Five small nitrogen rings spell out every gene on Earth
Adenine, cytosine, guanine, thymine and uracil are compact molecules built around rings of carbon and nitrogen. Pair a big one with a small one and the DNA ladder keeps a constant width. Thymine and uracil differ by a single methyl group, yet that tiny tag marks the line between DNA and RNA.
These nucleobases attach to sugars to form nucleosides, which gain phosphates to become nucleotides, the links of nucleic acid chains. DNA uses A, C, G and T, while RNA swaps thymine for uracil. Adenine and guanine have a double-ring shape derived from purine, and the other three have a single ring derived from pyrimidine. Some viruses even replace adenine with aminoadenine, called Z, whose extra amine group bonds more firmly to thymine.
In the double helix every rung joins one purine to one pyrimidine: A with T through two hydrogen bonds, or C with G through three. Pairing a large base with a small one is what keeps the helix uniform. The strands run in opposite chemical directions, which makes that matching possible and lets the information be copied or read. Sugar and phosphate groups alternate along the outside to form the backbone.
Cells also edit bases after a chain is built. The most common change in DNA is 5-methylcytosine, while RNA carries many modifications such as pseudouridine and inosine. Some alterations are damage: when mutagens strip off an amine group, adenine becomes hypoxanthine, guanine becomes xanthine and cytosine becomes uracil. The name base simply reflects their behaviour in acid–base chemistry, which matters little for their biology.
Scientists exploit look-alike molecules. Fluorescent versions label genetic material on microarrays, several teams are designing extra base pairs to expand the genetic alphabet, and nucleoside mimics serve as anticancer and antiviral drugs, delivered as nucleosides because charged nucleotides struggle to cross membranes. Several bases, including adenine and guanine, may have formed in space as well as on Earth, and experiments have joined bases to ribose in tiny water droplets, a possible early step toward RNA.
Source: Nucleotide base
