Concept Breakdown

1. TITLE

What is the "Beads-on-String" structure? The NEET explanation you actually need

2. CONCEPT EXPLANATION 

Think of your DNA like a 2.2-meter-long kite string that needs to fit into a microscopic box (the nucleus). You can’t just stuff it in; it would tangle. You need spools to wind it up.

In your cells, this packing happens through charge interaction. DNA is negatively charged (acidic) because of its phosphate backbone. To pack it, the cell uses histones, which are positively charged (basic) proteins rich in amino acids like lysine and arginine.

The negative DNA wraps around a unit of eight histone molecules (histone octamer) to form a Nucleosome. Under an electron microscope, these repeating nucleosomes look like "beads-on-string".

Common Misconception: Students often think the "bead" is just the protein core. The "bead" (Nucleosome) is the protein core PLUS the DNA wrapped around it.

3. MCQ 

In the formation of the nucleosome (the "bead"), what is the primary force that holds the DNA wrapped around the histone octamer?

A) Hydrogen bonds between complementary base pairs
B) Electrostatic attraction between acidic histones and basic DNA
C) Electrostatic attraction between basic histones and acidic DNA
D) Covalent bonding between the sugar-phosphate backbone and the protein

4. ANSWER + EXPLANATION

Correct Answer: (C) Electrostatic attraction between basic histones and acidic DNA

Explanation:
The stability of the "beads-on-string" structure relies on opposites attracting.
* DNA is an acid (Deoxyribonucleic Acid) and is negatively charged due to phosphate groups.
* Histones are basic proteins (rich in positive Lysine and Arginine residues).
The positive histones attract the negative DNA, holding the nucleosome together.

  • Why the trap (B) is wrong: It flips the chemical nature. Remember, DNA is acidic (negative), not basic.
  • Why the trap (A) is wrong: Hydrogen bonds hold the two strands of the DNA helix together, not the DNA to the protein.

5. KEY TAKEAWAY BOX

Remember:
The "Bead" (Nucleosome) forms because negatively charged DNA is attracted to and wraps around positively charged Histone proteins.

6. NCERT REFERENCE

NCERT Class 12, Chapter 5 (Molecular Basis of Inheritance), Page 83
"The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome... The nucleosomes in chromatin are seen as 'beads-on-string' structure when viewed under electron microscope (EM)."

7. CTA

Full chapter explanation → Genetics and Evolution Complete Guide