From The School of Biomedical Sciences Wiki
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. 
- ↑ Alberts, B et al. (2008). Molecular Biology of the Cell. 5th ed. US: Garland Science. page 303
- ↑ Berg, J.M. Tymoczko, J.L. Stryer, L. (2007). Biochemistry. 6th ed. New York: W.H. Freeman and Company. p109.