DNA packing, nucleosome model, chromatin, chromosome

Average length of human DNA is approx 2 meter (2n). How this length of DNA fit in cell which even we cannot seen by naked eyes. How histone are used, what is chromatin (euchromatin and heterochromatin). Chromosome which stage of cell cycle chromosome can be studies.Histone protein are mainly four type H1 H2 H3 H4, H2 protein is further two type H2A and H2B type. The H3 and H4 are arginine and lysine rich. These amino acid arginine and lysine are positive charge and the DNA have phosphate group in it's backbone which provide it a negative charge overall due to which the negative charge DNA coiled or raped around the histone. The histone protein are assemble into octamer unit in which each histones two unit are found H2A H2B H3 and H4 so that's why known as octamer. It produce a 6nm long and 11nm wide cylinderical structure known as histone octamer. While H1 protein act as linker protein raped around linker DNA. Linker DNA is dna fragment between two histone octamer.