Nucleus

The nucleus is the control centre of the eukaryotic cell — it directs the activities of the other organelles and carries the hereditary material. It was first described as a cell organelle by Robert Brown, as early as 1831.

Naming of chromatin

The nuclear material that takes up basic (staining) dyes was later given the name chromatin by Flemming. In a cell that is not dividing — the interphase nucleus — this chromatin exists as highly extended, elaborate nucleoprotein fibres. Along with these fibres, the interphase nucleus contains a nuclear matrix and one or more spherical bodies called nucleoli (singular: nucleolus).

The nuclear envelope

Electron microscopy reveals that the nucleus is bounded by a nuclear envelope made of two parallel membranes. Between these two membranes lies a gap called the perinuclear space, roughly ten to fifty nanometres wide. This double-membrane envelope acts as a barrier separating the contents of the nucleus from the cytoplasm.

Key features of the envelope include:

• The outer membrane is usually continuous with the endoplasmic reticulum and also bears ribosomes on its surface.
• At many points the envelope is interrupted by minute nuclear pores, each formed where the two membranes fuse together.
• These pores act as passageways through which RNA and protein molecules move in both directions between the nucleus and the cytoplasm.

Number of nuclei

Normally there is just one nucleus per cell, but variations occur — some cells have more than one nucleus. A few mature cells even lack a nucleus altogether; familiar examples are the erythrocytes (red blood cells) of many mammals and the sieve tube cells of the vascular tissue of plants.

Nucleoplasm and nucleolus

The nuclear matrix, also called the nucleoplasm, contains both the nucleolus and the chromatin. The nucleoli are spherical structures suspended in the nucleoplasm. A nucleolus is not a membrane-bound structure, so its contents are continuous with the surrounding nucleoplasm. Its main job is to serve as a site of active ribosomal RNA (rRNA) synthesis. Cells that are very active in protein synthesis tend to have larger and more numerous nucleoli, reflecting their greater need to make ribosomes.

Chromatin and chromosomes

The loose, indistinct network of nucleoprotein fibres seen in the interphase nucleus is the chromatin. During cell division, however, this material condenses so that well-defined chromosomes appear in place of the diffuse network.

Chromatin is made up of:

• DNA
• Histones (basic proteins)
• Non-histone proteins
• RNA

A single human cell contains about two metres of DNA thread in total, distributed among its forty-six chromosomes (that is, twenty-three pairs). The detailed way DNA is packaged into a chromosome is studied in a later class.

Parts of a chromosome

Every chromosome — visible only in dividing cells — has certain characteristic parts:

• A primary constriction, also called the centromere.
• On either side of the centromere are disc-shaped structures called kinetochores.
• The centromere is the region that holds the two chromatids of a chromosome together.

Types of chromosomes based on centromere position

Depending on where the centromere sits, chromosomes are grouped into four types: …