Uracil

Uracil is a nitrogen-containing base, one of the four chemical letters of RNA alongside adenine, guanine and cytosine. Like cytosine and thymine, it is a pyrimidine, which means its structure is a single ring of carbon and nitrogen atoms. In an RNA nucleotide, uracil is bonded to the sugar ribose, which in turn is linked to a phosphate group.

When a gene is transcribed, each adenine on the DNA template is matched by a uracil in the new RNA strand, and the two bases pair through two hydrogen bonds. DNA uses thymine in the same role, and thymine is simply uracil carrying one extra methyl group, a carbon atom bearing three hydrogens. The usual explanation is damage control: cytosine in DNA can lose its amino group and turn into uracil, one of the most frequent kinds of chemical damage. Because healthy DNA holds almost no uracil, repair enzymes called uracil-DNA glycosylases can find any that appears and cut it out before it causes a mutation.

Students meet uracil whenever they turn a DNA sequence into messenger RNA and read its codons, with U written wherever T would be. A close chemical copy of uracil, fluorouracil, is used as a cancer drug because it disrupts how fast-dividing cells make DNA and RNA.