Nucleosome

A nucleosome is the basic structural unit of chromatin in eukaryotic cells. It consists of a segment of DNA wrapped around a core of histone proteins, helping to package the long DNA strands into a compact, organized structure that fits within the nucleus.

Structure:

  • A nucleosome core particle is made of:
    • 8 histone proteins (an octamer): 2 copies each of H2A, H2B, H3, and H4
    • Around 147 base pairs of DNA wrapped in 1.65 turns around the histone core
  • Linker DNA (about 10–80 base pairs) connects one nucleosome to the next
  • Histone H1 (the linker histone) binds outside the core, helping stabilize the structure and compact the chromatin further

Function:

  1. DNA packaging: Reduces DNA length by ~7-fold at the nucleosome level, allowing the DNA (~2 meters long per human cell) to fit in the nucleus.
  2. Regulation of gene expression: Nucleosome positioning and histone modifications influence accessibility of DNA to transcription machinery.
  3. Genome stability: Organizes DNA and protects it from damage or random cleavage.
  4. Epigenetic control: Histone modifications (e.g., acetylation, methylation) help regulate genes without altering the DNA sequence.

Nucleosome and Chromatin:

  • Chromatin = DNA + histone proteins
  • Nucleosomes are the first level of chromatin organization, forming the “beads on a string” appearance.
  • These structures can then fold further into:
    • 30 nm fibers
    • Higher-order loops and scaffolds
    • Ultimately, condensed chromosomes during mitosis

Nucleosome Dynamics:

  • Nucleosomes are not static. They can:
    • Slide along DNA (nucleosome remodeling)
    • Be partially or fully displaced
    • Undergo histone exchange or modification
  • These changes affect how tightly DNA is packed and whether genes are active or silent

Clinical and Biological Importance:

  • Cancer: Abnormal histone modifications and nucleosome positioning can lead to dysregulated gene expression.
  • Development: Proper nucleosome arrangement is crucial during embryonic development and cell differentiation.
  • DNA repair: Chromatin remodeling is needed to allow repair proteins access to damaged DNA.

Summary Table:

FeatureDescription
Basic UnitDNA + histone core = nucleosome
Histones InvolvedH2A, H2B, H3, H4 (2 each) + H1 (linker)
DNA Wrapped~147 base pairs
FunctionDNA compaction, gene regulation, protection
Role in EpigeneticsModifications to histones influence gene expression
Seen As“Beads on a string” in chromatin