[quantum_toothpaste]

in DNA you have adenine (A), cytosine (C), guanine (G) and thymine (T) which pair up like this

G<->C
A<->T

But in RNA you have adenine  (A), cytosine  (C), guanine (G) and uracil (U) and they pair up with the DNA like this

G<->C
A<->U

How is it possible that A can pair with U or T?

Now if I have the chemical diagrams (do these sorts of diagram have a name?)


Can I use these to find out these pairings myself (if I didn't know them) and how do you do it?

thanks!

[Yggdrasil]

Look up the structures of thymine and uracil bases.  You should see that they are almost identically structurally.  The only difference between the two is a methyl group on carbon-5 of the thymine base.  This methyl group is on the opposite side of the molecule that is involved in base pairing, so it does not affect the base pairing.

DNA contains thymine instead of uracil bases because a specific type of DNA damage converts cytosine bases into uracil.  This way, DNA repair enzymes can recognize uracil bases as damaged cytosines then convert them back to cytosines.  If DNA regularly contained uracil, there would be no way to correctly identify which uracils are correct and which uracils were mutated cytosines.

[azumore]

It's something to do with the bond lenghts.

A--  -U
C-  --G

Can you see that, if it was for example G and A pairing up then it would go out of order.
Hope this helps