Q.50 The number of nucleosomes present in a 30 nm solenoid structure of a chromatin is
(A) 2 (B) 4 (C) 6 (D) 8
30 nm Solenoid Chromatin Nucleosomes
Chromatin packaging is fundamental to eukaryotic gene regulation, with the 30 nm solenoid representing higher-order folding beyond “beads-on-a-string.” Question 50 asks: “The number of nucleosomes present in a 30 nm solenoid structure of a chromatin is (A) 2 (B) 4 (C) 6 (D) 8.”
This tests understanding of nucleosome stacking in the solenoid model, crucial for molecular biology and bioinformatics.
Correct Answer: (C) 6
A 30 nm solenoid structure contains 6 nucleosomes per helical turn. The “beads-on-a-string” 10 nm fiber (one nucleosome every ~200 bp DNA) coils into a 30 nm fiber where linker histone H1 stabilizes 6 nucleosomes around ~11 nm axial length per turn, achieving 6-fold compaction.
This solenoid model, proposed by Finch & Klug, features nucleosomes tilted ~20-30° to the helix axis with bent linker DNA connecting adjacent cores.
Explanation of All Options
6 Nucleosomes (Correct Choice)
-
Standard for solenoid: 6 nucleosomes/turn, pitch ~11 nm.
-
Electric dichroism confirms across species (rat liver to sea urchin).
-
H1-mediated; mass per length = 6 nucleosomes/11 nm.
2 Nucleosomes (Option A)
-
Too few; represents minimal dimer stacking, not full solenoid.
-
Insufficient for observed fiber diameter (~30 nm).
-
Hypothetical crossed-linker models, not standard.
4 Nucleosomes (Option B)
-
Intermediate; seen in some zigzag models but not classical solenoid.
-
Lower compaction than observed 30 nm fiber mass.
-
Not matching experimental dichroism data.
8 Nucleosomes (Option D)
-
Overestimates; implies tighter helix not supported by fiber dimensions.
-
Two-start helix proposals debated, but solenoid standard is 6.
-
Exceeds typical nucleosome density measurements.
| Option | Nucleosomes/Turn | Model Type | Compaction Level | Evidence |
|---|---|---|---|---|
| 6 | 6 nucleosomes | Solenoid (1-start) | High (30 nm) | Strong |
| 2 | 2 nucleosomes | Minimal dimer | Low | Weak |
| 4 | 4 nucleosomes | Zigzag variant | Medium | Partial |
| 8 | 8 nucleosomes | Overpacked | Excessive | None |
Exam & Research Relevance
Mastering 30 nm solenoid chromatin nucleosomes is key for chromatin dynamics, epigenetics, and Hi-C bioinformatics analysis of chromosome territories. Frequently tested alongside histone variants and 300 nm looped domains in NEET/GATE.
This structure enables reversible compaction for transcription—critical for biotech applications in gene therapy vectors.
