One difference is that G pairs to C with 3 hydrogen bonds, whereas A pairs to T with 2 hydrogen bonds. This makes G-C pairing stronger than A-T pairing, and G-C-rich DNA more difficult to denature. Both types of pairing are important for stabilizing the double helical structure. This pairing is also important for replication of DNA.

Notice also that C at specific CpG sequences is the site of DNA methylation. This a type of epigenetic modification of the genome.