why does dna have thymine and rna have uracil
thymine has a methyl group which is not found in uracil. Having a methyl group means it is more stable. That is why thymine is more stable that uracil. DNA is more stable than RNA because DNA has deoxyribose sugar and RNA has ribose sugar. So DNA has thymine base and RNA has uracil. Also, all genetic info is stored in DNA, so DNA has to be stable and 'long-lived' to stored the information.