence

Paste your pure A,T,C,G primer sequence without spaces.

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2. Add Salt

Input Na⁺ and Mg²⁺ molarity for advanced modes.

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3. Set Temp

Instantly view the Tm and set your thermal cycler.

🧪 Real-World Application

A researcher designs a forward primer with 4 Adenines, 3 Thymines, 6 Guanines, and 5 Cytosines (18 bases). Using the basic Wallace rule:

Tm = 2(4 + 3) + 4(6 + 5) = 14 + 44 = 58°C

The researcher will typically set their PCR annealing temperature to 53°C—about 5 degrees below the Tm.

❓ FAQ

What is the ideal Tm for a PCR primer?

Generally, an ideal primer Tm falls between 52°C and 65°C. Sequences running hotter than 65°C can cause secondary structural issues like hairpins.

Why do G/C bonds matter more?

Guanine and Cytosine bind with three hydrogen bonds, whereas Adenine and Thymine bind with only two. Higher GC content = higher thermodynamic stability.