Ribosomes. Coloured transmission electron micrograph (TEM) of a section through ribosomes and polysomes from chick embryo fibroblast cells. Ribosomes (dark dots) are the sites of protein synthesis in the cell. Each consists of ribosomal RNA (ribonucleic acid) and 40 or more different types of protein. During protein synthesis a molecule of messenger RNA attaches to a groove in the ribosome and slides along. As it passes, amino acids are bound together in an order spelt out by the base code on the messenger RNA. Groups of ribosomes attached to the same messenger RNA molecule are called polysomes. Magnification: x40,000 at 6x7cm size. x135,000 at 10x8"~PORTRAIT"