You know from Translation that each codon names one amino acid. The full dictionary is the genetic code, usually printed as a codon table. Reading it is a skill worth practicing.
Key idea
To read a codon, find its first base on the left, its second base across the top, and its third base on the right. Translation starts at AUG (methionine, Met) and ends at UAA, UAG or UGA. There are 64 codons for 20 amino acids.
Why it works
Because 64 codons share 20 amino acids, most amino acids have more than one codon: leucine has six, glycine four, methionine only one. Codons for the same amino acid usually differ only in the third base. That redundancy protects the protein: many changes in a third base change nothing.
Worked example: One codon
Which amino acid does the codon GCU code for?
- First base G: the bottom row.
- Second base C: the second column.
- Third base U: the first line in that box.
Answer: Alanine (Ala).
Worked example: A whole message
Translate 5′-GGAUGUUUAAAUGA-3′.
- Find the first AUG: it starts at the third base. Ignore GG before it.
- Read in threes from AUG: AUG UUU AAA UGA.
- AUG = Met, UUU = Phe, AAA = Lys, UGA = stop.
Answer: Met-Phe-Lys
Worked example: From DNA letters
A gene's coding strand has the codon TGG. What amino acid does it code for?
- The table uses mRNA letters, so change T to U: UGG.
- UGG is tryptophan.
Answer: Tryptophan (Trp).
The code is nearly universal: bacteria, plants and people read the same codons as the same amino acids, with only a few small exceptions. That shared code is strong evidence that all life is related, and it is why a bacterium can make a human protein in Recombinant DNA.
Common mistake
Two errors are common: looking up DNA letters (T is not in the table; change T to U first) and starting at the first base of the message instead of at AUG. Find AUG, then read in threes, and stop at the first stop codon.