A structure or part that is enclosed within its own membrane inside a cell and has a particular function. Organelles are found only in eukaryotic cells and are absent from the cells of prokaryotes such as bacteria. The nucleus, the mitochondrion, the chloroplast, the Golgi apparatus, the lysosome, and the endoplasmic reticulum are all examples of organelles. Some organelles, such as mitochondria and chloroplasts, have their own genome (genetic material) separate from that found in the nucleus of the cell. Such organelles are thought to have their evolutionary origin in symbiotic bacteria or other organisms that have become a permanent part of the cell.