Question

Why is uracil not used as a nucleotide in DNA?

Answer 1

Hint: DNA is a type of nucleic acid, which is double-stranded. It is made from nucleotides, containing a five-carbon sugar backbone, nitrogen bases, and a phosphate group. DNA offers the code for the activities of the cell. The four nitrogenous bases present in DNA are adenine, thymine, guanine, and cytosine. Adenine and guanine belong to purines, while pyrimidines contain cytosine and thymine.

Complete answer:

                                    Fig: Pyrimidines and Purines in DNA and RNA
In RNA, pyrimidines are uracil and cytosine. DNA possesses thymine instead of uracil as thymine possesses greater resistance to photochemical mutation, providing more stability to genetic message. This is essential for holding all of the information required for life to function.
RNA utilizes uracil as the instability does not matter for RNA. Because mRNA is relatively short-lived and any potential errors do not lead to any lasting damage.
Thymine is easily oxidized. So it is protected from oxygen in the nucleus. Outside of the nucleus, it is quickly destroyed. Uracil is resistant to oxidation and is utilized in RNA, which exists outside of the nucleus.
Compared to RNA, DNA is more stable. A damaged nucleotide on one DNA strand is easily corrected by cell machinery as the complementary nucleotide provides a baseline for correction.
Deamination is one of the common ways of damaging DNA. The Deamination reaction of cytosine results in the formation of uracil. If DNA uses uracil, the deamination damage to cytosine will be essentially improbable to detect. Since DNA uses thymine, the formation of uracil is correctly recognized as a damaged one and it is repaired back to cytosine with thymine acting as a template.

Note:
Uracil is easily produced energetically and is present at the RNA level, as fidelity is not significant. But in the case of DNA, fidelity is important, which is enhanced by thymine. Since cytosine is usually deaminated to uracil, the repair systems in DNA find it hard to distinguish between the normal uracil and the uracil formed from deamination. So organisms evolved to possess thymine over the years, before which RNA dominated the world.