Why is uracil not used as a nucleotide in DNA?
See answer below:
DNA uses thymine instead of uracil because thymine has greater resistance to photochemical mutation, making the genetic message more stable. This is necessary for holding all of the information needed for life to function.
RNA, however, uses uracil - because the instability doesn't matter for RNA as much since the mRNA is comparatively short-lived and any potential errors don't lead to any lasting damage.
Also thymine is easily oxidized. Thymine is protected from oxygen in the nucleus. Outside of the nucleus, thymine is quickly destroyed. Uracil is resistant to oxidation and is used in the RNA that must exist outside of the nucleus.
All information was found at this website, which goes into more detailed information.