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Genetic Code

For medical students2 min readUpdated 2026-10-10

The genetic code (or biological code) is the universal system by which information regarding the amino acid sequence of proteins is stored. This vital information is encoded by a precise sequence of nucleotides within DNA or RNA molecules.

Code unitTriplet (codon) consisting of exactly 3 nucleotides
SpecificityA single triplet encodes only one specific amino acid
DegeneracyAn amino acid can be encoded by multiple triplets (ranging from 2 to 6)
Stop codonsTermination triplets UAA, UAG, and UGA signal the end of protein synthesis
UniversalityThe code is nearly identical across all living organisms, from bacteria to humans

Core Concept of the Biological Code

For polypeptide synthesis to occur within a cell, clear instructions are required. The genetic code serves as this exact set of instructions. Essentially, it is a translation mechanism from the "language" of nucleic acids (nucleotide sequences in DNA or RNA) into the "language" of proteins (amino acid sequences). Without this mechanism, the precise synthesis of structural and functional molecules in living systems would be impossible.

Key Properties of the Genetic Code

Biochemists outline several fundamental properties that describe the rules for reading genetic information:

  1. Triplet nature. The coding unit consists of three elements. This means the smallest unit of the code is a triplet (or codon), which consists of exactly three nucleotide residues. A single triplet encodes one amino acid.
  2. Specificity. The rule of unambiguity: each specific triplet encodes only one amino acid, without exceptions.
  3. Degeneracy (redundancy). Conversely, the reverse rule also applies: the same amino acid can be encoded by several different triplets. Typically, their number varies from 2 to 6. This provides an important buffer against mutations and errors.
  4. Universality. The biological code is nearly identical across almost all existing species. These rules operate identically in both bacterial and human cells.

Reading Rules and Synthesis Termination

The execution of genetic information follows strict directional and termination rules:

Mnemonic

The properties of the genetic code can be remembered by their initials: T (triplet), S (specificity), D (degeneracy), U (universality), U (unidirectionality), C (collinearity).

Frequently asked questions

Which amino acids are encoded by only a single codon (exceptions to degeneracy)?

Only methionine and tryptophan are encoded by a single codon, serving as exceptions to the degeneracy (redundancy) of the genetic code.

Degeneracy means that more than one triplet can correspond to a single amino acid. Most amino acids are encoded by multiple codons:

  • Leucine — encoded by 6 codons.
  • Serine — encoded by 6 codons.
  • Glycine — encoded by 4 codons.
  • Alanine — encoded by 4 codons.

When an amino acid is encoded by multiple codons, they most frequently differ at the third nucleotide position.

What are the exceptions to the universality of the genetic code?

Exceptions to the universality of the genetic code include mitochondrial DNA and a few other minor instances.

The property of universality implies that the genetic code is identical across almost all species, though mitochondrial DNA exhibits minor deviations.

What is the significance of the non-overlapping nature of the genetic code?

The non-overlapping nature means that codons in mRNA are read sequentially, one after another.

A single nucleotide does not simultaneously belong to two adjacent codons (triplets). Reading occurs triplet by triplet; each nucleotide belongs to one triplet only.

What is a start (initiation) codon, and what amino acid does it encode?

The start (initiation) codon is the AUG triplet, which encodes methionine and is located near the 5' end of the coding region to initiate translation.

This codon enters the peptidyl (P) site of the ribosome, where initiator tRNA delivers methionine. If AUG triplets are found internally within a gene, they simply code for internal methionines and are not initiation codons. In prokaryotes, the starting amino acid is N-formylmethionine.

How many total possible triplets (codons) exist, and how many of them are sense codons?

There are 64 possible three-nucleotide combinations (codons).

Out of these, 61 are sense (coding) codons capable of encoding the 20 standard proteinogenic amino acids. The remaining 3 codons (UAA, UAG, UGA in mRNA) are non-sense triplets. These are termination codons (stop codons) that do not code for amino acids, acting instead as punctuation marks to signal the termination of protein synthesis.

What is the difference between specificity and degeneracy of the code?

Specificity means that a single triplet encodes strictly one amino acid. Degeneracy means that a single amino acid can be encoded by multiple different triplets (from 2 to 6).

What are stop codons, and what are their types?

They are termination triplets signaling the end of protein synthesis. They do not encode amino acids. They include UAA, UAG, and UGA.

In which direction is the genetic code read?

Information (codons) in mature mRNA is read in only one direction — from the 5' end to the 3' end.

What does the term "collinearity" mean?

Collinearity is the principle where the sequence of triplets in a gene (and mature mRNA) directly matches the amino acid sequence in the protein.

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