Why does adenine pair with uracil in RNA ?

In RNA, uracil base-pairs with adenine and replaces thymine during DNA transcription. Methylation of uracil produces thymine.^[10] In DNA, the evolutionary substitution of thymine for uracil may have increased DNA stability and improved the efficiency of DNA replication (discussed below). Uracil pairs with adenine through hydrogen bonding. When base pairing with adenine, uracil acts as both a hydrogen bond acceptor and a hydrogen bond donor. In RNA, uracil binds with a ribose sugar to form the ribonucleosideuridine. When a phosphate attaches to uridine, uridine 5'-monophosphate is produced.^[6]

Uracil undergoes amide-imidic acid tautomeric shifts because any nuclear instability the molecule may have from the lack of formal aromaticity is compensated by the cyclic-amidic stability.^[5] The amide tautomer is referred to as the lactamstructure, while the imidic acid tautomer is referred to as the lactim structure. These tautomeric forms are predominant at pH 7. The lactam structure is the most common form of uracil.

Uracil tautomers: Amide or lactamstructure (left) and imide or lactimstructure (rightracil also recycles itself to form nucleotides by undergoing a series of phosphoribosyltransferase reactions.^[2]Degradation of uracil produces the substrates aspartate, carbon dioxide, and ammonia.^[2]

 → H₃NCH₂CH₂COO⁻ + NH₄⁺ + CO₂

Oxidative degradation of uracil produces urea and maleic acid in the presence of H₂O₂ and Fe²⁺ or in the presence of diatomic oxygenand Fe²⁺.

Uracil is a weak acid. The first site of ionization of uracil is not known.^[11] The negative charge is placed on the oxygen anion and produces a pK_a of less than or equal to 12. The basic pK_a = -3.4, while the acidic pK_a = 9.38₉. In the gas phase, uracil has 4 sites that are more acidic than water.^[12] a