Nucleolus. Transmission electron micrograph of the nucleolus (dark circular area) in the nucleus of a mouse liver cell. The nucleolus contains unravelled DNA (deoxyribonucleic acid) involved in making protein-manufacturing bodies called ribosomes. The rest of the nucleus contains a condensed form of DNA called chromatin. The double nuclear membrane can be seen in the lower part of the picture, with small pores that allow large molecules to pass out of the nucleus and into the cell cytoplasm. In the lower left is an energy- producing mitochondrion. Magnification x37,500 at 10x8 inch size.