Why is thymine not incorporated into mRNA?

Question

I am aware that in transcription uracil bonds to adenine and not thymine. But what is it that actually prevents thymine from bonding to adenine in transcription, that is not present in replication?

Answer 1

Thymine and Uracil and are identical structures except for Thymine having an additional methyl group. Similiar to trying to put two incompatible puzzle pieces together, the methyl group prevents thymines from being incorporated into mRNA Only the Uracil is the correct fit for the mRNA molecule.