What are those structures that appear as 'beads-on-string' in the...

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 $10 \mathrm{~nm}$ and a thickness of 5-5.7 nm. Its core is called nu-body. The latter is formed of four pairs of histone molecules- $\mathrm{H}_2 \mathrm{~A}$, $\mathrm{H}_2 \mathrm{~B}, \mathrm{H}_3$ and $\mathrm{H}_4$. DNA makes 1.75 turns over the octamer to form a nucleosome. Two adjacent nucleosomes are connected by a short segment of unbound DNA called linker DNA. A fifth type of histone called $\mathrm{H}_1$ is attached over the linker DNA. Nucleosomes appear as 'beads-on-string' in the chromosomes under electron microscope.