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: 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 breaking down.

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.

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