DNA Codon- The Genetic Code Explained
What Is a DNA Codon?
A codon is a sequence of three nucleotide bases in DNA or RNA. Each codon specifies a particular amino acid or signals the start or end of a protein sequence.
Think of it this way: DNA is written in a language. The alphabet has only four letters—A, T, G, and C (or A, U, G, and C in RNA). But proteins are built from 20 different amino acids. The genetic code solves this problem by reading three letters at a time.
Three letters × four bases = 64 possible combinations. That's more than enough to code for 20 amino acids, with plenty left over.
The Genetic Code Table
The standard genetic code maps each codon to an amino acid. Here's how it works:
| First Base | Second Base | Third Base | Amino Acid |
|---|---|---|---|
| U | U | U / C / A / G | Phenylalanine (Phe) |
| U | C | Any | Serine (Ser) |
| U | A | U / C | Tyrosine (Tyr) |
| U | A | A / G | Stop codon |
| U | G | U / C | Cysteine (Cys) |
| U | G | A / G | Stop (UGA) / Tryptophan (UGG) |
| A or G | U | Any | Isoleucine (Ile) / Leucine (Leu) |
The table has 64 entries. Most amino acids are coded by multiple codons. Only tryptophan and methionine have single codons.
Start and Stop Codons
The codon AUG does double duty. It's the code for methionine, and in most organisms, it's also the start codon. Translation begins here.
Three codons are stop signals:
- UAA — ochre
- UAG — amber
- UGA — opal
When a ribosome hits a stop codon, there's no tRNA with that anticodon. Release factors bind instead, and the protein chain is released.
Degeneracy and the Wobble Position
The third base of a codon is less specific. This is called the wobble position. For example, GCU, GCC, GCA, and GCG all code for alanine. The first two positions matter most.
This redundancy protects against some mutations. If a point mutation changes the third base, the protein might still be correct.
How Mutations Affect Codons
Mutations in codon regions have three possible outcomes:
- Silent mutations — the codon changes, but the amino acid stays the same. Example: CCU → CCC (both code for proline)
- Missense mutations — the codon changes to a different amino acid. This can alter protein function
- Nonsense mutations — the codon changes to a stop codon, truncating the protein prematurely
Frameshift mutations are worse. Inserting or deleting a single nucleotide shifts the reading frame. Every codon downstream is wrong.
Practical: How to Read a Codon Table
Step 1: Identify the mRNA Sequence
DNA is double-stranded. The coding strand is what you're reading. The template strand is what gets transcribed into mRNA. Remember: DNA uses T, RNA uses U.
Step 2: Group Into Triplets
Starting from the 5' end, divide your sequence into groups of three. The reading frame matters. Wrong frame = completely wrong protein.
Step 3: Look Up Each Codon
Match your triplet to the codon table. For AUG, that's methionine and the start signal. For UAA, UAG, or UGA, you've hit a stop.
Step 4: Translate to Amino Acids
Each codon adds one amino acid to the growing chain. Continue until you hit a stop codon or the end of your sequence.
Comparing Genetic Code Variations
The standard code applies to most organisms. Mitochondria and some protists use slightly different tables. Here's a quick comparison:
| System | UGA Codes For | Other Differences |
|---|---|---|
| Standard (Nuclear) | Stop | — |
| Human Mitochondria | Tryptophan | AGA, AGG = Stop (not Arg) |
| Yeast Mitochondria | Tryptophan | CUN = Threonine (not Leucine) |
| Some Ciliates | Tryptophan or Stop | UAA, UAG = Glutamine |
These variations are why you can't always apply the standard table to organelle genomes.
The Bottom Line
Codons are the bridge between nucleic acids and proteins. The genetic code is nearly universal, degenerate, and ordered. It has a clear start signal, clear stop signals, and redundancy that provides some mutation protection.
You don't need to memorize all 64 codons. Know the start codon, the three stops, and the general pattern. The rest is reference material.