Which among the following are seen as ‘beads-on-string’ structure when viewed under electron microscope?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The nucleosomes
In eukaryotes, the long DNA is packed with the help of histones. Eight histone molecules form a histone octamer, and the negatively charged DNA wraps around this positively charged core. One such unit of DNA and histone octamer is a nucleosome. A chain of nucleosomes joined by DNA looks like beads on a string under the electron microscope.
Option (1) Histone only — Incorrect. Histones are positively charged basic proteins. They form only the core of each bead; histones alone, without the DNA wrapped round them, do not give the beads-on-string look.
Option (2) DNA only — Incorrect. DNA is the string that wraps round each core and links one bead to the next. DNA alone, without the histone cores, has no beads.
Option (3) The nucleosomes — Correct. The nucleosomes in chromatin are the beads, and the DNA between them is the string. This repeating unit gives chromatin its beads-on-string look under the electron microscope.
NCERT Extract:
Packaging of DNA Helix – “… Histones are organised to form a unit of eight molecules called histone octamer. The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome (Figure 5.4 a). A typical nucleosome contains 200 bp of DNA helix. Nucleosomes constitute the repeating unit of a structure in nucleus called chromatin, threadlike stained (coloured) bodies seen in nucleus. The nucleosomes in chromatin are seen as ‘beads-on-string’ structure when viewed under electron microscope (EM) (Figure 5.4 b).”