Is there an evolutionary purpose/advantage of having uracil in RNA

rather than thymine (as seen in DNA)?

 

Why does only thymine get replaced in RNA?

 

 

-cheers

I think that was answered a couple of times already. Short answer: DNA does have thymine instead of uracil (you actually have to see it that way) because thymine (which is basically methylated uracil) ensures higher fidelity in DNA as compared to using uracil (which is more promiscuous with regards to base pairing, in addition cytosine can deaminate to uracil, which, if uracil instead of thymine was used in DNA, would lead to a mutation).

Fidelity is less a problem in RNA than in DNA.

Does the uracil at all help with the RNA folding, or is that all the different sugar-phosphate backbone?

I would expect that uracil RNA would be slightly more flexible than thymine RNA, but I am not sure. There are ways to calculate that, though, if you really wanted.