Q.24 Nuclease-hypersensitive sites in the chromosomes are sites that appear to be
- (A) H2 and H4 histone free
- (B) H1 and H2 histone free
- (C) H3 and H4 histone free
- (D) Nucleosome free
Nuclease-Hypersensitive Sites: Nucleosome-Free Chromatin Regions
Nuclease-hypersensitive sites mark open chromatin areas crucial for gene regulation in molecular biology. The correct answer is (D) Nucleosome free, as these sites lack nucleosomes to enable nuclease access.
Core Concept
These sites appear ~10-200 bp long where DNase I cuts 100-fold more than bulk chromatin. They signal regulatory elements like promoters where transcription factors bind after displacing nucleosomes.
Positive supercoiling or non-histone proteins maintain this open state for dynamic access.
Correct Answer: (D) Nucleosome Free
Nuclease-hypersensitive sites form when histone octamers (H2A, H2B, H3, H4) detach entirely from DNA. This nucleosome-free DNA becomes highly accessible, marking active genes in eukaryotes.
Option Analysis
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(A) H2 and H4 histone free: Incorrect; hypersensitivity requires full nucleosome removal, not selective core histone loss.
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(B) H1 and H2 histone free: Incorrect; linker histone H1 absence aids openness but insufficient alone—core nucleosome displacement defines the site.
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(C) H3 and H4 histone free: Incorrect; H3/H4 tetramer stability persists unless total octamer eviction occurs.
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(D) Nucleosome free: Correct; complete absence of ~147 bp-wrapped histone octamer creates the hypersensitive gap.
Option Histone Status Defines Hypersensitivity? Role in Chromatin (A) H2, H4 absent No Partial unwrap (B) H1, H2 absent No Linker loosening (C) H3, H4 absent No Tetramer focus (D) Nucleosome free Yes Full access Biological Relevance
Found near enhancers/promoters, these sites drive cell-specific expression in biotech studies. Techniques like DNase-seq map them genome-wide for epigenetics research.
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