The solenoid defines the packing of DNA as a 30 nm fiber of chromatin and results from the helical winding of at least five nucleosome strands.
In eukaryotic cells, 146 bp of DNA are wrapped approximately 1.65 times 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 eight 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.
DNA packed into solenoids, unlike DNA in nucleosome form, is not transcriptionally active. With more packing, solenoids are able to become increasingly more packed, forming chromosomes. At this point, solenoids coil around each other to form a loop (anywhere from 20 to 80,000 base pairs), followed by a rosette (consisting of six connected loops), then a coil, and at last, two chromatids. The end result is the metaphase chromosome. The completely condensed chromatin has a diameter of up to 600 nm.