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The solenoid model refers to a theoretical structure of nucleosome arrangement and compaction to form a 30nm fiber of chromatin deduced from X-ray diffractions. In eukaryotic cells, 146 bp of DNA are twice wrapped around a histone octamer (each histone consists of 2 H2A, H2B dimers, and H3, H4 tetramer) which together are called a nucleosome. Histone H1, which is not part of the binding histones, tightens the DNA bound to the 8 protein complex. The nucleosomes, which at this point resemble beads on a string, are further compacted into a helical shape via the NH2 terminal protein interactions of the octameric histones, called a solenoid.
