What are those structures that appear as ‘beads-on-string’ in the chromosomes when viewed under electron microscope
a) Nucleotides b) Nucleosomes c) Base pairs d)Genes
AA
BB
CC
DD
A.
(b) 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 andH_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_1is attached over the linker DNA. Nucleosomes appear as ‘beads-on-string’ in the chromosomes under electron microscope.
