Nucleus. Structure of the nucleus. Chromatin. Chromosomes. Nucleolus

The nucleus is the command centre and genetic material container of the cell, and the organelle of cell division. It is membrane-enclosed and occupies about 10 per cent of cell volume. Its main components are the nuclear envelope, chromatin, nucleolus and nuclear matrix.

The nuclear envelope is a double membrane: an outer membrane of 6 nm, rough, connected with the rER, and an inner membrane of 6 nm, smooth, lined by the nuclear lamina. Between them is the perinuclear space, 10 to 40 nm. The two are connected at the nuclear pore complex.

Chromatin is a complex of DNA and associated proteins, the histones. It comes in two types: heterochromatin, condensed and darkly staining, 90 per cent; and euchromatin, extended and lightly staining, the most active portion of the genome.

A human cell has 46 chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes.

The nucleolus is a spherical, highly basophilic structure whose function is producing ribosomes.

The nucleus is the command centre, or genetic material container, of the cell, and the organelle of cell division.

• It is a membrane-enclosed organelle, occupying about 10 per cent of cell volume.
• Its main components are the nuclear envelope, chromatin, nucleolus and nuclear matrix, also called the nucleoplasm.

Its tasks:

• It contains the code for all the cell's enzymes and other proteins.
• It contains molecular machinery to replicate DNA and to synthesise and process all types of RNA.

The nuclear envelope is a double membrane, a selectively permeable barrier between the nuclear and cytoplasmic content.

The two membranes are separated by the perinuclear space, 10 to 40 nm.

The outer nuclear membrane is 6 nm and rough:

• it carries ribosomes, and is connected with the rER;
• it carries vimentin filaments.

The inner nuclear membrane is 6 nm and smooth, with no ribosomes:

• it is lined by the nuclear lamina, a layer of specific proteins, 100 to 300 nm thick;
• those proteins are lamins A, B and C.

The connection between the outer and inner membrane is called the nuclear pore complex.

Ions and small molecules can pass through the membranes by simple diffusion. The nuclear pore complex regulates the movement of macromolecules between nucleus and cytoplasm.

So the envelope is not a wall with holes in it. Small things cross freely; the pore is a checkpoint for everything large enough to matter.

The nuclear matrix, or nucleoskeleton, is a network of fibres inside the cell nucleus.

• It is a highly dynamic structure, and an amorphous structure.
• Its composition is proteins and RNA, the nucleoskeleton, together with lamins A, B and C, the nuclear lamina, plus numerous enzymes, metabolites and ions.

Chromatin is a complex of DNA and associated proteins in the nucleus, the histones.

Each human cell's chromatin, except in sperm and eggs, is divided into 46 chromosomes: 22 pairs of autosomes and 2 sex chromosomes.

• Coiled strands of DNA, 146 base pairs of DNA.
• Proteins:
  • non-histone proteins, 20 per cent, which are acid proteins;
  • histones, 80 per cent, which are basic proteins.

H1 is the larger histone, associated with both the wrapped DNA and the surface of the core.

The nucleosome core is a core of 8 histones, two copies of each of H2A, H2B, H3 and H4.

The nucleosome is the structural unit of chromatin.

It is a core of histones with two loops of DNA, 146 base pairs of DNA.

Heterochromatin is condensed, or darkly staining, chromatin. It is 90 per cent of the total.

It can be subdivided depending on its location:

• marginal chromatin, which attaches to the nuclear membrane;
• chromocenters, in the nuclear matrix;
• nuclear-associated chromatin, either around the nucleolus, which is perinucleolar, or inside the nucleolus, which is intranucleolar.

Functionally it divides differently:

• constitutive heterochromatin is inactive, and lies around the chromosome, at centromeres and near telomeres;
• facultative heterochromatin is condensed and silent in some cells or at some stages, but can be reactivated; the classic example is the sex chromatin, the Barr body, the inactivated X chromosome in female cells.

Euchromatin is extended, or lightly staining, chromatin.

• It is the most active portion of the genome.
• It is involved in DNA replication, transcription and the formation of functional proteins.

The two types are told apart by how tightly the DNA is packed, and that packing is what decides whether the DNA can be read. Condensed means silent; extended means in use.

A chromosome is a very long DNA molecule containing many genes, with associated proteins, carrying portions of genetic material.

The total number in humans is 46, that is 2n, forming 23 homologous pairs:

• 22 pairs of autosomes;
• 1 pair of gonosomes, the sex chromosomes, XX and XY, which determine the sex of an organism.

• Arms: a short and a long arm.
• The primary constriction is the centromere, which is a microtubule organising centre.
• The ends are the telomeres.
• The secondary constriction carries the satellites, which are nucleolar organizers.

The nucleolus is a spherical, highly basophilic structure in the nucleus.

It contains DNA, RNA and proteins. It is basophilic because it is full of nucleic acid, which is negatively charged and takes up basic dyes.

• The fibrillar centre is where ribosomal proteins are formed.
• The granular components contain rRNA binding to ribosomal proteins.
• The fibrillar components have newly synthesised RNA connecting the ribosomal proteins.

Its function is producing ribosomes, that is ribosomal RNA.

Those are then transported to the cytoplasm, where they serve as the sites for protein synthesis.