DNA packaging into chromosome
Three levels of condensation are required to package the DNA in chromosome
Nucleosomal Level of packaging (11 nm beads on string): It involves packaging DNA as a negative supercoil into nucleosomes, to produce the 11-nm-diameter interphase chromatin fiber. This involves an octamer of histone molecules, two each of histones H2A, H2B, H3, and H4.
Solenoid (30 nm) level of packaging: The second level of packaging involves an additional folding or supercoiling of the 11-nm nucleosome fiber, to produce the 30-nm chromatin fiber. Histone H1 is involved in this supercoiling .
Higher (300 -1400 nm) order level of packaging: Higher order packaging involves nonhistone chromosomal proteins which form a scaffold that is involved in condensing the 30-nm chromatin fiber into the tightly packed metaphase chromosomes (1400 nm). This third level of condensation appears to involve the separation of segments of the giant DNA molecules present in eukaryotic chromosomes into independently supercoiled domains or loops.
