Cell nucleus. Colored Transmission Electron Micrograph (TEM) of a cell nucleus. The nucleus is large and irregularly shaped, and is bounded by a nuclear membrane (dark blue) containing gaps (nuclear pores) that allow large molecules to pass out of the nucleus. The dark, circular body inside the nucleus is the nucleolus (center right). The nucleus of a cell contains a complete set of the organism's genes, which are encoded into DNA molecules on chromosomes. The chromosomes are normally in a condensed form called chromatin but in the nucleolus they are unraveled in order to produce protein-manufacturing bodies called ribosomes. Magnification unknown.