What Is a Codon? Definition, Function, and Examples

A codon is a three-letter “word” in a gene’s message that tells a cell which protein building block to add next, or when to start and stop. Life writes its genes with only four chemical letters (A, C, G, and U in RNA, with T in place of U in DNA), yet uses them to build every protein in your body. What makes the system remarkable is how much sense it makes: it is built to shrug off many small errors, and almost every living thing reads it the same way.

How a Codon Turns Genetic Letters Into Protein

Genes are stored in DNA, but the cell first copies a gene into messenger RNA, or mRNA, a working copy that leaves the gene’s library. That is why codons are usually written in RNA letters, like UUU or AUG, even though they start out in DNA.

The cell copies the gene because DNA is the master copy, and in a human cell it stays protected in the nucleus. Many copies can come from one gene, so the cell can make a lot of a protein when it needs it. Messenger RNA is also broken down after a while, so the cell can turn production down partly by letting the messages fade.

Proteins are chains of amino acids, small building blocks of which life uses 20 kinds.1National Institutes of Health. Frameshift Mutation The cell reads the mRNA three letters at a time, and each triplet corresponds to one amino acid.2National Institutes of Health. Frameshift Mutation A gene’s sequence of codons therefore spells out the exact order of amino acids in its protein. Some codons act as punctuation, marking where a protein begins and ends.3The Tech Interactive. Why does a codon have three letters?

Take the strand AUG-UUU-UAA and split it into three codons. AUG starts the chain and adds methionine, and UUU adds phenylalanine. UAA tells the cell to stop, releasing a chain two amino acids long. Real genes run hundreds of codons, but the reading works the same way.

“Reading” is physical. A machine moves along the mRNA strand and handles three letters at a time, with no understanding involved. There are no spaces or commas between codons, so the grouping into threes depends on where reading starts. That makes the first codon read matter for every codon after it.

How the Cell Reads Each Codon

Inside the ribosome, the cell’s protein-building machine, molecules called transfer RNAs (tRNAs) read the mRNA in three-letter groups.4PubMed Central. Expanding codon size Each tRNA carries a matching three-letter patch, its anticodon, that pairs with a codon, and it holds the amino acid that codon calls for. A correct match sets off steps that let the ribosome accept that tRNA and attach its amino acid to the chain.5PubMed Central. An active role for tRNA in decoding beyond codon:anticodon pairing Near-matches tend to fall off the ribosome faster and are not sped along, which is how the cell keeps errors rare.6PubMed Central. An active role for tRNA in decoding beyond codon:anticodon pairing

The process runs as a cycle: the ribosome exposes one codon, a tRNA whose anticodon fits stays put, its amino acid joins the chain, and the ribosome slides three letters on. This repeats until a stop codon is reached.

The anticodon is the codon’s mirror because letters pair with fixed partners: A with U, and G with C. So codon UUU is read by an anticodon AAA, apart from the looser pairing at the third position described below.

A tRNA is a short RNA strand folded into a compact shape, with the anticodon at one end and the amino acid attached at the other. A dedicated enzyme must load each tRNA with the correct amino acid, and that loading step is another place where the cell checks for errors.

Why Codons Have Three Letters

With single letters, four bases could name only four amino acids.7University of Nebraska-Lincoln. What Is a Codon? Definition, Function, and Examples Pairs give 4 × 4 = 16 combinations, still short of 20.8University of Nebraska-Lincoln. What Is a Codon? Definition, Function, and Examples Triplets give 4 × 4 × 4 = 64 combinations, plenty for all 20 amino acids.9University of Nebraska-Lincoln. What Is a Codon? Definition, Function, and Examples The spare combinations leave room for signals that start and end a protein.10The Tech Interactive. Why does a codon have three letters? Sixty-four combinations minus the three stop signals leaves 61 codons for 20 amino acids, so the code has spare capacity.

Scientists proved the triplet code by experiment. In 1961 Nirenberg and Matthaei fed a cell extract RNA made only of U. It produced a chain of only phenylalanine, so UUU means phenylalanine. That was the first codon decoded, and others followed the same approach. By 1966 the full code was worked out.

Codon Examples Including Start and Stop Codons

Start is not always AUG. In bacteria, GUG can also serve as a start codon.14PubMed Central. Initiation at AUGUG and GUGUG sequences can lead to translation of overlapping reading frames in E. coli A nearby landing sequence on the mRNA helps the ribosome choose which start to use.15PubMed Central. Initiation at AUGUG and GUGUG sequences can lead to translation of overlapping reading frames in E. coli

A codon chart lays all 64 codons in a grid. Find the first letter on the left, the second along the top, and the third on the right. The box where they meet names the amino acid. Stop codons are usually marked “Stop” rather than an amino acid. Charts use RNA letters, so swap U for T if you are reading DNA.

To find GCU, start with G in the first-letter group, then choose C as the second letter, then U as the third. The box says alanine.

Why Several Codons Mean the Same Amino Acid

Because many amino acids have more than one codon, a changed letter often still gives the same amino acid.16GenScript. What Is a Codon? Definition, Function, and Examples Single-letter changes usually cause no change or a swap to a chemically similar amino acid.17PubMed. Genetic code: aspects of organization18PubMed. Genetic code: aspects of organization

“Redundant” here means several codons map to one amino acid. It never means one codon maps to several amino acids. That one-way rule keeps the code unambiguous: reading a codon always gives a single answer.

The third letter follows a pattern. For many amino acids, such as alanine, the first two letters decide the amino acid and the third can be anything. Other amino acids split by the third letter, with A or G giving one answer and C or U giving another in many cases. So a mistake in the third position is the least likely to change the protein, while a change in the first or second is more likely to.

Cells have fewer tRNA types than codons, so one tRNA can read more than one codon.19PubMed. tRNA’s wobble decoding of the genome: 40 years of modification The third position pairs loosely, an idea Francis Crick proposed in 1966.20PubMed. tRNA’s wobble decoding of the genome: 40 years of modification Small chemical tweaks to tRNAs let one tRNA read several codons, such as both AAA and AAG for lysine.21PubMed Central. Anticodon stem-loop tRNA modifications influence codon decoding and frame maintenance during translation One tweak lets a tRNA read up to three codons, for example three of arginine’s codons.22PubMed Central. Anticodon stem-loop tRNA modifications influence codon decoding and frame maintenance during translation

Silent changes are not always harmless. A “silent” change keeps the amino acid but can alter how fast the codon is read. In one cystic fibrosis gene, such a change altered how disease mutations behaved.23PubMed Central. Positive epistasis between disease-causing missense mutations and silent polymorphism with effect on mRNA translation velocity

What Happens When a Codon Changes

A single-letter change comes in three kinds. In a silent change, the new codon names the same amino acid. In a missense change, it names a different amino acid, as in many disease mutations. In a nonsense change, the codon becomes a stop, cutting the protein short. Cells often destroy messages with early stops, but not always.

The cell groups letters in threes from a fixed starting point, called the reading frame.24National Institutes of Health. Frameshift Mutation Inserting or deleting letters in a number that is not a multiple of three shifts that frame, a change known as a frameshift.25National Institutes of Health. Frameshift Mutation Every codon after the mutation is then misread, producing a very different protein.26Cold Spring Harbor Symposia on Quantitative Biology. Frameshift Mutations and the Genetic Code

That is why a three-letter deletion is gentler. It removes one whole codon, so the frame stays intact and the amino acids after the gap are still correct. The protein ends up missing one amino acid, which can matter a great deal or very little depending on where it sits. Losing one or two letters changes the grouping of every later codon, so the damage is much greater.

A frameshift often ends in a stop as well, since stop codons occur by chance among the misread codons. So many frameshifted proteins have wrong amino acids up to the point where they end early.

This vocabulary turns up on genetic test reports, which may name a variant as frameshift, missense or nonsense. Frameshift and nonsense variants often, though not always, mean a protein that does not work. Missense effects range from none to severe, depending on the swap.

Is the Codon Code the Same in All Life

Over 50 natural variations of the standard code have been described in organisms and their mitochondria.27PubMed. Natural and artificial variations of the standard genetic code Some arthropod mitochondria read AGG as lysine, where the standard code says arginine.28PubMed Central. Parallel Evolution of the Genetic Code in Arthropod Mitochondrial Genomes Some single-celled ciliates read the stop codon UAG as leucine or glutamine.29PubMed Central. Multiple independent genetic code reassignments of the UAG stop codon in phyllopharyngean ciliates In many organisms UGA, normally a stop, can insert a rare amino acid called selenocysteine when a special signal follows it.30PubMed Central. Recoding of the selenocysteine UGA codon by cysteine in the presence of a non-canonical tRNACys and elongation factor SelB

Your own cells include an example. Mitochondria, the cell’s energy parts, carry their own small genome and use a code that differs slightly from the main one. Reading a mitochondrial gene needs a different chart.

The standard code is shared so widely because all life likely descends from a common ancestor that used it.31PubMed Central. Origin and evolution of the genetic code: the universal enigma Changing a codon’s meaning would garble thousands of proteins at once.32PubMed Central. Origin and evolution of the genetic code: the universal enigma Scientists debate whether the code’s error-resistant layout was shaped by natural selection or largely fixed by chance early on.33PubMed Central. Origin and evolution of the genetic code: the universal enigma

How Codons Matter in mRNA Vaccines and Medicine

Cells favour certain codons in busy genes, matching the tRNAs they have most of.34PubMed Central. Codon usage bias from tRNA’s point of view: Redundancy, specialization, and efficient decoding for translation optimization Codons whose tRNAs are scarce are read more slowly. Reading speed can change how a protein folds, and swapping rare codons for common ones once left about 20% of an enzyme misfolded.35PubMed Central. Translation Rates and Protein Folding

A protein begins folding as it leaves the ribosome, so pauses may help or hinder the folding. Designers therefore cannot simply choose the fastest codon everywhere and expect the best result.

mRNA vaccines can be rewritten with chosen codons while still coding the same protein.36PubMed Central. Optimization and Deoptimization of Codons in SARS‐CoV‐2 and Related Implications for Vaccine Development In mice, one design method raised antibody levels up to 128 times over standard codon-optimized COVID-19 and shingles vaccine designs.37PubMed Central. Algorithm for optimized mRNA design improves stability and immunogenicity In mice, an optimized flu mRNA gave about ten times stronger protective antibodies.38PubMed Central. Deep generative optimization of mRNA codon sequences for enhanced mRNA translation and therapeutic efficacy Mouse results do not translate directly into human dose or effect.

Some labs go further and add brand-new codons. Scientists can assign a codon to an amino acid outside the usual 20.39PubMed Central. Expanding the Genetic Code for Biological Studies It needs its own matched tRNA and loading enzyme that don’t interfere with the cell’s.40PubMed Central. Expanding the Genetic Code for Biological Studies Labs have even made extra artificial DNA letters to create new codons.41PubMed Central. Genetic Code Engineering by Natural and Unnatural Base Pair Systems for the Site-Specific Incorporation of Non-Standard Amino Acids Into Proteins