dna - Why does A pair with T and G with C?

DNA is made up of pairs on AT and GC base pairs.

I know that A only pairs with T and G only with C. Does this apply just to humans, or are there animals where T will pair with G?

Also, surely there would be more possible combinations if A could pair with G or T or C, and DNA would have a better information density - less would be needed to hold the information?

Answer

There is no difference in base pairing between different kinds of organisms. Humans, animals and bacteria all share the same fundamental mechanisms as they all use DNA. Which bases can pair is determined by the chemistry of the individual bases.

The bases in DNA form the following hydrogen bonds when they are paired:

If you would try to combine other bases than the canonical Watson-Crick pairs, the hydrogen bond donors and acceptors wouldn't match. You need a donor/acceptor pair at each position.

There are different base pairs possible, but these situations don't really occur in DNA but in RNA. You can find G/U wobble base pairs and even base pairs that use different parts of the bases than the canonical base pairs, e.g. Hoogsteen base pairs.