codon

Codons are read in the 5' to 3' direction on mRNA during translation, with each triplet corresponding to a specific tRNA anticodon that delivers the appropriate amino acid to the growing polypeptide chain via ribosomes. The genetic code is nearly universal across organisms, unambiguous (each codon specifies only one amino acid), and degenerate (most amino acids are encoded by multiple codons, differing often in the third 'wobble' position, which reduces the impact of some point mutations). AUG is the start codon, coding for methionine and establishing the reading frame; UAA, UAG, and UGA are stop codons that signal termination of translation (mnemonics: UAA = 'U Are Away,' UAG = 'U Are Gone,' UGA = 'U Go Away'). Clinically, understanding codons is essential for interpreting mutation types: silent mutations (codon change doesn't alter amino acid, often due to wobble), missense mutations (codon change results in different amino acid, e.g., sickle cell anemia from a single codon change GAG→GTG causing Glu→Val substitution in beta-globin), nonsense mutations (codon changes to a premature stop codon, truncating protein, e.g., some thalassemias), and frameshift mutations (insertions/deletions not in multiples of three shift the entire reading frame downstream, typically causing more severe dysfunction than point mutations). Trinucleotide repeat expansions (e.g., CAG repeats in Huntington disease, CGG in Fragile X) involve pathogenic amplification of specific codons/repeat units. This topic is foundational for molecular biology, genetics, and understanding molecular mechanisms of inherited diseases tested extensively on USMLE Step 1.