Super Coiling of DNA

The DNA double helix wraps around a nucleosome. Six nucleosomes form one turn of a solenoid. Loops of solenoid coils are attached to a non-histone protein scaffold. At the fourth level, the scaffold forms a spiral supercoil, which constitutes a chromatin fiber. During metaphase of the cell cycle, the chromatin condenses in an orderly manner to form microscaopically-visible chromosomes with distinctive sizes and morphologies. The chromatin that makes up a single chromosome appears to be a single, continuous DNA helix.

Figure © 2000 by Griffiths et al. ; text © 2008 by Steven M. Carr