Start vs. Stop Codons: Key Differences & Roles in Gene Expression
Start codons are three-base sequences (AUG) that kick off protein synthesis; stop codons (UAA, UAG, UGA) are three-base sequences that slam on the brakes and end the chain.
Students often confuse them because both are short, triplet “words” in mRNA and both appear near the beginning and end of textbook diagrams—making it easy to skim past their opposite roles.
Key Differences
Start codons pair with initiator tRNA carrying methionine, launching ribosome assembly. Stop codons recruit release factors, not tRNA, ejecting the ribosome and freeing the finished protein.
Which One Should You Choose?
You don’t choose; your gene does. A mutation that swaps a start for a stop—or vice versa—can truncate or extend a protein, leading to disorders like thalassemia or cancers.
Examples and Daily Life
COVID PCR tests detect viral genes by looking for start-to-stop signatures. In mRNA vaccines, engineers keep the start codon but replace the viral stop with a stronger one for better stability.
What happens if a start codon mutates?
Translation may begin at the next downstream AUG, producing a shorter, often non-functional protein that can cause disease.
Can one sequence act as both?
Rare “dual-function” codons exist in some viruses, but in humans AUG never doubles as a stop.