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Uracil (/ ˈ j ʊər ə s ɪ l /) (symbol U or Ura) is one of the four nucleotide bases in the nucleic acid RNA. The others are adenine (A), cytosine (C), and guanine (G). In RNA, uracil binds to adenine via two hydrogen bonds. In DNA, the uracil nucleobase is replaced by thymine (T). Uracil is a demethylated form of thymine.
Adenine (/ ˈ æ d ɪ n iː n /, / ˈ æ d ɪ n ɪ n /) (symbol A or Ade) is a purine nucleotide base. It is one of the nucleobases in the nucleic acids, DNA and RNA. The shape of adenine is complementary to either thymine in DNA or uracil in RNA. In cells adenine, as an independent molecule, is rare.
The ability of nucleobases to form base pairs and to stack one upon another leads directly to long-chain helical structures such as ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Five nucleobases—adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U)—are called primary or canonical.
Watson-Crick base pairs in a siRNA.Hydrogen atoms are not shown. Each nucleotide in RNA contains a ribose sugar, with carbons numbered 1' through 5'. A base is attached to the 1' position, in general, adenine (A), cytosine (C), guanine (G), or uracil (U).
The purine bases adenine and guanine and pyrimidine base cytosine occur in both DNA and RNA, while the pyrimidine bases thymine (in DNA) and uracil (in RNA) occur in just one. Adenine forms a base pair with thymine with two hydrogen bonds, while guanine pairs with cytosine with three hydrogen bonds.
The general structure of a ribonucleotide consists of a phosphate group, a ribose sugar group, and a nucleobase, in which the nucleobase can either be adenine, guanine, cytosine, or uracil. Without the phosphate group, the composition of the nucleobase and sugar is known as a nucleoside.
RNA also contains adenine, guanine, and cytosine, but replaces thymine with uracil. [15] Thus, DNA synthesis requires dATP, dGTP, dCTP, and dTTP as substrates, while RNA synthesis requires ATP, GTP, CTP, and UTP. Nucleic acid synthesis is catalyzed by either DNA polymerase or RNA polymerase for DNA and RNA synthesis respectively. [16]
The double helical structures of DNA or RNA are generally known to have base pairs between complementary bases, Adenine:Thymine (Adenine:Uracil in RNA) or Guanine:Cytosine. They involve specific hydrogen bonding patterns corresponding to their respective Watson-Crick edges, and are considered as Canonical Base Pairs.