What causes duplex dna with a certain (a + t): (g + c) ratio to melt at a higher temperature than comparable length duplex dna with a greater (a + t): (g + c) ratio?

If two duplex DNA are of comparable length then the duplex DNA with more GC content will have higher melting point as compared to the duplex DNA with less GC content. So, more GC content as compared to AT content causes the melting point of DNA double helix to be higher.

Explanation:

In a DNA double helix, nucleotide G is paired with nucleotide C with the help of 3 bonds while nucleotide A is paired with nucleotide T with the help of 2 bonds. More energy is required to break 3 bonds as compared to break 2 bonds. So, the duplex DNA which has more (G + C) content than (A + T) i.e. greater (G + C):(A + T) ratio will have more melting temperature as compared to the duplex DNA with greater  (A + T):(G + C) ratio.

As per this question, we can easily infer that the duplex DNA which has greater melting temperature than comparable length duplex DNA with a greater (A + T):(G + C) ratio must have more (G + C) than (A + T).

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