RNA splicing occurs in eukaryotes but not prokaryotes. Eukaryotic pre-mRNA contains non-coding introns that must be removed by the spliceosome before translation. Prokaryotes lack introns in their genes (with rare exceptions) and therefore do not require splicing. Mitochondrial RNA synthesis occurs only in eukaryotes (which have mitochondria), but splicing is the more fundamental distinction as a nuclear process.