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Why does A always go with T?

And C with G in DNA molecules :confused:
Reply 1
Avatar for madelinesux
madelinesux
the way i've always been taught is that they're shapes fit together, but i guess that's too simplistic...
Reply 2
Avatar for overmyhead
overmyhead
A and T each have 2 sites that can hydrogen bond to each other, and G and C have 3.
Reply 3
Avatar for Iscariot
Iscariot
11
Just for the sake of being pedantic :P

http://en.wikipedia.org/wiki/Pyrimidine_dimers is an example of non-AT/CG binding.

Anyway... Cytosine and guanine have 3 hydrogen bonds between them while adenine and thymine only have 2.

Edit: Sorry someone already answered it.
usually because they're complementary shapes?
Reply 5
Avatar for J.Farrell
J.Farrell
Width of DNA has to be kept constant. A and G are purines which have 1 pentose sugar and one hexose, while T and C are pyrimadines which have only 1 pentose sugar. So to keep the width constant A must bind with T and G must bind with C.
Reply 6
Avatar for GregoryJL
GregoryJL
I'm pretty sure you are confusing the nitrogenous base (purine or pyrimadine) with sugars. Purines have two rings in their nitrogenous base, pyrimadines one. The width thing is correct - it is quite a good way of detecting transversions (so GA, CT binding.) It may be that the very specific width of the helix makes transitions (AC or AU, GT) detectable by length - however, purines are different from each other (Guanine, if memory serves, has an O= on carbon 7 (?) which in Adenine is just an H.)

I'm not convinced that the number of hydrogen bonds is sufficient to explain complimentary base pairing - there is no reason why that alone would prohibit AC, GT binding, if for no other reason than these modes (and even less favourable ones) can exist happily in a cell unless they are detected and corrected. I suspect that DNA pol whatever can detect the bases with varying degrees of fidelity and 'permit' binding only to the correct nucleotide - as an aside, DNA pols vary in their fidelity (some are particularly error prone, like eta), which would support the idea that they are involved with 'ensuring' complimentary base pairing.

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