Coloured transmission electron micrograph (TEM) of a section through a nucleus (green) and nucleolus (yellow). The nucleolus is responsible for producing components of ribosomes, the cell's protein-manufacturing organelles. The lighter material at the edge of the nucleolus is RNA (ribonucleic acid), which forms into ribosomal particles once exported to the cytoplasm (bottom left). The fine, dense, granular regions within the nucleolus are sites at which RNA synthesis is occurring on the ribosomal RNA genes. The coarser granular material is ribosomal RNA awaiting export to the cytoplasm. Magnification: x4,500 when printed at 10 centimetres tall.