In AP Biology, the start codon is the AUG triplet in mRNA that signals where translation begins. It positions the ribosome and tRNA so the first amino acid added is methionine in eukaryotes (formylmethionine in prokaryotes).
The start codon is a three-nucleotide sequence in mRNA, almost always 5'-AUG-3', that tells the ribosome exactly where to start building a protein. Translation kicks off when the rRNA inside the ribosome lines up with the mRNA at this codon (EK 6.4.A.3). That single base-pairing event sets the reading frame, meaning every codon after it gets read in groups of three from that fixed starting point.
Because AUG codes for methionine, the very first amino acid in a freshly made polypeptide is methionine in eukaryotes (formylmethionine in prokaryotes). The matching tRNA carries an anticodon that base-pairs with AUG and delivers that first amino acid (EK 6.3.A.1). So the start codon does two jobs at once: it marks the launch point and it codes for the first residue in the chain.
The start codon lives in Unit 6: Gene Expression and Regulation, specifically topic 6.4 Translation, and ties back to 6.3 on how information flows from DNA to RNA to protein. It directly supports AP Bio 6.4.A (how genotype determines phenotype) because the start codon is where the genetic message physically becomes a protein. It also anchors AP Bio 6.3.A on the DNA to RNA to protein pathway. On the exam, the start codon is your evidence that translation is a precise, sequence-driven process. Mess up where it sits and the whole protein comes out wrong, which is exactly the kind of cause-and-effect reasoning Unit 6 questions reward.
Keep studying AP Biology Unit 6
Codon and Reading Frame (Unit 6)
The start codon is just one special codon, but it's the one that decides how every other codon gets grouped. Pick the wrong AUG and you shift the reading frame, so all the codons downstream get misread.
Translation Initiation (Unit 6)
Initiation is the first of translation's three steps, and the start codon is the trigger. The ribosome's rRNA finds the mRNA at AUG, the first tRNA loads in, and only then can elongation begin (EK 6.4.A.3).
Methionine (Unit 6)
AUG codes for methionine, so the start codon doubles as the code for the first amino acid. That's why new proteins start with methionine before any later processing trims it off.
Prokaryotes vs. Eukaryotes (Unit 6)
The start codon works in both, but the first amino acid differs (formylmethionine in prokaryotes, methionine in eukaryotes), and in prokaryotes translation can begin at the start codon while the mRNA is still being transcribed (EK 6.4.A.2).
Expect the start codon in multiple-choice stems about translation initiation. A classic version gives you an mRNA with 5'-AUG-3' followed by more codons and asks what that AUG does, the answer being that it marks where translation begins. Other stems probe initiation more broadly, like asking what happens if a eukaryotic mRNA loses its 5' cap (the ribosome can't load and scan to the start codon) or what process is disrupted if 16S rRNA in the small subunit is mutated (initiation, because rRNA-mRNA pairing at the start codon fails). No released FRQ uses the exact phrase "start codon," but it supports any free-response argument about how a sequence change alters a protein, since shifting or destroying the start codon breaks the whole translation step.
The start codon (AUG) tells the ribosome where to begin and codes for an amino acid. A stop codon (UAA, UAG, or UGA) tells the ribosome where to quit during termination and does NOT code for any amino acid. One opens the reading frame, the other closes it.
The start codon is almost always AUG and signals where the ribosome begins translating an mRNA.
Translation initiates when the ribosome's rRNA base-pairs with the mRNA at the start codon (EK 6.4.A.3).
AUG codes for methionine, so the first amino acid in a new polypeptide is methionine in eukaryotes (formylmethionine in prokaryotes).
The start codon sets the reading frame, so every codon after it is read in correct groups of three.
Unlike a stop codon, the start codon both marks a position and codes for an amino acid.
It's the three-nucleotide sequence in mRNA, almost always AUG, that signals where translation begins. The ribosome's rRNA recognizes it during initiation, and it codes for methionine as the first amino acid.
No. The start codon (AUG) marks where translation begins and codes for methionine, while a stop codon (UAA, UAG, or UGA) marks where translation ends and codes for no amino acid at all.
Yes for the first amino acid. AUG codes for methionine in eukaryotes and formylmethionine in prokaryotes. Note that AUG can also appear mid-sequence and code for ordinary methionine there too.
The rRNA in the ribosome's small subunit base-pairs with the mRNA at the start codon during initiation (EK 6.4.A.3). In eukaryotes the 5' cap helps the ribosome load and locate AUG, which is why losing the cap blocks initiation.
Translation can't begin in the right place, so the protein either isn't made or gets built from a wrong starting point, shifting the reading frame and producing a faulty polypeptide. That's a direct link between a sequence change and an altered phenotype (AP Bio 6.4.A).
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