Illustration of the structure of a typical animal cell. The cell nucleus is at centre left. It holds the cell's genetic material in the form of DNA (deoxyribonucleic acid). Surrounding the nucleus is the endoplasmic reticulum (ER, orange). Some parts of the ER are studded with ribosomes (purple spheres), sites of active protein synthesis. Vesicles (spheres) leave the rough ER, releasing their protein cargo into the Golgi apparatus (purple at bottom right) where the proteins are stored before being re-packaged for transport. Mitochondria (red at bottom) are the sites of energy synthesis within the cell. The two red rod-like structures at lower centre are a centrosome, which is made up of two centrioles at right angles to each other. Centrosomes are involved in assembling the spindle that pulls cells apart during mitosis (nuclear division) and the production of cilia and flagella.