What is Nucleosome, chromatin?

In a eukaryotic nucleus the DNA is wound on proteins: $147\,$ base pairs make 1.7 turns around an octamer of eight histones (two each of H2A, H2B, H3, H4, small proteins rich in lysine and arginine that neutralise the phosphates), forming a nucleosome $11\,\mathrm{nm}$ across; nucleosomes follow one another every $200\,$ pairs or so, like beads on a string, with a fifth histone, H1, sealing each bead. This chromatin folds further: into a fibre of $30\,\mathrm{nm}$, into loops of tens of thousands of pairs anchored on a protein scaffold, and — at division — into the compact rods of the metaphase chromosome, ten thousand times shorter than the naked DNA. Euchromatin is the looser, gene-rich, transcribed form of interphase; heterochromatin the condensed, gene-poor form that stays compact (around centromeres, at the ends, and on a switched-off X chromosome).