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Why only H2A, H2B, H3 and H4 histones form the octamer? Why H1 does not take part in octamer formation?

Histone octamer includes 2 copy of each H2A ,H2B, H3 and H4.Dna is wrapped around this core octamer to form nucleosome. H1 histone is a linker histone which connects 2 nucleosome.

Moreover mathematically octamer means 8. As i said earlier 2 copy of each H2A ,H2B, H3 and H4.Therefore 4*2=8. octamer is completed and thus H1 forms linker histone.

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Histone octamer consists of a pair of each H2A,H2B,H3 and H4 which form an octamer around which the DNA gets wrapped whereas H1 is a linker histone which is like a pinning structure that holds the DNA in place with the histone octamer.

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Unlike the other histones, H1 does not make up thenucleosome"bead". Instead, it sits on top of the structure, keeping in place the DNA that has wrapped around the nucleosome. H1 is present in half the amount of the other four histones, which contribute two molecules to each nucleosome bead. In addition to binding to the nucleosome, the H1 protein binds to the "linker DNA" (approximately 20-80 nucleotides in length) region between nucleosomes, helping stabilize the zig-zagged 30nm chromatin fiber.

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