Q.59 Given below are two statements:
Statement–I: 146 base pairs of DNA are wrapped around the histone octamer to form a nucleosome.
Statement–II: Nucleosomes play important role in gene expression.
In the light of the above statements, choose the most appropriate answer from the options given below.
- Both Statement–I and Statement–II are correct
- Both Statement–I and Statement–II are incorrect
- Statement–I is correct but Statement–II is incorrect
- Statement–I is incorrect but Statement–II is correct
Correct Answer: Both Statement–I and Statement–II are correct.
Nucleosomes form the basic unit of chromatin packaging, and both statements accurately describe their structure and function in eukaryotic cells.
Statement–I Analysis
Exactly 146 base pairs of B-form DNA make ~1.65 left-handed superhelical turns around the histone octamer (2 each of H2A, H2B, H3, H4). This core particle, plus linker histone H1 binding ~20 additional bp, forms the 10-nm “beads-on-a-string” structure.
Statement–II Analysis
Nucleosomes regulate gene expression by controlling DNA accessibility. Histone modifications (acetylation opens chromatin for transcription; methylation often represses), positioning (NFRs at active promoters), and variants (H2A.Z at inducible genes) modulate TF binding and Pol II recruitment.
Option Breakdown
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Both correct: Selected; structural fact (146 bp) pairs with functional role in transcription regulation.
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Both incorrect: Wrong; both represent textbook molecular biology.
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Statement–I correct, Statement–II incorrect: Invalid; nucleosome positioning directly affects ~80% of protein-coding genes.
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Statement–I incorrect, Statement–II correct: False; 146 bp is the precisely determined crystal structure value.
Introduction to Nucleosome 146 Base Pairs Gene Expression
Nucleosome 146 base pairs gene expression regulation defines chromatin biology: precisely 146 bp DNA wraps histone octamer core, while positioning/modifications control transcription access. Both statements correctly capture structure-function relationship.
Nucleosome Core Particle Structure
Crystal structure reveals 147 bp maximum (commonly cited 146 bp excludes entry/exit bp), forming flat disk ~11 nm diameter. H3-H4 tetramer binds first, then H2A-H2B dimers; superhelix stabilized by histone tails.
Gene Regulation Mechanisms
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Positioning: NFRs (~150 bp) at active promoters enable TF binding
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Modifications: H3K4me3 (active), H3K27me3 (repressive)
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Variants: H2A.Z destabilizes nucleosomes for rapid induction
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Remodelers: SWI/SNF slides/evicts nucleosomes at enhancers
Exam Relevance for Molecular Biology
GATE Life Sciences tests “beads-on-a-string” (10 nm) vs. 30-nm fiber, plus epigenetic marks. Your focus area: plant nucleosome dynamics control flowering genes (FT/FLC via H3K27me3), directly linking structure to physiology.
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