We know that dna is an acid which is elaborated by deoxyribo nucleic acid.It has 4 nucleo bases.Now,the question arises, why dna has no uracil base?
First of all, uracil can be in DNA. Cytosine can spontaneously convert to uracil by a process called hydrolytic desamination. This causes guanine, which was originally bound to this cytosine, to be bound to uracil instead (remember: uracil normally binds to adenine). The next time the cell replicates its DNA, the place opposite this uracil would be occupied by adenine instead of guanine, correctly changing the information on this DNA section. This process of cytosine deamination is one of the most common DNA damage, but it is usually effectively corrected.
In DNA, thymine replaces uracil. Uracil can be produced relatively easily from cytosine by deamination and hydrolysis, which then alters (mutates) the base sequence and possibly alters the information genetically encoded in the nucleotide sequence.
Desamination of cytosine to uracil Thymine, on the other hand, differs from uracil in that it has an additional methyl group and thus cannot easily be formed from cytosine. Uracil present in the DNA can thus be recognized as a mutation and exchanged for cytosine by base excision repair.