A stop codon is a sequence of three nucleotides in mRNA (UAA, UAG, or UGA) that signals the ribosome to terminate translation, releasing the finished polypeptide. Unlike other codons, it does not code for an amino acid.
A stop codon is the genetic code's period at the end of a sentence. During translation (CED topic 6.4), the ribosome reads mRNA three nucleotides at a time, and each three-letter codon usually specifies one amino acid. But three of them, UAA, UAG, and UGA, don't code for any amino acid. When the ribosome hits one, translation stops. The completed polypeptide is released and the ribosome lets go of the mRNA.
This is the termination step of translation. Per EK 6.4.A.3, translation runs through initiation, elongation, and termination in sequence. Initiation happens at the start codon (AUG); termination happens at a stop codon. Everything in between is the chain of amino acids that becomes your protein. Because translation works the same way in prokaryotes and eukaryotes (EK 6.4.A.1), stop codons function the same way in both, even though prokaryotes can translate mRNA while it's still being transcribed (EK 6.4.A.2).
Stop codons live in Unit 6: Gene Expression and Regulation, specifically topic 6.4 Translation, and they support learning objective AP Bio 6.4.A: explaining how genotype determines phenotype. Here's the connection. The DNA sequence (genotype) sets where the ribosome starts and stops, which sets the length and sequence of the protein (phenotype). Move a stop codon, and you change the protein, which can change the trait. That's the whole genotype-to-phenotype pipeline in one nucleotide change, which is exactly why the AP exam loves to test mutations that create or destroy stop codons.
Keep studying AP Biology Unit 6
Start Codon (Unit 6)
Start and stop codons are the bookends of translation. AUG kicks the ribosome off at the start, a stop codon shuts it down at the end, and everything between them gets translated into amino acids. Knowing both lets you count the length of a polypeptide from a given mRNA.
Genetic Code (Unit 6)
The genetic code is the lookup table of all 64 codons. Three of those 64 are stop codons (UAA, UAG, UGA) that map to no amino acid. So the code isn't just about which amino acid you get, it also tells the ribosome when to quit.
Mutation and Phenotype (Unit 6)
A point mutation that turns an amino-acid codon into a stop codon (a nonsense mutation) cuts the protein short and usually breaks its function. This is the cleanest example of how a single base change in the genotype rewrites the phenotype, the core idea behind objective 6.4.A.
Stop codons show up most often in MCQ stems built around mutations and mRNA reading. One common move gives you a full mRNA sequence like 5'-AUGCCCGGGAAAUAG-3' and asks how many amino acids end up in the polypeptide. The trick is that the stop codon (UAG here) does NOT add an amino acid, so you count the coding codons after AUG and stop before the stop codon. Another classic stem describes a point mutation that converts an amino-acid codon (like the cysteine codon UGU) into a stop codon (UGA), then asks what happens to the protein. The answer: translation ends early, producing a truncated, usually nonfunctional protein. You won't see a released FRQ that names "stop codon" verbatim, but the same reasoning, tracing how a base change alters the protein and therefore the phenotype, is exactly the genotype-to-phenotype logic free-response questions reward.
A start codon (AUG) tells the ribosome where to BEGIN translation and it codes for the amino acid methionine. A stop codon (UAA, UAG, UGA) tells the ribosome where to END and codes for no amino acid at all. Easy memory hook: AUG is one specific codon that adds an amino acid; the three stop codons add nothing and just release the protein.
A stop codon is one of three mRNA codons (UAA, UAG, UGA) that ends translation and does not code for any amino acid.
Stop codons trigger termination, the final step of translation after initiation and elongation (EK 6.4.A.3).
When counting amino acids in a polypeptide, do not count the stop codon, because it adds nothing to the chain.
A nonsense mutation creates a premature stop codon, cutting the protein short and usually destroying its function.
Stop codons work the same way in prokaryotes and eukaryotes, which is why they're a clean example of genotype determining phenotype (objective 6.4.A).
A stop codon is a three-nucleotide sequence in mRNA (UAA, UAG, or UGA) that signals the ribosome to stop translation and release the finished polypeptide. It's the termination step of protein synthesis in topic 6.4.
No. This is the most common mistake. Unlike the other 61 codons, the three stop codons code for no amino acid. They only tell the ribosome to terminate translation, so you never count them when figuring out how many amino acids a polypeptide has.
A start codon (AUG) marks where translation begins and codes for methionine. A stop codon (UAA, UAG, UGA) marks where translation ends and codes for no amino acid. One opens the protein and adds an amino acid; the other closes it and adds nothing.
Translation stops too soon, producing a shortened (truncated) protein that's usually nonfunctional. This is called a nonsense mutation, and AP questions often use it, like changing the cysteine codon UGU to the stop codon UGA, to test how a single base change wrecks a protein.
Find the start codon (AUG), then read in three-letter chunks until you hit a stop codon. Count every codon from AUG up to but NOT including the stop codon. For 5'-AUGCCCGGGAAAUAG-3', AUG-CCC-GGG-AAA gives 4 amino acids, and UAG is the stop.