The human genome consists of approximately 3X10^6 basepairs (3 000 mega bases). Through a number of levels of specific folding this large quantity of double stranded DNA is tightly packed into discrete chromosomes. Every 200 nucleotides, double-stranded DNA is coiled twice around a complex of histones, small basic proteins rich in the armino acids arginine and lysine, forming a nucleosome. The histone structure is remarkably conserved throughout eukaryotic organisms, and nucleosomes are always found to contain an octamer of two units cach of the histones H2A, H2B, H3 and H4. Further tight winding, 36-fold, of the string of nucleosomes forms a solenoid structure, 30 nm in diameter, which is stabilized by the 111 histone forming a central scaffold. This chromatin fibre is further supercoiled 50 000~100 000 times to form the structures we know as chromosomes. Depending on the stage of the cell cycle and the region of the chromosome the chromatin can be highly condensed forming heterochromatin, areas of DNA usually inactive, or loosely packed (except during vell division) forming euchromatin, where functional DNA is actively expressed.
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