Nucleus and chromosomes

The nucleus was described by Robert Brown (1831). Inside it are chromatin (a name given by Flemming), a fluid nuclear matrix, and one or more round nucleoli. It is wrapped in a nuclear envelope of two membranes with a gap of 10 to 50 nm between them (the perinuclear space). The outer membrane continues into the ER and carries ribosomes. Nuclear pores let RNA and proteins move in and out.

Most cells have one nucleus. Some have several. A few mature cells have none: mammalian red blood cells and the sieve tube cells of plants.

Nucleoli have no membrane. They make ribosomal RNA, so they are larger and more numerous in cells that make a lot of protein. Chromatin is DNA with basic proteins called histones, plus some non-histone proteins and RNA. A single human cell holds about 2 metres of DNA, packed into 46 chromosomes.

Each chromosome has a narrow point, the primary constriction or centromere, holding its two halves (chromatids) together. Disc-shaped kinetochores on the sides of the centromere are where spindle fibres attach. Chromosomes are named by where the centromere sits:

• Metacentric: centromere in the middle, two equal arms.
• Sub-metacentric: slightly away from the middle, one arm shorter.
• Acrocentric: very near one end, one arm extremely short.
• Telocentric: at the very end.

Some chromosomes also have a secondary constriction, which marks off a small piece called a satellite.

Microbodies are tiny membrane-bound sacs containing enzymes, found in both plant and animal cells.