If the temperature of a solution containing a helical nucleic acid is raised sufficiently, strand separation, or “melting,” occurs (
1 –
3 ) (Fig. 1 ) (
see Note 1 ) The temperature that marks the midpoint of the melting process is called the melting temperature (or
T m ) (Fig. 2 ). At the
T m , half of the nucleic acid exists in the helical state and the other half exists in the single-stranded state and the two species are in equilibrium (
Eq. 1 ).
Fig 1 Scheme descrlbing the melting process of a DN A or RN A double-helix A-T-rich regions melt first, then additional basepairs melt and unwind the helix Finally G-C-regions melt and the twist is taken up by the single-stranded regions
Fig 2 The effect of increasing temperature on the 260 nm absorbance of polyd(A-T)
2 The left ordinate shows normalized absorbance (—), where
A norlnallzed = (
A ss –
A t )/ (
A T –
A h );
A ss is the absorbance for the single-stranded state (occurs at the htghest temperature),
A h is the absorbance for the helical state (occurs at the lowest temperature), and
A T is the absorbance at temperature
T . The right ordinate shows an approxrmation of the first derivative of normalized absorbance (Δ
A normalized /Δ
T ,

) The temperature that marks the midpoint of the melting process is called the melting temperature (
T m )