Nucleosome and chromatin কী?

Chromatin is the complex of DNA with proteins in which the genome exists in a eukaryotic nucleus. Its repeating unit is the nucleosome: $147\,\mathrm{bp}$ of DNA wound in $1.65$ left-handed turns around a histone octamer — two copies each of the small, very basic proteins H2A, H2B, H3 and H4 — followed by a linker of $20\text{ to }60\,\mathrm{bp}$ to the next nucleosome. A fifth histone, H1, binds the linker where it enters and leaves the core and helps the beads pack against one another. Each core histone carries an unstructured amino-terminal tail of some $20\text{ to }40$ residues that projects out of the particle. Chromatin that stains lightly, is loosely packed and is transcribed is euchromatin; densely packed, late-replicating and largely silent chromatin, at centromeres, telomeres and repeated sequences, is heterochromatin.