False colour transmission electron micrograph (TEM) of circular DNA from a mitochondrion, the site of synthesis of chemical energy within the cell. The filament seen in image was collected from a fragmented mitochondrion. Such circular DNA molecules are 5-6 micrometers long, with a molecular weight of around 10 million; each mitochondrion has between 3 to 6. This relatively tiny amount of DNA is estimated to have a coding capacity of about 5,000 amino acids, and suggests that the mitochondrion needs to rely on a supplement of external DNA (from the cell nucleus) to synthesise its constituents. Magnification: X 150,000 at 35mm size.