Figure 1.

Chromatin organization. Chromatin is arranged into nucleosomes as its first level of organization. A nucleosome is made up of histone proteins and double-stranded deoxyribonucleic acid (DNA) that wraps around a histone octomer 1.7 times in a left-handed coil. Hydrogen bonds hold the phosphodiester backbone of DNA to the amino acid side chains of the histones. Each nucleosome is separated from the next by a short segment of DNA. The conformation resembles a “beads-on-a-string” structure. Further condensation of chromatin structure is facilitated by histone “tails” that extend from the nucleosome and help to stack nucleosomes into a more compact chromatin structure by linking them to neighboring nucleosomes. These circular stacks of nucleosomes compact further to form a solenoid that results in a chromatin fiber that measures 30 nanometers in diameter. As the final chromatin structure, chromosomes are thought to be the result of a mesh formed through fibers connected by cross-linked proteins or as a result of hierarchical packaging of chromatin around a central axis mediated by structural maintenance of chromosome proteins. Reprinted by permission from Macmillan Publishers Ltd: Felsenfeld & Groudine, 2003.