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The eukaryotic genome is enormous,
not only in its information content,
but also in its sheer physical size.
The eukaryotic nucleus is only about five microns in diameter.
It has to contain a genome that,
if all pieced together,
would be about two meters in length.
As a simple analogy,
if the nucleus were the size of a golf ball,
it would contain about 10 miles of DNA.
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As a consequence, each eukaryotic cell is faced with
a very daunting proposition of compacting its DNA to a massive degree.
The requirements are not only to package DNA into the nucleus,
but also to protect the genome while at the same time allowing
access to the information at the appropriate time by the cellular machinery.
DNA is found in complex with an equal mass of proteins to
form protein DNA complex collectively termed chromatin.
Histone proteins form the nucleosomes and
non-histone proteins are supposed to aid in further compaction.
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DNA packaging in the eukaryotic cell occurs at multiple levels.
At the first organizational level,
DNA is wrapped around disk-shaped assembly of eight histones,
the so-called histone octamer,
to form the nucleosome.
Beads on a string-like structures of nucleosomes are
then further folded into nucleosome arrays,
or the so-called, 30-nanometre fiber.
This occurs via short-range inter-nucleosome interaction.
Linker histones then aid in the further compaction of
30-nanometer fibers to form higher-order structures and stably folded chromatin fibers.
Long-range fiber-fiber interactions are
responsible to form even higher condensed arrays of nucleosomes.
At the peak of the compaction,
the metaphase chromosome is found.
