trinucleotide

A trinucleotide is any sequence of three nucleotides within a DNA or RNA molecule. Nucleotides consist of a sugar, a phosphate group, and a nitrogenous base (adenine, cytosine, guanine, and thymine in DNA or uracil in RNA). Trinucleotides are the basic units that can carry genetic information or participate in genome structure.

In protein-coding regions, trinucleotides in messenger RNA form codons. Each codon specifies an amino acid or a stop signal in the genetic code. There are 64 possible codons; the code is read in triplets, starting from a start codon (AUG) and ending at stop codons (UAA, UAG, UGA). Several codons encode the same amino acid, providing redundancy in the code.

A related concept is trinucleotide repeats, where a three-nucleotide motif is repeated in tandem in the genome. The number of repeats varies among individuals, and expansions of these repeats can disrupt gene function, leading to disease. Notable examples include CAG repeats in Huntington disease, CGG repeats in Fragile X syndrome, and GAA repeats in Friedreich ataxia. Expansions are associated with mechanisms such as replication slippage and other genome instability processes.

In genetics research and clinical diagnostics, trinucleotide repeats are used as markers and as a focus of tests for certain disorders. They are also studied for their roles in genome evolution and, in some contexts, the regulation of gene expression.