Which amino acid does the codon UGG code for?
- First base U picks the top block, second base G the fourth row of that block.
- Third base G picks the last column: Trp.
Answer: Tryptophan (Trp, W). UGG is its only codon.
The genetic code, in one table
Codon chart: mRNA codons to amino acids
The codon chart (genetic code table) shows which amino acid each three-letter mRNA codon codes for. Type a codon such as AUG or an amino acid such as leucine to find it, then use the amino acid table to see every codon that codes for it.
Type three bases (mRNA or DNA letters) or an amino acid name, three-letter code, or one-letter code.
The standard codon chart
Read a codon from its 5′ end. Find the first base in the left column, the second base in the next column, and the third base along the top. The cell where they meet is the amino acid. 61 codons code for the 20 amino acids and 3 are stop signals.
| 1st base (5′) | 2nd base | 3rd base: U | 3rd base: C | 3rd base: A | 3rd base: G |
|---|---|---|---|---|---|
| U | U | UUU Phe (F) | UUC Phe (F) | UUA Leu (L) | UUG Leu (L) |
| C | UCU Ser (S) | UCC Ser (S) | UCA Ser (S) | UCG Ser (S) | |
| A | UAU Tyr (Y) | UAC Tyr (Y) | UAA Stop | UAG Stop | |
| G | UGU Cys (C) | UGC Cys (C) | UGA Stop | UGG Trp (W) | |
| C | U | CUU Leu (L) | CUC Leu (L) | CUA Leu (L) | CUG Leu (L) |
| C | CCU Pro (P) | CCC Pro (P) | CCA Pro (P) | CCG Pro (P) | |
| A | CAU His (H) | CAC His (H) | CAA Gln (Q) | CAG Gln (Q) | |
| G | CGU Arg (R) | CGC Arg (R) | CGA Arg (R) | CGG Arg (R) | |
| A | U | AUU Ile (I) | AUC Ile (I) | AUA Ile (I) | AUG Met (M) |
| C | ACU Thr (T) | ACC Thr (T) | ACA Thr (T) | ACG Thr (T) | |
| A | AAU Asn (N) | AAC Asn (N) | AAA Lys (K) | AAG Lys (K) | |
| G | AGU Ser (S) | AGC Ser (S) | AGA Arg (R) | AGG Arg (R) | |
| G | U | GUU Val (V) | GUC Val (V) | GUA Val (V) | GUG Val (V) |
| C | GCU Ala (A) | GCC Ala (A) | GCA Ala (A) | GCG Ala (A) | |
| A | GAU Asp (D) | GAC Asp (D) | GAA Glu (E) | GAG Glu (E) | |
| G | GGU Gly (G) | GGC Gly (G) | GGA Gly (G) | GGG Gly (G) |
Amino acid to codon table
The reverse lookup: every amino acid with its abbreviations, all the mRNA codons that code for it, and its side-chain class. Use it when a question asks which codons could code for a given amino acid, or how many.
| Amino acid | 3-letter | 1-letter | mRNA codons | Codons | Side chain |
|---|---|---|---|---|---|
| Alanine | Ala | A | GCU GCC GCA GCG | 4 | Nonpolar (hydrophobic) |
| Arginine | Arg | R | CGU CGC CGA CGG AGA AGG | 6 | Positively charged (basic) |
| Asparagine | Asn | N | AAU AAC | 2 | Polar, uncharged |
| Aspartic acid | Asp | D | GAU GAC | 2 | Negatively charged (acidic) |
| Cysteine | Cys | C | UGU UGC | 2 | Polar, uncharged |
| Glutamine | Gln | Q | CAA CAG | 2 | Polar, uncharged |
| Glutamic acid | Glu | E | GAA GAG | 2 | Negatively charged (acidic) |
| Glycine | Gly | G | GGU GGC GGA GGG | 4 | Nonpolar (hydrophobic) |
| Histidine | His | H | CAU CAC | 2 | Positively charged (basic) |
| Isoleucine | Ile | I | AUU AUC AUA | 3 | Nonpolar (hydrophobic) |
| Leucine | Leu | L | UUA UUG CUU CUC CUA CUG | 6 | Nonpolar (hydrophobic) |
| Lysine | Lys | K | AAA AAG | 2 | Positively charged (basic) |
| Methionine | Met | M | AUG | 1 | Nonpolar (hydrophobic) |
| Phenylalanine | Phe | F | UUU UUC | 2 | Nonpolar (hydrophobic) |
| Proline | Pro | P | CCU CCC CCA CCG | 4 | Nonpolar (hydrophobic) |
| Serine | Ser | S | UCU UCC UCA UCG AGU AGC | 6 | Polar, uncharged |
| Threonine | Thr | T | ACU ACC ACA ACG | 4 | Polar, uncharged |
| Tryptophan | Trp | W | UGG | 1 | Nonpolar (hydrophobic) |
| Tyrosine | Tyr | Y | UAU UAC | 2 | Polar, uncharged |
| Valine | Val | V | GUU GUC GUA GUG | 4 | Nonpolar (hydrophobic) |
| Stop | Stop | * | UAA UAG UGA | 3 | Stop signal |
Start codon, stop codons, and DNA codons
Start codon. AUG codes for methionine and is where ribosomes begin translating, so almost every new protein starts with Met. AUG inside a gene simply adds another methionine.
Stop codons. UAA, UAG, and UGA match no tRNA. A release factor binds instead and the finished chain is let go. A mutation that creates one early is a nonsense mutation.
DNA codons. The chart uses mRNA letters. A coding-strand DNA triplet has the same letters with T for U, so ATG means AUG. A template-strand triplet must be complemented first: 3′-TAC-5′ becomes AUG. The DNA to mRNA converter does both for a whole gene.
Codons and anticodons. A codon is on the mRNA. The anticodon is the complementary triplet on the tRNA that reads it, written 3′ → 5′: AUG is read by UAC.
Worked examples
Reading the codon chart: worked examples
Four questions that come up in every genetics unit, worked from the chart above.
What amino acid sequence does the mRNA 5′-AUG GCA UUC UAG-3′ code for?
- AUG = Met (start), GCA = Ala, UUC = Phe.
- UAG is a stop codon, so the chain ends there.
Answer: Met-Ala-Phe, a 3-amino-acid peptide (one-letter code MAF).
Which codons code for leucine?
- Scan the chart for Leu: four in the CU_ row and two in the UU_ row.
- Only the third base differs within the CU_ group, a typical pattern of the code.
Answer: UUA, UUG, CUU, CUC, CUA, CUG: 6 codons.
The DNA template strand reads 3′-TAC-5′. Which amino acid does it code for?
- Transcribe first: the complement of TAC with U for T is AUG.
- Look up AUG in the chart, not TAC: the chart uses mRNA letters.
Answer: Methionine (Met), the start codon.
Convert a longer DNA sequenceA little question. A bigger picture.
Codon chart questions
Wondering about the why? Start here.
How do you read a codon chart?
Read the mRNA codon from its 5′ end. Find its first base in the left column, its second base in the next column, and its third base along the top; the cell where the three meet is the amino acid. For GCA: first base G, second base C, third base A gives alanine (Ala). If you have DNA, convert it to mRNA first: swap T for U on the coding strand, or take the complement of the template strand.
Convert DNA to mRNAHow many codons are there?
There are 64 codons, every possible combination of three of the four bases (4 × 4 × 4). Sixty-one code for the 20 amino acids and three (UAA, UAG, UGA) are stop signals. Because 61 codons share 20 amino acids, most amino acids have several: leucine, serine, and arginine have six each, while methionine (AUG) and tryptophan (UGG) have only one.
See the amino acid tableWhat are the start and stop codons?
AUG is the start codon: it codes for methionine and marks where the ribosome begins translating, which is why new proteins almost always begin with Met. UAA, UAG, and UGA are the stop codons: no tRNA matches them, so a release factor binds and the finished polypeptide is released. In DNA coding-strand letters these are ATG, and TAA, TAG, and TGA.
Meet the ribosomeWhat is the difference between a codon and an anticodon?
A codon is a three-base sequence on the mRNA that specifies one amino acid. An anticodon is the complementary three-base sequence on a tRNA molecule that pairs with it during translation, antiparallel, so it is written 3′ → 5′. The codon 5′-AUG-3′ pairs with the anticodon 3′-UAC-5′ on the tRNA that carries methionine.
Read: transcription and translation