The nucleus of each cell in our bodies contains approximately 1.8
metres of DNA in total, although each strand is less than one millionth
of a centimetre thick. This DNA is tightly packed into structures called
chromosomes, which consist of long chains of DNA and associated
proteins. In eukaryotes, DNA molecules are tightly wound around
proteins - called histone proteins - which provide structural support and
play a role in controlling the activities of the genes. A strand 150 to 200
nucleotides long is wrapped twice around a core of eight histone
proteins to form a structure called a nucleosome. The histone octamer
at the centre of the nucleosome is formed from two units each of
histones H2A, H2B, H3, and H4. The chains of histones are coiled in turn
to form a solenoid, which is stabilised by the histone H1. Further coiling
of the solenoids forms the structure of the chromosome proper.