Q.40 In nucleosome model, the dsDNA is wound on which molecular structure to give organisation to DNA into
chromatin
1. H1 molecule
2. Histone protein Octamer
3. Nuclear membrane
4. Lysosomes
Nucleosome Model: Histone Octamer Organizes Chromatin DNA
In the nucleosome model, double-stranded DNA wraps around histone octamers to form the basic chromatin unit. The correct answer is option 2: Histone protein Octamer.
Correct Answer
Option 2: Histone protein Octamer.
The histone octamer, composed of two copies each of H2A, H2B, H3, and H4, forms the nucleosome core around which ~147 bp of dsDNA wraps in 1.67 left-handed superhelical turns. This “beads-on-a-string” structure compacts DNA into 10 nm chromatin fibers.
Option Analysis
Option 1: H1 Molecule
Histone H1 is a linker histone binding DNA between nucleosomes to stabilize higher-order 30 nm fibers. It stabilizes but does not serve as the core winding structure.
Option 2: Histone Protein Octamer (Correct)
Core histones form the octamer spool; positively charged tails interact electrostatically with DNA backbone, enabling compaction and gene regulation.
Option 3: Nuclear Membrane
The nuclear envelope compartmentalizes eukaryotic DNA but plays no direct role in nucleosome assembly or chromatin organization.
Option 4: Lysosomes
Lysosomes are digestive organelles unrelated to chromatin; they degrade macromolecules via hydrolytic enzymes.
| Option | Structure | DNA Winding Core? | Reason |
|---|---|---|---|
| 1 | H1 molecule | No | Linker stabilizer |
| 2 | Histone octamer | Yes | 147 bp superhelix |
| 3 | Nuclear membrane | No | Compartmental barrier |
| 4 | Lysosomes | No | Waste degradation |
Exam Relevance
Nucleosome questions test chromatin packaging hierarchy; distinguish core octamer (H2A-H2B-H3-H4) from linker H1.
