startkodon

A start codon is a nucleotide triplet in messenger RNA (mRNA) that signals the beginning of translation, the process by which a ribosome reads an mRNA sequence to synthesize a protein. The most common start codon is AUG, which encodes the amino acid methionine in eukaryotes and n-formylmethionine in many bacteria and organelles. The start codon also establishes the reading frame for the entire coding sequence.

In bacteria, archaea, and organelles such as mitochondria and chloroplasts, translation initiation often depends on additional signals that position the ribosome at the start codon. In bacteria, a Shine-Dalgarno sequence upstream of the start codon helps align the ribosome with the mRNA. The initiating tRNA typically carries formylmethionine in bacteria and methionine in eukaryotes and many organelles.

In eukaryotes, translation generally begins at the first AUG downstream of the 5' cap, with initiation guided by a Kozak consensus sequence surrounding the start codon that influences efficiency. Eukaryotic initiator tRNA is methionyl-tRNAi, which partners with initiation factors to start protein synthesis.

While AUG is the predominant start codon, alternative start codons such as CUG, GUG, or others can be used in some bacteria and yeast, often with lower efficiency and context-dependent regulation. In mitochondrial and chloroplast genomes, start codons and initiation mechanisms can differ from cytoplasmic systems.

Misannotation or shifting of the start codon can change the reading frame and alter the resulting protein. Understanding start codons is essential for studying gene expression, protein synthesis, and genome annotation.