Genetic code
The genetic code is the rule set cells use to translate nucleotide sequences into protein sequences. It connects codons in messenger RNA with amino acids, start signals, stop signals, and the molecular machinery that builds proteins.
What the genetic code is
The genetic code is the set of rules that links nucleotide sequences to amino acid sequences. During protein production, a cell reads messenger RNA in three-letter codons. Each codon either specifies an amino acid or signals the end of translation.
Codons
A codon is a sequence of three nucleotides in DNA or RNA. Because RNA uses four bases, there are 64 possible three-base codons. This is enough to encode the 20 standard amino acids plus stop signals. The order of codons in an mRNA helps determine the order of amino acids in a protein.
From DNA to mRNA
Protein-coding information is stored in DNA, but ribosomes read messenger RNA. In transcription, a gene's DNA sequence is copied into RNA. In translation, the ribosome reads the mRNA codons and builds a chain of amino acids using transfer RNAs that match codons with the correct amino acids.
Start and stop signals
Translation usually begins at an AUG codon, which sets the reading frame and codes for methionine. Stop codons do not code for amino acids in the standard code. Instead, they recruit release factors that help end translation and release the finished protein chain.
Reading frames
A reading frame is the way a nucleotide sequence is divided into triplets. Shifting the starting point by one base changes every codon downstream. This is why insertions or deletions that are not in multiples of three can cause frameshift mutations with large effects on a protein.
Redundancy and wobble
The genetic code is redundant, meaning more than one codon can specify the same amino acid. Often the third base in a codon can vary without changing the amino acid. This flexibility, sometimes called wobble, helps explain why some DNA changes are silent at the protein level.
Nearly universal, with exceptions
The standard genetic code is used across much of life, which is one clue that living organisms share deep evolutionary ancestry. It is not absolutely universal, however. Mitochondria and some microorganisms use slightly different codon meanings.
Mutations and protein changes
Mutations can change codons. A substitution may leave the amino acid unchanged, replace one amino acid with another, or create a stop codon. Insertions and deletions can alter the reading frame. The biological effect depends on the protein, the position of the change, and the organism.
Why it matters
The genetic code connects DNA, RNA, and proteins into one readable system. It is essential for understanding genes, mutations, protein synthesis, biotechnology, genetic engineering, disease variants, antibiotics, and how cells turn stored information into working molecules.