Uracil (U) is a pyrimidine base. It is one of the four bases found in RNA where it replaces Thymine (T) which is found in DNA. The structure of uracil differs from thymine in that thymine contains an extra methyl group on the 5-C carbon atom whereas uracil just has a hydrogen atom. However, the base-pairing is unaffected by the lack of this methyl group, therefore the U-A base pairs are very similar to T-A base pairs  e.g. there are 2 hydrogen bonds formed between the bases .