The genetic code translates nucleotide sequences into proteins
During translation, a ribosome reads an mRNA sequence in groups of three nucleotides called codons. Each codon specifies an amino acid or a stop signal. Four RNA bases can form different triplets; 61 normally encode amino acids and three normally signal termination.
The code is also degenerate: more than one codon can specify the same amino acid. Degeneracy does not make the code ambiguous, because each individual codon has one standard meaning in a given translation system.
A shared mapping preserves evolutionary information
The mapping between codons and amino acids is nearly the same across bacteria, archaea, and eukaryotes. That conservation matters because chemistry permits many conceivable mappings. Once an early translation system became established, descendants that inherited it could continue reading the same genetic information.
Consider a hypothetical gene moved from one species into another. If the recipient uses the standard code and the needed expression machinery is present, its ribosomes can interpret the donor's codons using the same basic amino-acid assignments. This cross-species compatibility reflects a deeply shared biological system.
“Nearly universal” is more accurate than “universal”
Some mitochondria and a small number of microorganisms reassign particular codons. For example, a codon that is normally a stop signal may specify an amino acid in a mitochondrial translation system. These exceptions show that the code can evolve, especially in small, specialized genomes.
The exceptions do not erase the dominant pattern. Instead, they support a precise conclusion: most life shares a highly conserved standard code inherited through evolutionary history, with a limited number of later modifications.
Use the code as one line of evidence
The shared genetic code is powerful evidence for common ancestry, but evolutionary relationships are evaluated with multiple independent observations, including homologous genes, protein sequences, cellular structures, and the fossil record. The code is most informative when interpreted as part of that converging evidence.
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Mastering Biology Chapter 1 Answer Key: Why Is the Genetic Code Nearly Universal?Sources
These references support the core concepts and interpretation boundaries explained above.