Life sciences · Reference
What is a codon?
A codon is a sequence of three nucleotide bases in messenger RNA that specifies a particular amino acid, or signals the start or stop of protein synthesis — the basic unit of the genetic code.
The triplet code
The genetic instructions in messenger RNA are read in units of three bases. Each such triplet is a codon, and because there are four possible bases at each of three positions, there are 4 × 4 × 4 = 64 possible codons. Sixty-one of these specify one of the twenty standard amino acids, while three — UAA, UAG, and UGA — are stop codons that signal the end of a protein. The codon AUG both starts most proteins and codes for the amino acid methionine.
Reading codons during translation
Codons are decoded at the ribosome during translation. Each codon on the mRNA is matched by a transfer RNA carrying a complementary three-base anticodon and the corresponding amino acid.
The ribosome moves along the mRNA one codon at a time, adding amino acids in the order the codons dictate. The reading frame — where the counting of triplets begins — is fixed by the start codon and must be maintained for the protein to be built correctly.
A nearly universal, redundant code
The genetic code is described as degenerate or redundant because most amino acids are specified by more than one codon. It is also nearly universal: the same codons mean the same amino acids in almost all organisms, a unity that reflects the common ancestry of life and underpins the transfer of genes between species in biotechnology. The code was deciphered in the 1960s, work recognised with the 1968 Nobel Prize in Physiology or Medicine.
Key facts
At a glance
- Definition: a three-base unit of mRNA specifying an amino acid or stop
- Total possible codons: 64
- Amino-acid-coding codons: 61
- Stop codons: 3 (UAA, UAG, UGA)
- Start codon: AUG (also codes methionine)
- Property: degenerate (redundant) and nearly universal
Common questions
FAQ
What is a codon?+
A codon is a sequence of three bases in messenger RNA that specifies a particular amino acid or a stop signal. Codons are the units in which the ribosome reads the genetic code during translation.
How many codons are there?+
There are 64 possible codons. Sixty-one specify amino acids and three are stop codons that end protein synthesis. Because there are only twenty standard amino acids, most are coded by more than one codon.
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