The nucleus is by far the most visually prominent organelle in eukaryotic cells.
It was discovered early in the study of cell struc ture and was shown by Robert Brown in 1831 to be a constant feature of eukaryotic cells.
The nucleus is the repository for the cell's genetic information.
Nuclei are membrane-delimited spherical bodies about 5 to 7 11m in diameter .
They contain more than one chromosome; the exact number depends on the organism, cell type, and stage in the life cycle.
Each eukaryotic chromosome is composed of chromatin.
Chromatin is a complex of DNA and proteins, including histones.
Histones are small basic proteins rich in the amino acids lysine, arginine, or both.
There are five types of histones in most eukaryotic cells: HI, H2A, H2B, H3, and H4.
Eight histone molecules form an ellipsoid about 11 nm long and 6.5 to 7 nm in diameter around which the DNA wraps to form a "beads-on-a-string" formation .
Each bead is called a nucleosome.
Chromosomes are very dynamic and vary in terms of their degree of compaction.
When the cell is not dividing, there is less compaction.
The highest degree of compaction occurs dur ing cell division.
Compaction is brought about in part byhistones and other proteins, including condensins.
Recall that condensins are also used by bacterial and archaeal cells to compact their chromosomes during cell division.
Tethering of one region of the chromosome to another and tethering of the chromosome to nuclear structures also contribute to chromo some folding and compaction.
a complex structure consisting of inner and outer membranes separated by a perinuclear space.
The nuclear envelope is contin uous with the ER at several points, and its outer membrane is covered with ribosomes.
A network of intermediate filaments, called the nuclear lamina, is observed in animal cells.
It lies against the inner surface of the nuclear envelope and supports it.
Many nuclear pores penetrate the envelope, and each pore is formed by about 30 proteins; each pore plus the associated pro teins is called a nuclear pore complex .
Pores are about 70 nm in diameter and collectively occupy about 10 to 25% of the nuclear surface.
The nuclear pore complexes serve as trans port routes between the nucleus and surrounding cytoplasm.
Small molecules move through the nuclear pore complex un aided.
However, large molecules are transported through the nuclear pore complex. Some nuclear pore complex proteins are involved in these transport processes.
Often the most noticeable structure within the nucleus is the nucleolus .
A nucleus may contain from one to many nucleoli.
Although the nucleolus is not membrane-enclosed, it is a complex organelle with separate granular and fibrillar regions.
It is present in nondividing cells but frequently disappears during mi tosis. After mitosis, the nucleolus reforms around the nucleolar organizer, a particular part of a specific chromosome.
The nucleolus plays a major role in ribosome synthesis.
The DNA of the nucleolar organizer directs the production of ribo somal RNA (rRNA).
This RNA is synthesized in a single long piece that is cut to form the final rRNA molecules.
The processed rRNAs combine with ribosomal proteins (which have been syn thesized in the cytoplasm) to form partially completed ribosomal subunits.
The granules seen in the nucleolus are probably these subunits.
Immature ribosomal subunits then leave the nucleus, presumably by way of the nuclear pore complexes, and mature in the cytoplasm.
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