Molecular Genetics—Chromatin Structure Chromosomal DNA in eukaryotes complexes with core and linker histones to form nucleosomes. Which complete set correctly lists the canonical histone types?

Introduction / Context:
Eukaryotic chromosomal DNA is packaged into chromatin, whose fundamental unit is the nucleosome. Proper identification of histone types is essential to understand gene regulation, DNA replication, and epigenetics.

Given Data / Assumptions:

• Nucleosome core contains an octamer: (H2A-H2B)2 and (H3-H4)2.
• Linker histone H1 associates with DNA at the entry/exit site to stabilize higher-order structure.
• Question asks for the complete canonical set of histones in eukaryotic chromatin.

Concept / Approach:
Recall the core histones (H2A, H2B, H3, H4) and the linker histone (H1). While variant histones exist (e.g., H2A.Z, H3.3), the canonical list remains five types. Histone H5 is specific to avian erythrocytes and is not a canonical eukaryotic histone across species.

Step-by-Step Solution:

Verification / Alternative check:
Chromatin reconstitution experiments and crystal structures of nucleosome cores confirm the histone composition and the role of H1 in chromatin compaction.

Why Other Options Are Wrong:

• Three or two histones: incomplete; missing core members.
• Four with H1 but missing H2B: still incomplete.
• Including H5: not a ubiquitous canonical histone; mainly avian erythrocytes.

Common Pitfalls:
Confusing histone variants with canonical types; forgetting the linker histone H1.

Final Answer:
Five histones: H1, H2A, H2B, H3, and H4