Why do A and T form pairs and C and G form pairs? Why are AC pairs rare or nonex

[DRfej]

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Why do A and T form pairs and C and G form pairs? Why are AC pairs rare or nonexistent?

 

[drm85]

Hydrogen bonding, a double ring nucleotide bonds with a single ring nucleotide.

 

[camshaft]

If you check in an orgo textbook (not all bio books show it) they should have a diagram showing the actual molecular structures of the base pairs and it'll be easier to see it there rather than trying to explain it I think.

 

[drpsuedonym]

C and T are pyrimidines (1 ring)
A and G are purines (2 rings)

You need to always have a pyrimidine bond with a purine to keep the double helix a uniform width.

If you're asking why not AC and TG, I don't know, but the good news is that you don't have to know either!

 

[doc3232]

Ya, one has 2 Hydrogen bonds the other has 3...