Human cell, cutaway illustration. The cell membrane (blue) surrounds the internal structures (organelles). At centre is the nucleus (surrounded by transparent membrane), which contains the nucleolus (orange) and chromosomes (blue). DNA (deoxyribonucleic acid, helices) is transcribed to mRNA (messenger ribonucleic acid, green) in the nucleus by the enzyme RNA polymerase II (purple). Surrounding the nucleus are the rough (purple) and smooth (blue) endoplasmic reticulum (ER). The rough ER is studded with ribosomes (dark purple spheres), the sites of active protein synthesis. Vesicles (gold spheres) leave the rough ER, releasing their protein cargo into the Golgi apparatus (blue and purple at bottom) where the proteins are stored before being re-packaged for transport. Mitochondria (transparent, one at bottom centre) create energy for the cell. Lysosomes (green spheres) contain enzymes for digesting foreign particles. At upper right a kinesin motor protein carries a vesicle along a microtubule (yellow).