"Thymine and Uracil differ by the extra methyl group on thymine. The affect of this extra methyl group is to create extra surface tension in the water. It helps to form the DNA double helix. Without this methyl group RNA, is able to form both single and double helixes. The methyl group is like a little drop of oil and is better shield in the double helix. RNA would be under extra tension in the water if it contained the methyl group and tried to be a single helix. It would be messed up and would not function properly."