Explanation

Nucleosome is sub-microscopic sub-unit of chromatin which is formed by wrapping of DNA over a core of histone proteins. The term was coined by Oudet $et.\ al$.., (1975). It is oblate structure with a length of 10nm and a thickness of 5-5.7nm. Its core is called nu-body. The latter is formed of four pairs of histone molecules $H_{2},A,H_{2}B,H_{3}$ and$H_{4}$. DNA makes 1.75 turns over the octamer to form a nucleosome. Two adjacent nucleosomes are connected by a short segment of unboud DNA called linker DNA. A fifth type of histone called $H_{1}$is attached over the linker DNA. Nucleosomes appear as ‘beads-on-string’ in the chromosomes under electron microscope.