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Well, first off, it doesn't. A pairs with T in DNA, but with U in RNA. That aside, it's basically for the same reason your key opens your door, but not your neighbor's door. Adenine just doesn't "fit" with cytosine or guanine the same way as it does with thymine and uracil; the hydrogen bonds don't line up right.

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16y ago

What else can I help you with?