Structure of Chromosomes:
The term “chromosome” is used to refer to a nucleic
acid molecule that is the repository of genetic information
in a virus, a bacterium, a eukaryotic cell, or an organelle.
It also refers to the densely colored bodies seen
in the nuclei of dye-stained eukaryotic cells, as visualized 
using the light microscope.

Chromatin Consists of DNA and Proteins:

The eukaryotic cell cycle produces remarkable changes in the structure in the chromosome. In nondividing eukaryotic cells (in G0) and those
in interphase (G1, S, and G2), the chromosomal material,
chromatin, is amorphous and seems to be randomly dispersed
in certain parts of the nucleus. In the S phase of
interphase the DNA in this amorphous state replicates,
each chromosome producing two sister chromosomes
(called sister chromatids) that remain associated with
each other after replication is complete. The chromosomes
become much more condensed during prophase of
mitosis, taking the form of a species-specific number of
well-defined pairs of sister chromatids.

Chromatin consists of fibers containing protein and
DNA in approximately equal proportions (by mass),
along with a small amount of RNA. The DNA in the chromatin
is very tightly associated with proteins called histones which package and order the DNA into 
structural unit called nucleosomes. Also
found in chromatin are many nonhistone proteins, some
of which help maintain chromosome structure and others
that regulate the expression of specific genes . Beginning with nucleosomes, eukaryotic chromosomal
DNA is packaged into a succession of higher-order
structures that ultimately yield the compact chromosome
seen with the light microscope. We now turn to a description
of this structure in eukaryotes and compare it with
the packaging of DNA in bacterial cells.

Histones Are Small, Basic Proteins:
Found in the chromatin of all eukaryotic cells, histones
have molecular weights between 11,000 and 21,000 and
are very rich in the basic amino acids
arginine and lysine (together these
make up about one-fourth of the
amino acid residues). All eukaryotic
cells have five major classes of histones,
differing in molecular weight

and amino acid composition.  The H3 histones are nearly
identical in amino acid sequence in all
eukaryotes, as are the H4 histones,
suggesting strict conservation of their
functions. For example, only 2 of 102
amino acid residues differ between
the H4 histone molecules of peas and  humans
and yeast. Histones H1, H2A, and H2B show less
sequence similarity among eukaryotic species.
Each type of histone is subject to enzymatic modification
by methylation, acetylation, ADP-ribosylation,
phosphorylation, glycosylation, sumoylation, or ubiquitination.
Such modifications affect the net electric charge,
shape, and other properties of histones, as well as the
structural and functional properties of the chromatin,
and they play a role in the regulation of transcription.
In addition, eukaryotes generally have several variant
forms of certain histones, most notably histones
H2A and H3, described in more detail below. The variant
forms, along with their modifications, have specialized
roles in DNA metabolism.

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