Q.63 The histone that links two histone octamers is
1. H1
2. H4
3. H2A
4. H2B
Nucleosomes form the basic unit of chromatin with DNA wrapped around histone octamers; linker histones stabilize higher-order structure.
Correct Answer
1. H1 – Linker histone binds DNA between adjacent nucleosomes, linking two histone octamers.
Chromatin Structure Basics
Each nucleosome contains a histone octamer (2 each of H2A, H2B, H3, H4) with ~147 bp DNA wrapped around it. H1 binds the linker DNA (40-60 bp) between nucleosomes, stabilizing the 30 nm fiber.
Option Explanations
1. H1
Linker histone (H1/H5) binds entry/exit DNA sites on nucleosome surface and linker DNA, physically connecting adjacent octamers into solenoid structure.
✅ Correct: Bridges nucleosomes.
2. H4
Core histone in octamer (H3₂-H4₂ tetramer core). H4 tail contacts H2A/H2B acidic patch on SAME or adjacent nucleosomes, but doesn’t “link two octamers” – part of octamer assembly.
❌ Incorrect: Intra-nucleosomal role.
3. H2A
Core histone forming H2A-H2B dimers that flank H3-H4 tetramer in octamer. Contributes to DNA wrapping, not inter-nucleosome linking.[prior knowledge]
❌ Incorrect: Octamer component only.
4. H2B
Paired with H2A in octamer. H2B tail influences transcription but doesn’t span between nucleosomes.[prior knowledge]
❌ Incorrect: Core histone function.
| Option | Histone Type | Role | Links Octamers? |
|---|---|---|---|
| H1 | Linker | Binds linker DNA between nucleosomes | ✅ Yes |
| H4 | Core | Octamer assembly, tail interactions | ❌ No |
| H2A | Core | DNA wrapping in nucleosome | ❌ No |
| H2B | Core | Dimer formation in octamer | ❌ No |
Exam Mnemonic: H1 = “Hi, I’m the linker!” (links nucleosomes). Core histones H2A/H2B/H3/H4 stay inside one octamer.
