Uracil in RNA replaces thymine in DNA, according to ScienceDaily. Both uracil and thymine bond with adenine, the complementary base found in both the RNA and DNA structures.
Both RNA and DNA have four bases. In DNA, these are adenine, cytosine, guanine, and thymine. RNA has adenine, cytosine, guanine and uracil, which replaces thymine. ScienceDaily states that the reason may be that uracil synthesis requires less energy from the body. Uracil also occurs in greater quantities in cells. Thymine has a longer lifespan, making it the better choice for DNA. In DNA, genetic sequences must be maintained accurately to avoid mutations.