Definition: Uracil is a pyrimidine base (nitrogenous base) and constituent of nucleotides and as such one member of the base pair A-U (adenine-uracil). It is normally found in RNA but not DNA.

Uracil is one of the four major RNA nucleobases, and replaces the DNA base thymine. Similar to thymine, uracil can form a base pair with adenine via two hydrogen bonds, but it lacks the methyl group present in thymine.

Uracil is not normally incorporated into DNA and is only very rarely observed in DNA. Incorporation of uracil by polymerase chain reactions (PCR) coupled with the enzyme, uracil DNA glycosylase (UDG), is used as a method of cross-over contamination prevention, which important in clinical diagnostic essays.

Replacement of thymine

Uracil is thought to replace thymine in RNA for a number of reasons. First, it requires less energy to produce. RNA is present in large quantities and only exists transiently in cells. Thymine contains a methyl group that is not present in uracil and can provide more stability. DNA needs to last longer than RNA and uses thymine; the stability outweighs the increased energy required to produce thymine.

Uracil can be produced by the deamination of cytosine, so if it were present in DNA, the genetic code could be corrupted undetectably. Methylating uracil to producing thymine makes it possible to distinguish between thymine and a cytosine to uracil mutation. UDG, an enzyme that follows DNA polymerase during DNA repair can check DNA for uracil and allow base excision repair to take place.

Uracil is also more likely to form non Watson-Crick bases pairs than thymine, which can be highly desireable in RNA due to its high degree of secondary structure. However, this can be detrimental in DNA, which requires great precision in basepairing to reduce the chances for mutations.

See also

 nucleobases