Here's why:

* Hydrogen Bonding: Guanine (G) and cytosine (C) form three hydrogen bonds with each other, while adenine (A) and thymine (T) form only two hydrogen bonds.

* Stacking Interactions: The planar bases in DNA stack on top of each other. GC pairs have more efficient stacking interactions due to their larger surface area and stronger hydrophobic interactions.

* Stability: The stronger hydrogen bonding and more favorable stacking interactions in GC pairs contribute to a higher overall stability of the DNA molecule.

Therefore, a higher GC content leads to a higher melting temperature.

Practical Implications:

* PCR: In polymerase chain reaction (PCR), primers with higher GC content are often preferred because they have a higher Tm, which can improve the specificity and efficiency of the reaction.

* DNA Hybridization: Higher GC content in probes used for DNA hybridization can increase their binding affinity and specificity.

* Genomics: Regions of the genome with high GC content are often associated with gene-rich regions and have a higher level of transcriptional activity.

In summary, GC-rich DNA is more stable than AT-rich DNA, and therefore has a higher melting temperature.