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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?

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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:

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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.

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    $\begingroup$ My understanding is that even if there were more combos in DNA, this would not increase information density (the questioner suggests that). Indeed, only one "side" of the DNA is read. Worse, if there wasn't a one-to-one pairing, then one would get garbage when copying DNA, no? $\endgroup$
    – Bruno Le Floch
    Commented Nov 23, 2014 at 23:06
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    $\begingroup$ Some pedantry: To be sure, they pair because it is energetically favorable for them to bind. It so happens that in this case, respectively two and three H-bonds are the most energetically favorable, but strictly speaking the bonds are not the cause of the pairing. $\endgroup$
    – Superbest
    Commented Nov 24, 2014 at 1:09
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    $\begingroup$ @OP The reason GT pairs don't exist in DNA because it can distort the structure. See the bonding patterns for details $\endgroup$
    – WYSIWYG
    Commented Nov 24, 2014 at 4:43

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