Trinucleotides

Trinucleotides are sequences of three nucleotides in DNA or RNA. They serve as the fundamental units of the genetic code in coding regions, where each trinucleotide, or codon, specifies a particular amino acid or a signal to terminate translation. With four nucleotides (adenine, thymine or uracil, cytosine, guanine), there are 64 possible codons, which are read in a single, nonoverlapping reading frame from a start codon to a stop codon. In messenger RNA, the start codon is typically AUG, encoding methionine, while stop codons UAA, UAG, and UGA mark the end of translation.

Not all trinucleotides occur as codons, however; trinucleotides also appear as short sequence motifs in noncoding regions and as elements used in research and biotechnology. The genetic code is largely universal but exhibits redundancy, so most amino acids are specified by more than one codon.

In addition to standard codons, there are trinucleotide repeats, tandem arrays of a single trinucleotide motif, such as CAG or CGG. Variations in repeat number among individuals can lead to trinucleotide repeat expansion diseases, including Huntington disease (CAG repeats), fragile X syndrome (CGG repeats), myotonic dystrophy type 1 (CTG repeats), and Friedreich ataxia (GAA repeats). Expansions are thought to arise through replication slippage and repair defects and can affect gene function by altering protein length or RNA structure.