When calculating the melting temperature (Tm) of a short probe, which formula is used?

The formula for calculating the melting temperature (Tm) of a short nucleic acid probe is fundamental for understanding its stability and hybridization characteristics. The correct formula takes into account the contributions of GC and AT base pairs to the overall stability of the DNA duplex.

For a short probe, the melting temperature can be influenced significantly by the GC content because guanine and cytosine pairs hold three hydrogen bonds, compared to the two hydrogen bonds in adenine and thymine pairs. This differential bonding strength leads to higher stability for sequences with more GC pairs, making them require a higher temperature to denature. Hence, the formula considers this ratio by giving more weight to the contribution of GC pairs.

In short, the melting temperature is calculated as a function of the number of GC pairs multiplied by a coefficient, and the number of AT pairs multiplied by a different coefficient. This highlights that each base pair's contribution is not equal—thus the choice that correctly encapsulates this relationship reflects how the stability of the probe's hybridization is primarily determined by the specific base pair composition. This choice emphasizes the significance of the distinct contributions of GC and AT pairs in the calculation of Tm.

The other choices either incorrectly combine the factors or use inappropriate operations that do not represent the