Codon
A codon is a sequence of three consecutive nucleotides (a triplet) in mRNA that specifies a particular amino acid or signals the start or stop of protein synthesis. With four possible bases (A, U, G, C) at each of three positions, there are 4³ = 64 possible codons, which encode 20 standard amino acids plus three stop codons and one start codon (AUG, which codes for methionine). The redundancy of the genetic code — where multiple codons can specify the same amino acid — is called degeneracy and provides robustness against point mutations.
Key Formula
Number of codons = 4^3 = 64
LaTeX: N_{\text{codons}} = 4^3 = 64
| Symbol | Meaning | Unit |
|---|---|---|
| N | Total number of possible codons | dimensionless |
| 4 | Number of possible bases at each position (A, U, G, C) | dimensionless |
| 3 | Number of nucleotide positions per codon | dimensionless |
Selected Codons, Their Amino Acids, and Function
| Codon | Amino Acid / Function | Type | Notes |
|---|---|---|---|
| AUG | Methionine (Met) | Start codon | Initiates translation |
| UUU / UUC | Phenylalanine (Phe) | Sense codon | Example of degeneracy |
| UAA | Stop (Ochre) | Stop codon | Terminates translation |
| UAG | Stop (Amber) | Stop codon | Terminates translation |
| UGA | Stop (Opal) | Stop codon | Terminates translation |
| GGG / GGA / GGC / GGU | Glycine (Gly) | Sense codon | Fourfold degenerate |
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Related Terms
Anticodon
An anticodon is a three-nucleotide sequence located in the anticodon loop of a transfer RNA (tRNA) molecule that is complementary and antiparallel to a specific mRNA codon during translation. Base pairing between the codon and anticodon — following Watson-Crick rules (A-U and G-C) and sometimes wobble base pairing at the third position — ensures that the correct amino acid is added to the growing polypeptide chain. Each tRNA carries a specific amino acid at its 3' CCA end that corresponds to its anticodon, making tRNA the physical link between the nucleotide and amino acid languages of the cell.
Translation (biology)
Translation is the process by which a ribosome decodes the nucleotide sequence of a messenger RNA (mRNA) and synthesizes the corresponding sequence of amino acids to produce a polypeptide chain. It occurs in three phases — initiation, elongation, and termination — and takes place at ribosomes in the cytoplasm of both prokaryotes and eukaryotes. The genetic code, read in triplets called codons, determines which amino acid is incorporated at each step, with transfer RNA (tRNA) molecules acting as adaptors between the mRNA codons and the amino acids.
Messenger RNA (mRNA)
Messenger RNA (mRNA) is a single-stranded RNA molecule that carries genetic information transcribed from DNA and serves as the template for protein synthesis during translation. In eukaryotes, the primary transcript (pre-mRNA) is processed in the nucleus by addition of a 5' 7-methylguanosine cap, a 3' poly-adenylate tail, and removal of non-coding introns by the spliceosome before being exported to the cytoplasm. The lifespan of an mRNA molecule varies from minutes to hours and is tightly regulated, making mRNA abundance a key determinant of gene expression levels.
From the English word "code," meaning a system of signals or symbols. The term codon was coined by Francis Crick in 1963 to describe the three-base unit of the genetic code, reflecting the idea that DNA "encodes" protein sequence information.