Genetic code

The rules by which messenger RNA nucleotide sequences specify amino acids or start and stop signals during protein synthesis. Its codons are read as three-nucleotide units.

Codon: A sequence of three nucleotides in messenger RNA that specifies an amino acid or a translation signal. Each codon is the basic unit that the genetic code assigns a meaning.
Nucleotide: A molecule composed of a sugar, phosphate group, and nitrogenous base that forms DNA and RNA. Nucleotides are the symbols arranged into codons.
Francis Crick: A British molecular biologist who helped establish DNA’s structure and proposed key principles of protein synthesis. His adaptor hypothesis anticipated the molecules that connect codons with amino acids.
Universal genetic code: The near-universality of codon assignments across organisms, with a small number of known exceptions. Its broad conservation makes deviations in particular genomes especially informative.
Codon degeneracy: The property that multiple codons can specify the same amino acid. Redundant assignments explain why some nucleotide substitutions leave a protein unchanged.
Transfer RNA: An RNA molecule that carries an amino acid and pairs its anticodon with a messenger RNA codon. Its anticodon matches a codon while delivering the assigned amino acid.
Messenger RNA: An RNA molecule that carries protein-coding information from DNA to the cellular translation machinery. The code is conventionally written as assignments for messenger RNA codons.
Marshall Nirenberg: An American biochemist who helped decipher the genetic code using synthetic RNA and cell-free protein synthesis. His poly-U experiment showed that UUU specifies phenylalanine.
Mitochondrial genetic code: A set of codon assignments used in mitochondria that differs from the standard code in some organisms. Some mitochondrial codons encode different amino acids or signals than in the standard code.
Synonymous substitution: A nucleotide substitution that changes a codon without changing its encoded amino acid. It illustrates one consequence of redundant codon assignments.

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