Rough endoplasmic reticulum, coloured transmission electron micrograph (TEM). This section shows the rough endoplasmic reticulum (ER, folds, centre), a membranous structure that occurs in cells. The cell nucleus is partially seen at left. Rough ER has ribosomes on its surface where protein synthesis occurs. The instructions for the protein synthesis come from the DNA (deoxyribonucleic acid) in the nucleus. The rounded structures may be vesicles that have broken off the rough ER to transport the proteins elsewhere in the cell. That would mean that this is the Golgi apparatus. Magnification: x20,000 when printed 10cm wide.