Nucleus

The structure of a eukaryotic cell's nucleus becomes clearly distinct in a non-dividing cell, or during interphase. Such an interphase nucleus is made up of four main parts: the nuclear envelope, the nucleoplasm, the nucleolus, and the chromatin network.

Nuclear envelope

The nuclear envelope is a double-layered membrane that delimits the nucleus. Its two membranes are kept apart by a perinuclear space, roughly 10 to 50 nanometres wide. The outer membrane connects with the endoplasmic reticulum at certain points and also carries ribosomes on its surface, while the inner membrane is lined on its inner face by the nuclear lamina, a network of protein fibres that helps the nucleus keep its shape. Together, the two membranes and the perinuclear space between them separate the nucleoplasm from the surrounding cytoplasm. This separation is not total, however -- at various places the envelope has small openings called nucleopores, guarded by pore complexes that regulate the flow of substances between the nucleus and the cytoplasm in both directions.

Nucleoplasm and nucleolus

The nucleoplasm, also called karyolymph, contains a range of substances -- nucleic acids, protein molecules, minerals and salts -- along with the chromatin network and the nucleolus. The nucleolus itself has no membrane of its own; it is best known as the site where ribosomes are assembled (ribosome biogenesis), and it is made up of ribosomal RNA and ribosomal proteins. Depending on how synthetically active a cell is, there may be one or several nucleoli present -- for example, an oocyte's nucleus contains a large nucleolus, while a sperm cell's nucleus contains only a small, inconspicuous one. Nucleoli appear as dense, spherical bodies close to the chromatin network, and they produce the ribosomal RNA and ribosomal proteins that are later transported into the cytoplasm, where they are assembled into complete ribosomes.

Chromatin material …