Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

→ Nucleosome:
→ The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome.
→ A typical nucleosome contains 200 bp of DNA helix.
→ Nucleosomes constitute the repeating unit of a structure in nucleus called chromatin, thread- like stained (coloured) bodies seen in nucleus.
→ The nucleosomes in chromatin are seen as 'beads-on-string' structure when viewed under electron microscope (EM).
→ The beads-on-string structure in chromatin is packaged to form chromatin fibers that are further coiled and condensed at metaphase stage of cell division to form chromosomes.
→ Non-histone Chromosomal (NHC) proteins:
→ The packaging of chromatin at higher level requires additional set of proteins that collectively are referred to as Non-histone Chromosomal (NHC) proteins.
→ Euchromatin :
→ In a typical nucleus, some region of chromatin are loosely packed (and stains light) and are referred to as euchromatin.
→ Heterochromatin:
→ The chromatin that is more densely packed and stains dark are known as heterochromatin. Euchromatin is said to be transcriptionally active chromatin, whereas heterochromatin is inactive.