Why is \(T_{m}\) related to base composition?

DNA, or deoxyribonucleic acid, is a double-stranded molecule composed of nucleotides, which consist of a sugar, a phosphate group, and a nitrogenous base. There are four types of nucleotide bases in DNA: adenine (A), guanine (G), cytosine (C), and thymine (T). The two strands of DNA are complementarily paired based on specific base pairing rules, where adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C).

The melting temperature (Tm) is the temperature at which half of the DNA strands are separated, or "melted," from their complementary strands. The stability of a DNA double helix depends on various factors such as the sequence of base pairs, the presence of salts in the solution, and the temperature.

The stability of the DNA double helix is mainly determined by the hydrogen bonds that form between the complementary base pairs. Adenine (A) forms 2 hydrogen bonds with Thymine (T), while Guanine (G) forms 3 hydrogen bonds with Cytosine (C). As a result, G-C base pairs are generally stronger and more stable than A-T base pairs.

The melting temperature (Tm) depends on the number and type of hydrogen bonds holding the complementary strands together. DNA molecules with a higher GC content have more G-C base pairs, which are stronger and more stable due to three hydrogen bonds. Therefore, they require a higher temperature to break these bonds and separate the strands, resulting in an increased melting temperature (Tm). In contrast, DNA molecules with a higher AT content have more A-T base pairs, which are less stable and can be separated at a lower temperature, resulting in a lower melting temperature (Tm). In conclusion, the melting temperature (Tm) of a DNA molecule is related to its base composition because the stability of the DNA double helix is determined by the hydrogen bonding between the complementary base pairs, and the strength of these bonds is directly affected by the proportion of each type of base pair present in the molecule.