Nucleus and microbodies

Robert Brown described the nucleus as a cell organelle in 1831; Flemming later named the material stained by basic dyes chromatin.

A non-dividing (interphase) nucleus contains chromatin, an extended network of nucleoprotein fibres, along with the nuclear matrix and one or more spherical nucleoli.

The nuclear envelope has two parallel membranes separated by a 10–50 nm perinuclear space. The outer membrane is usually continuous with the ER and carries ribosomes.

Nuclear pores, formed where the two membranes fuse, allow RNA and protein to move in both directions between nucleus and cytoplasm.

A cell usually has a single nucleus, though the number can vary. Some cells lack one altogether, such as sieve tube cells in vascular plants and mature erythrocytes in many mammals.

Nucleoli are not membrane-bound, so their contents are continuous with the nucleoplasm. They are sites of active ribosomal RNA synthesis, and cells that make a lot of protein have larger and more numerous nucleoli.

Along with DNA, chromatin has histones (basic proteins), RNA and some non-histone proteins. About two metres of DNA, distributed among 46 chromosomes, is packed into a single human cell.

Every chromosome has a primary constriction, the centromere, with disc-shaped kinetochores on its sides. By centromere position chromosomes are metacentric (middle; two equal arms), sub-metacentric (slightly off-centre; one shorter and one longer arm), acrocentric (close to one end; one very short and one very long arm) and telocentric (at the tip).

Some chromosomes have a secondary constriction at a fixed location that sets off a small fragment called the satellite.

Microbodies are tiny membrane-bound vesicles packed with a variety of enzymes; both plant and animal cells have many of them.