Why does uracil replace thymine in RNA?

Answer 1

Funnily enough, it is probably the other way round!

Scientists believe, for various reasons, that RNA came first, and that there was an "RNA world" before DNA evolved. If this idea is correct, uracil was a component of nucleic acids before thymine.

When DNA evolved, thymine may have proved a preferable material for storing genetic information because of its much greater stability; RNA breaks down relatively quickly, but DNA is stabilized by its double-stranded form. RNA is easily hydrolised than DNA.

It is interesting to note that in our own bodies we can synthesize RNA from simpler compounds, but to make DNA we first build RNA nucleotides, then convert them. We remove one oxygen atom from the ribose component of the nucleotide, to form deoxyribose. Then, if the base is uracil, we add a methyl group to it to form thymine.

But this leaves the question: what advantage does thymine have over uracil in DNA? One suggestion is this: cytosine (C) occasionally converts into uracil (U) by deamination. If this U is not removed, at the next replication it will act as a template for an adenine (A) on the new strand, and there will have been a mutation from G to A. Having thymine (T) as the regular base in DNA makes it easy for a cell to spot a deamination, because U should not be there at all. The cell then removes the U with a DNA repair enzyme (e.g. uracil glycosylase).

Answer 2

Uracil replaces thymine in RNA because thymine contains a methyl group not present in uracil. This difference makes thymine less stable in RNA, and the presence of uracil allows RNA to be more dynamic and flexible in structure and function.