Nucleobases (or nucleotide bases/nitrogenous bases) are the parts of DNA and RNA that may be involved in pairing (see also base pairs). The primary nucleobases are cytosine, guanine, adenine (DNA and RNA), thymine (DNA) and uracil (RNA), abbreviated as C, G, A, T, and U, respectively. They are usually simply called bases in genetics. Because A, G, C, and T appear in the DNA, these molecules are called DNA-bases; A, G, C, and U are called RNA-bases.
Uracil replaces thymine in RNA. These two bases are identical except that uracil lacks the 5' methyl group. Adenine and guanine belong to the double-ringed class of molecules called purines (abbreviated as R). Cytosine, thymine, and uracil are all pyrimidines (abbreviated as Y).
The system of a base forms a Glycosidic bond with the 1' carbon of a ribose or deoxyribose is called a nucleoside, and a nucleoside with one or more phosphate groups attached at the 5' carbon is called a nucleotide.
Apart from adenine (A), cytosine (C), guanine (G), thymine (T) and uracil (U), DNA and RNA also contain bases that have been modified after the nucleic acid chain has been formed. In DNA, the most common modified base is 5-methylcytidine (m5C). In RNA, there are many modified bases, including pseudouridine (Ψ), dihydrouridine (D), inosine (I), ribothymidine (rT) and 7-methylguanosine (m7G).
Hypoxanthine and xanthine are two of the many bases created through mutagen presence, both of them through deamination (replacement of the amine-group with a carbonyl-group). Hypoxanthine is produced from adenine, xanthine from guanine. In similar manner, deamination of cytosine results in uracil.
- The "skeleton" of adenine and guanine is purine, hence the name purine-bases.
- The "skeleton" of cytosine, uracil, and thymine is pyrimidine, hence pyrimidine-bases.
These are incorporated into the growing chain during RNA and/or DNA synthesis.
Modified purine bases
These are examples of modified adenosine or guanosine.
Modified pyrimidine bases
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