What are those structures that appear as 'beads-on-string' in the chromosomes when viewed under electron microscope?

Step-by-step solution

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 ~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- 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 unbound DNA called linker DNA. A fifth ty