In AP Biology, a codon is a sequence of three nucleotides on an mRNA molecule that specifies one amino acid or a stop signal during translation (Topic 6.4), making it the unit that connects the genetic code to the polypeptide a cell builds.
A codon is a group of three nucleotides on mRNA read together during translation. The ribosome scans the mRNA three letters at a time, and each three-letter "word" (the codon) tells the cell which amino acid to add next, or when to stop. That's the whole genetic code in a nutshell: triplets of bases mapped to amino acids.
During translation (Topic 6.4), the ribosome starts at the start codon (AUG) and reads codons one after another, adding an amino acid for each one until it hits a stop codon, which has no amino acid and ends the chain. A transfer RNA carries the matching amino acid and pairs its anticodon to the codon, so the codon on the mRNA and the anticodon on the tRNA are complementary partners. Because there are 64 possible codons but only 20 amino acids, multiple codons can code for the same amino acid. That redundancy is why some mutations don't change the protein at all.
Codons live in Unit 6 (Gene Expression and Regulation) under Topic 6.4 Translation, and they're the mechanism behind learning objective AP Bio 6.4.A: explaining how an organism's phenotype comes from its genotype. The codon is the literal bridge. DNA stores the sequence, mRNA carries it, and codons translate that sequence into the amino acid chain that becomes a functional protein. EK 6.4.A.3 lists translation's steps (initiation, elongation, termination), and a codon is what the ribosome reads at each step, starting when the rRNA in the ribosome locks onto the start codon on the mRNA. If you understand codons, you understand why a single base change can be harmless, harmful, or catastrophic, which is the genotype-to-phenotype story the exam keeps coming back to.
Anticodon (Unit 6)
An anticodon is the codon's mirror image on a tRNA. The tRNA brings the right amino acid and base-pairs its anticodon to the mRNA codon, so codon and anticodon are the lock and key that make translation accurate.
Genetic Code (Unit 6)
The genetic code is just the full lookup table of which codon means which amino acid. A codon is one entry in that table, and because 64 codons map to only 20 amino acids, the code is redundant, which sets up why silent mutations exist.
Start Codon / Stop Codon (Unit 6)
These are special codons that punctuate the message. AUG (start) tells the ribosome where to begin reading, and stop codons (UAA, UAG, UGA) carry no amino acid and end the chain, so they bookend every polypeptide.
Phenotype and Mutation (Units 6-7)
Because codons read in groups of three, deleting one nucleotide shifts the reading frame and scrambles every codon downstream. That's how a tiny change in genotype can wreck a whole protein and change the phenotype natural selection acts on.
Codons show up in both multiple-choice and free-response, usually as a reading puzzle. A classic MCQ hands you an mRNA sequence like 5'-AUGCCCGGGAAAUAG-3' and asks how many amino acids the polypeptide has. You count codons (5), then subtract the stop codon, so the answer is 4. Another favorite tests the reading frame: a single nucleotide deletion causes a frameshift that garbles every codon after it, almost always destroying the protein. You'll also see the silent-mutation question, where a single base change leaves protein function normal because the new codon still codes for the same amino acid (genetic code redundancy). On the FRQ side, the 2024 short FRQ used ribosome profiling to measure how long a ribosome pauses at each codon as it moves along the mRNA, so be ready to reason about codons in the context of translation speed and data, not just memorize the definition.
A codon is on the mRNA; an anticodon is on the tRNA. They are complementary three-base sequences that pair up during translation. Mix-up tip: the codon carries the message from the gene, and the anticodon "answers" it by delivering the matching amino acid.
A codon is three nucleotides on mRNA that specify one amino acid or a stop signal.
The ribosome reads codons in non-overlapping groups of three, starting at the AUG start codon.
There are 64 codons but only 20 amino acids, so the genetic code is redundant and many amino acids have more than one codon.
That redundancy is why a silent mutation can change a nucleotide without changing the protein.
Deleting or inserting one nucleotide causes a frameshift that scrambles every codon downstream and usually breaks the protein.
The codon (mRNA) pairs with the anticodon (tRNA), which links the genetic code to the amino acid being added.
A codon is a sequence of three nucleotides on mRNA that the ribosome reads during translation to add a specific amino acid to a polypeptide, or to signal a stop. It's the basic unit of the genetic code (Topic 6.4).
No. Stop codons (UAA, UAG, UGA) don't code for any amino acid; they tell the ribosome to end translation. So if an mRNA has 5 codons and one is a stop codon, the polypeptide has only 4 amino acids.
A codon sits on the mRNA, while an anticodon sits on the tRNA. They are complementary three-base sequences that pair up during translation, so the anticodon delivers the amino acid the codon calls for.
Because the genetic code is redundant: 64 codons code for only 20 amino acids. If a mutation changes a codon to a different codon that codes for the same amino acid (a silent mutation), the protein stays the same and function is normal.
You'll often be given an mRNA sequence and asked to count codons, predict the number of amino acids, or explain what a frameshift from an insertion or deletion does. The 2024 ribosome-profiling FRQ also used codons in a data context, measuring ribosome pausing as it moves codon to codon.