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(a) What is the composition of chromatin? (b) What is the function of histone proteins?

GeneralClass 12AllAnswered 27 Mar 2026
Answer

(a) Chromatin composition:

  1. DNA (30–40%): Genetic material
  2. Histone proteins (50–60%): 5 types
    • Core histones: H2A, H2B, H3, H4 (form nucleosome octamer)
    • Linker histone: H1 (stabilizes higher-order structure)
  3. Non-histone proteins (10%): Structural (scaffolding), regulatory (transcription factors), enzymatic (polymerases)
  4. RNA (trace amounts): Nascent transcripts, regulatory RNAs

(b) Functions of histone proteins:

  1. DNA Packaging:
    • Compact 2 billion bp of human DNA into nucleus (~10 µm diameter)
    • Nucleosome formation: DNA wraps 1.65 turns around histone octamer
  2. Gene Regulation:
    • Histone modifications (acetylation, methylation, phosphorylation) regulate:
      • Chromatin accessibility
      • Gene expression (epigenetic control)
    • Euchromatin (loosely packed, active genes) vs heterochromatin (tightly packed, inactive)
  3. Chromosome Structure:
    • Provide structural scaffold for chromosome condensation during mitosis

Highly conserved: Histone sequences are remarkably similar across eukaryotes, indicating fundamental importance.

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