what is the start codon
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)
| Feature | Start codon | Stop codon |
|---|---|---|
| Basic role | Signals where translation begins. | [7][1]Signals where translation ends. | [1][7]
| Typical sequence | AUG (methionine). | [5][7][1]UAA, UAG, UGA (no amino acid). | [10][7]
| Effect of mutation | May prevent translation or shift the start to the wrong place, altering the protein. | [5][7]May cause premature stop or overly long proteins. | [7]
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.