The organelles likely evolved from bacteria that were endocytosed more than one billion years ago. Although the organelles maintain their own genomes, many genes encoding mitochondrial and chloroplast proteins are found in the cell nucleus.
Mitochondria are small, oval shaped organelles surrounded by two highly specialized membranes.
Mitochondria are the sites of aerobic respiration, and are generally the major energy production center in eukaryotes. Plant mitochondrial genomes tend to be 10 - 150 times larger and contain additional genes. Many organisms use one genetic code to translate nuclear mRNAs, and a second one for their mitochondrial mRNAs. Chloroplasts are larger than mitochondria, and are surrounded by three specialized membranes. Chloroplast mRNAs are translated with the standard genetic code, although they often undergo extensive RNA editing, so it is difficult to predict the protein translations from genomic sequence.