(a) How are the following formed and involved in DNA packaging in a nucleus of a cell?
(i) Histone octomer
(iii) Chromatin (3)
(b) Differentitate between Euchromatin and Heterochromatin. (2)
(a) In eukaryotes there is a set positively charged, basic proteins called histones present in the nucleus. Histones are rich in the basic amino acid residues lysines and arginines. Both the amino acid residues carry positive charges in their side chains. Histones are organised to form a unit of eight molecules called histone octamer. 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. In chromatin nucleosome forms the repeating unit as seen as 'beads-on-string' structure.
(b) Difference between Euchromatin and Heterochromatin
1. The chromatin that is loosely packed 1. The chromatin that is densely packed and stains
and stains light is called euchromatin. dark is called heterochromatin.
2. This is transcriptionally active chromatin. 2. This is inactive chromatin. (5)