Q.87 In eukaryotic chromatin organization, which one of the histones seals off the nucleosome at
the location at which linker DNA enters and leaves the nucleosome?
(A) H1 (B) H2A-H2B (C) H3 (D) H4
Histone H1 seals off the nucleosome at the entry and exit points of linker DNA in eukaryotic chromatin organization.
The correct answer is (A) H1.
Option Analysis
H1 binds externally to the nucleosome core particle, interacting with linker DNA where it enters and exits, stabilizing the structure and protecting additional DNA from nuclease digestion. This sealing forms a chromatosome, essential for higher-order chromatin folding.
H2A-H2B form heterodimers that integrate into the core histone octamer, aiding DNA wrapping but not interacting with linker DNA entry/exit sites.
H3 contributes to the central tetramer (with H4) in the nucleosome core, positioning DNA superhelically without sealing linker regions.
H4 pairs with H3 in the tetramer and interacts internally with other core histones, lacking direct involvement in linker DNA sealing.
In eukaryotic chromatin organization, the histone that seals off the nucleosome at the location where linker DNA enters and leaves plays a pivotal role in stabilizing DNA packaging. This process ensures compact chromatin structure for gene regulation.
Nucleosome Structure Basics
Nucleosomes consist of DNA wrapped around a core octamer of histones H2A, H2B, H3, and H4, with ~147 bp of DNA forming 1.65 superhelical turns. Linker DNA (~20-80 bp) connects adjacent nucleosomes, forming the “beads-on-a-string” motif.
Role of Linker Histone H1
Histone H1, a linker histone, binds externally at the dyad axis, sealing the DNA entry/exit points and protecting linker DNA. Its globular domain anchors near the core, while tails bridge nucleosomes for 30-nm fiber formation.
Core Histones’ Functions
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H2A-H2B dimers flank the H3-H4 tetramer, stabilizing internal DNA contacts without linker involvement.
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H3 and H4 form the central wedge, enabling DNA wrapping but not external sealing.
| Histone | Location | Key Function | Seals Linker DNA? |
|---|---|---|---|
| H1 | External, linker | Seals entry/exit, compacts chromatin | Yes |
| H2A-H2B | Core dimer | DNA-histone contacts | No |
| H3 | Core tetramer | Central stability | No |
| H4 | Core tetramer | Tetramer assembly | No |
Exam Relevance for CSIR NET
This concept tests chromatin hierarchy knowledge, with H1 distinguishing from core histones in questions on nucleosome sealing. Understanding aids topics like gene expression and epigenetics.
