Codons are the information units that connect nucleotide sequences in mRNA to amino acid sequences in proteins. Understanding codon structure, reading rules, and genetic-code universality explains both accurate translation and evolutionary relationships.

Codons as the units of the genetic code

mRNA is interpreted during translation in groups of three nucleotides. Each three-nucleotide “word” is a codon that corresponds to a specific meaning in protein synthesis.

Standard genetic code table showing how each mRNA triplet codon specifies an amino acid or a stop signal. The layout makes degeneracy obvious: many amino acids correspond to multiple codons, often differing at the third (“wobble”) position. Start (AUG→Met) and stop codons (UAA, UAG, UGA) are explicitly indicated. Source

Codons are read by the ribosome in a defined orientation and grouping:

• The ribosome reads mRNA 5' → 3'.

• Codons are non-overlapping: each nucleotide is part of only one codon within a reading frame.

• Codons are read continuously: there are no “commas” between codons; the ribosome establishes a starting point and then proceeds in triplets.

Start and stop signals within codons

Not all codons specify amino acids. Some codons provide punctuation for translation:

• A start codon establishes the reading frame and initiates translation.

  • In most cases, the start codon is AUG, which specifies methionine in the genetic code.

• Stop codons signal termination of translation rather than adding an amino acid.

  • The standard stop codons are UAA, UAG, and UGA.

Overview diagram of translation showing initiation at a start codon (AUG), elongation as tRNAs add amino acids, and termination when a stop codon is reached. It reinforces that the ribosome reads the mRNA in the 5'→3' direction and that start/stop codons define where protein synthesis begins and ends. The labeled components (ribosomal subunits, tRNA, release factor) connect codon signals to the mechanics of translation. Source

The presence of defined start and stop codons ensures that the same mRNA can produce a predictable polypeptide when translation begins at the correct site.

Reading frame and why triplets matter

Because codons are read in triplets, the grouping of nucleotides into codons is essential. The same RNA sequence can encode completely different amino acid sequences if the reading frame shifts.

Key implications of triplet reading:

• A shift in the reading frame changes every downstream codon.

Example of a frameshift mutation caused by adding a nucleotide, which changes how the sequence is partitioned into triplet codons. The figure shows that once the grouping shifts, the amino acids translated downstream are altered, often producing a drastically different polypeptide. This visual links the abstract idea of “reading frame” to the practical consequences of insertions/deletions. Source

• The ribosome’s selection of the correct frame is crucial for producing the intended polypeptide.

• Triplets provide enough combinations to encode all amino acids:

  • With four RNA bases (A, U, C, G), there are $4^3 = 64$ possible codons.

Even though there are 64 codons, there are fewer standard amino acids, leading to redundancy in the code.

Redundancy (degeneracy) and specificity

The genetic code is degenerate: multiple codons can specify the same amino acid. This redundancy is not random; often the first two nucleotides are most important, while the third base may vary.

High-utility points for AP Biology:

• Degeneracy can reduce the impact of some base substitutions on protein sequence because a changed codon may still specify the same amino acid.

• Despite degeneracy, the code is unambiguous: each codon specifies only one amino acid or a stop signal (a given codon does not mean two different amino acids).

These properties help explain how translation can be both robust (tolerating some changes) and precise (maintaining consistent meaning for each codon).

Nearly universal genetic code and common ancestry

The AP focus emphasises that “the genetic code is nearly universal,” meaning that the same codons specify the same amino acids in most organisms. This broad conservation has major biological significance.

Why universality matters:

• Shared translation “dictionary”: bacteria, plants, and animals interpret codons in essentially the same way.

• Evidence for common ancestry: a conserved code across life strongly supports the idea that all organisms descended from a common ancestor with an early, established coding system.

• Functional constraint: changing codon meanings would disrupt many proteins at once, so strong selection maintains codon assignments over evolutionary time.

Exceptions and what “nearly” means

“Nearly universal” allows for limited deviations from the standard code:

• Some mitochondria and certain unicellular eukaryotes use alternative codon assignments.

• These exceptions are typically constrained to particular genomes or organelles and do not overturn the overall conservation of the code.

The existence of rare variants is consistent with the idea that the code is ancient and strongly conserved, yet capable of limited evolutionary modification in isolated contexts.

Codon–amino acid mapping as information flow

The central point for this subsubtopic is the mapping from mRNA triplet codons to amino acids:

• Input: an mRNA sequence read in triplets.

• Output: an ordered amino acid sequence in a polypeptide.

• Core principle: codon meaning is (1) consistent across nearly all organisms and (2) sufficient to encode all amino acids plus stop signals.

This codon-based system is foundational for understanding how genotype (nucleotide sequence) determines phenotype (protein structure and function), and why the shared coding scheme across life is powerful evidence of evolutionary relatedness.