why thymine is substituted by uracil in RNA?

Answer 1

I imagine this make it less likely for RNAs to slip into DNA duplex since uracil lack methyl group. Thymine is uracil with one methyl group that is used in DNA base pairing. Without this RNA won't pair up with DNAs. This would be good thing considering RNAs have different function than of DNA.

Answer 2

In RNA thymine is replaced by uracil as the base complementary to adenine. This base is also a pyrimidine and is very similar to thymine. Uracil is energetically less expensive to produce than thymine, which may account for its use in RNA. In DNA, however, uracil is readily produced by chemical degradation of cytosine, so having thymine as the normal base makes detection and repair of such incipient mutations more efficient. Thus, uracil is appropriate for RNA, where quantity is important but lifespan is not, whereas thymine is appropriate for DNA where maintaining sequence with high fidelity is more critical.

Answer 3

Uracil is energetically less expensive to produce than thymine, which accounts for its use in RNA.