Key structures within the cell nucleus: In the cell nucleus the key structures are: the nucleolus, an optically dense area, spherical shaped, where there are concentrated ribosomal RNA (that is, rRNA) related to proteins (there might be more than one nucleolus in a nucleus); the chromatin, made up of DNA molecules dispersed in nuclear matrix throughout the cell inter phase; the karyotecha, or nuclear membrane, the membrane which delimits the nucleus.