Chromatin structure: what is the correct composition of the nucleosome core particle with respect to histones and H1?

Introduction / Context:
Nucleosomes are the fundamental repeating units of eukaryotic chromatin. Understanding their precise composition is critical to interpreting chromatin packaging, accessibility, and epigenetic regulation.

Given Data / Assumptions:

• Focus on the nucleosome core particle.
• Consider whether linker histone H1 is part of the core.

Concept / Approach:
The nucleosome core particle is an octamer comprising two copies each of H2A, H2B, H3, and H4. About 146 base pairs of DNA wrap around this core. H1 is a linker histone that binds at the DNA entry/exit site to stabilize higher-order structure but is not part of the core octamer.

Step-by-Step Solution:
1) Enumerate histones in the core: H2A, H2B, H3, H4 (two of each).2) Confirm H1 function: binds linker DNA, not within the octameric core.3) Therefore, the correct composition is an octamer of four histones without H1.

Verification / Alternative check:
Biochemical reconstitution experiments form stable core particles with H2A/H2B/H3/H4 alone; addition of H1 shifts to chromatosome (~166 bp DNA), evidencing H1 as linker-associated.

Why Other Options Are Wrong:
Octamer with H1: incorrectly includes H1 in the core.Hexamer options: do not match established octameric stoichiometry.

Common Pitfalls:
Confusing the chromatosome (core + H1) with the nucleosome core particle, or miscounting histone copies.

Final Answer:
An octamer of four histones without H1 protein