The nucleus, being the largest sub-cellular compartment, varies in diameter from 10 to 20 micrometres. It is surrounded by a double membrane forming the nuclear envelope, about 30 nm wide. This selectively allows molecules to enter and leave the nucleus, and separates chemical reactions taking place in cytoplasm from reactions happening within the nucleus. The outer membrane has ribosomes. The inner and outer membrane fuse at regular spaces, forming nuclear pores.
Similar to the cytoplasm of a cell, the nucleus contains nucleoplasm - a highly viscous solid containing the chromosomes and nucleoli. Chromosones contain information encoded in DNA attached to proteins called histones and are usually arranged in to a dense network called chromatin. Nucleoli are granular structures which make ribonucleic DNA (rDNA) and assemble it with proteins.