mRNACodons

mRNACodons are sequences of three nucleotides within messenger RNA that specify which amino acid will be incorporated into a growing polypeptide during translation, or signal termination. Codons are read by ribosomes in the 5' to 3' direction, one codon at a time. There are 64 possible codons, formed from the four nucleotides.

Each codon maps to an amino acid or to a stop signal through the genetic code. The start codon, AUG, not only codes for methionine but also marks the beginning of translation. The three stop codons UAA, UAG, and UGA do not code for amino acids but terminate translation.

Codon recognition relies on transfer RNA molecules that carry amino acids. Each tRNA has an anticodon that is complementary to a codon, enabling base pairing, typically via A-U and C-G interactions. Because of wobble base pairing at the third codon position, a single tRNA can recognize more than one codon for the same amino acid, increasing decoding efficiency.

In translation, the ribosome binds the mRNA and assembles tRNAs to build a polypeptide chain in the sequence dictated by the codon order. Initiation uses the start codon, elongation adds amino acids, and termination occurs at a stop codon with release of the finished protein.

The genetic code is nearly universal among organisms, with some exceptions in mitochondria and certain organelles. Codon usage varies by organism and gene, a phenomenon known as codon usage bias, which can influence translation efficiency and protein folding.

Mutations in codons can alter the amino acid sequence (missense), create a stop signal (nonsense), or be silent (synonymous). Synonymous changes can still affect expression levels due to tRNA abundance and translational kinetics. In biotechnology, codon optimization uses organism-specific codon preferences to improve expression of heterologous genes.