Nucleosome: Subunit of chromatin composed of a short length of DNA wrapped
around a core of histone proteins.
The human genome contains about 3 billion nucleotide pairs organized as 23 chromosomes pairs. If uncoiled, the DNA contained
by each of those chromosomes would measure between 1.7 and 8.5 cm (0.67
to 3.35 inches) long. This is too long to fit into a cell. Moreover, if
chromosomes were composed of extended DNA, it is difficult to imagine how
the DNA could be replicated and segregated into two daughter cells without
In fact chromosomal DNA is packaged into a compact structure with the
help of specialized proteins called histones. The complex DNA plus
histones in eucaryotic cells is called chromatin.
The fundamental packing unit is known as a nucleosome. Each nucleosome
is about 11nm in diameter. The DNA double helix wraps around a central core
of eight histone protein molecules (an octamer) to form a single nucleosome.
A second histone (H1 in the illustration) fastens the DNA to the nucleosome
core. The total mass of this complex is about 100,000 daltons.
Nucleosomes are usually packed together, with the aid of a histone (H1,)
to form a 30nm large fiber. As a 30nm fiber, the typical human chromosome
would be about 0.1cm in length and would span the nucleus 100 times. This
suggests higher orders of packaging, to give a chromosome the compact structure
seen in a typical karyotype (metaphase) cell.