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#nucleosome core

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Nucleosome Packing

Which histone pair forms dimers in nucleosome core?

Structural analysis shows octamer built from central tetramer plus peripheral dimers. H2A and H2B heterodimerize via histone fold domains creating handshake arrangement stabilized by antiparallel helices and hydrophobic core. Two such dimers flank tetramer symmetrically contacting outer DNA superhelical turns near entry exit. This dimeric form exchanges dynamically during transcription, removed by remodeling complexes and chaperones NAP1, FACT enabling polymerase passage. H3-H4 interaction forms tetramer, cross combinations like H2B-H3 or H1-H2A incompatible due to sequence specificity, confirming H2A-H2B as physiological dimer.

Ref: Arents and Moudrianakis 1995 PNAS Histone Fold; Alberts et al., Chapter 4: H2A-H2B Dimer Pair

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Nucleosome Packing

Which histones form tetramer in nucleosome core?

Histone octamer assembly follows ordered hierarchical pathway in vitro and via chaperones in vivo. H3 and H4 first form heterodimer via histone fold helices interaction, two dimers associate via H3-H3 four-helix bundle to form stable H3-H4 tetramer (H3-H4)2. This tetramer binds DNA first during replication-coupled assembly, serving as kernel onto which two H2A-H2B dimers deposit to complete octamer. H2A-H2B alone cannot tetramerize; H1 associates externally after nucleosome formation. Conservation of tetramer during replication ensures epigenetic inheritance of H3-H4 modifications to daughter strands, maintaining heterochromatin memory.

Ref: Tagami H. et al., Cell 2004; Lodish et al., Molecular Cell Biology, Chapter 8: H3-H4 Tetramer Formation

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Nucleosome Packing

The nucleosome core particle contains how many histone proteins?

Biochemical reconstitution and high resolution crystal structures confirm nucleosome core particle comprises exactly eight histone proteins forming disc. Two copies each of canonical H2A, H2B, H3, H4 assemble as octamer shaping 11 nanometer particle. Central kernel forms H3-H4 tetramer via four-helix bundle through H3-H3 interface, while two H2A-H2B dimers dock on sides contacting outer DNA wraps. Six histones would destabilize wrapping, ten or twelve exceed structural capacity supported by stoichiometry measurements. Micrococcal nuclease protection and ultracentrifugation correspond to octamer alone, fundamental compaction unit conserved throughout eukaryotes.

Ref: Luger K. et al., Nature 1997 2.8 Å Structure; Alberts et al., Chapter 4: Nucleosome Eight Histones Octamer

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