The start codon is the first codon on an mRNA that tells the ribosome where to begin translating a protein, and in almost all organisms this codon is AUG , which codes for the amino acid methionine.

What is the start codon?

  • A start codon is a specific three‑nucleotide sequence on messenger RNA (mRNA) that marks the beginning of the protein‑coding region.
  • It acts like a “start here” signal so the ribosome knows exactly where to begin building the polypeptide chain.

Which sequence is the start codon?

  • In standard genetic code, the main start codon is AUG in mRNA.
  • AUG codes for methionine in eukaryotes and for formyl‑methionine in bacteria, so new proteins usually begin with methionine at their N‑terminus.
  • Rarely, some organisms can use alternative start codons (like certain non‑AUG codons), but AUG is still the dominant, “universal” signal.

What does the start codon do?

  • It recruits a special initiator tRNA carrying methionine to the ribosome, which locks in the reading frame (groups of three bases) for the entire protein.
  • Once the ribosome is positioned at the start codon, it moves along the mRNA three bases at a time, adding amino acids until it reaches a stop codon.

Start codon vs stop codon (quick view)

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Feature Start codon Stop codon
Basic role Signals where translation begins.Signals where translation ends.
Typical sequence AUG (methionine).UAA, UAG, UGA (no amino acid).
Effect of mutation May prevent translation or shift the start to the wrong place, altering the protein.May cause premature stop or overly long proteins.

A quick mental picture

You can think of the start codon as the capital letter at the beginning of a sentence in the genetic text: it tells the ribosome, “Begin reading here, three letters at a time, to make a protein.”

TL;DR:
The start codon is the first three‑base “word” in mRNA where translation begins, almost always AUG , which codes for methionine and sets the correct reading frame for protein synthesis.

Information gathered from public forums or data available on the internet and portrayed here.