Why does the mitochondrial genetic code differ from the universal code?

• A. Mitochondria have their own ribosomes and tRNAs; some codons are reinterpreted (e.g., AUA codes for methionine, UGA codes for tryptophan in some mitochondria).
• B. The universal code applies unchanged to mitochondria.
• C. Mitochondria do not use codons.
• D. Mitochondria use an RNA-only translation system.

Answer: (A)

Mitochondria carry their own translation machinery separate from the cytoplasmic system, which allows certain codons to be reinterpreted. This difference arises from their evolutionary origin as endosymbiotic bacteria and from having a limited set of tRNAs and ribosomes tuned to mitochondrial needs. Because their translation components are distinct, some codons are read differently than in the universal code.

For example, in many mitochondria, AUA does not code for isoleucine as it does in the standard code; instead it codes for methionine. Likewise, UGA, which is a stop signal in the universal code, can be read as tryptophan in mitochondria. These reassignments reflect the specialized mitochondrial translation system and its reduced tRNA repertoire.

The universal code isn’t unchanged in mitochondria because their genetic translation context is different, and they do produce proteins using their own ribosomes and tRNAs. The idea that mitochondria use only RNA or that the universal code applies without change to mitochondria isn’t correct; the evidence shows codon reassignments driven by their distinct, mitochondria-specific translation machinery.