(i) Eight molecules of (positively charged basic proteins called) histones are organised to form histone octamer.
(ii) Negatively charged DNA wrapped around positively charged histone octamer to give rise to nucleosome.
(iii) Nucteosome constitute the repeating unit of a structure called chromatin.
Detailed answer:
(i) Histone Octamer: It is a unit comprising two molecules each of four histones namely, H2A, H2B, H3 and H4. These histone molecules are positively charged and interacts with negatively charged DNA to keep it associated with itself.
(ii) Nucleosome : The negatively charged DNA wrapped around the histone octamer (positively charged) to form a nucleosome. Oudit (1975) called it as the nu-body. It consists of 200 base pairs and a histone octamer. Nucleosome act as building block for packaging the DNA into chromatin.
(iii) Chromatin : It is thread like and a complex of DNA and histone protein. It helps in packing of DNA into the nucleus. The nucleosomes in chromatin look like beads on a strong. The nucleosomes are the repeating units of chromatin and separated from one-another by the region of DNA which is coiled as linker DNA which is composed of 60 base pairs and one histone molecule H1. The nucleosomes are further coiled and packed into a structure of higher organization which is known as solenoid. Each solenoid six nucleosomes per burn. Further supercoiling lends to form chromatin and fiber and then chromatid. This further coils and condense at metaphase a stage to form chromosomes.
