Which of the following probes will have the highest melting temperature (Tm)?

The melting temperature (Tm) of a DNA probe is influenced by several factors, including the length and nucleotide composition of the sequence. Longer and more GC-rich sequences typically have higher melting temperatures due to the stronger hydrogen bonding between guanine (G) and cytosine (C) compared to adenine (A) and thymine (T).

In this case, the sequence represented by the correct answer is the longest among the options provided. It consists of 20 nucleotides, which contributes significantly to its Tm due to increased base stacking interactions and more hydrogen bonds. Additionally, the presence of more G and C nucleotides further elevates the Tm, as GC pairs form three hydrogen bonds compared to the two formed by AT pairs.

The other sequences are shorter, and their GC content may be lower compared to the selected option. Though other factors could influence Tm, like mismatches or secondary structures, the fundamental principle of length and GC content strongly supports the selection of the longest and most GC-rich probe as having the highest melting temperature.