Final answer:
The UGA codon is recognized as a stop codon in eukaryotic cells but codes for tryptophan in mitochondria due to differences in their genetic codes, which stem from the evolutionary origins of mitochondria having unique tRNAs and codon recognition patterns.
Step-by-step explanation:
In eukaryotic cells, the codon UGA is generally recognized as a stop codon during the process of translation in the cytoplasm. Translation termination occurs when this codon is encountered by the ribosome, which then prompts the release of the polypeptide chain from the translation machinery.
However, the same UGA codon codes for the amino acid tryptophan in the mitochondria. This phenomenon is due to the mitochondrial genome having a slightly different genetic code than the nuclear genome.
The different genetic codes are a result of the evolutionary origin of mitochondria, which are thought to have originated from a symbiotic relationship with a prokaryotic organism, leading to a unique set of tRNAs and codon recognition patterns in mitochondrial DNA.