Cell structure. Illustration of the ultrastructure of a typical cell. Components of this cell are in three-dimensions and colour-coded. At centre, the nucleus (pink) has a nucleolus (brown) containing the DNA genetic material. Closely associated with the nucleus is endoplasmic reticulum (ER, purple), a membrane system in the cytoplasm studded with ribosomes (red dots, sites of protein synthesis). Golgi bodies (yellow membranes) package products from the ER into spherical lysosomes (orange). Above the nucleus, two centrioles made of micro- tubules play a role in cell division. Mitochondria (green) store energy for the cell. The cytoplasm (blue) is bordered by a white cell membrane.