Chromatin

0.34 nm	=	Distance between base pairs in B-form DNA.
3 · 10⁹ bp	=	Human Haploid Genome.
~1 m	=	Length of Haploid Genome.
3-10 · 10^-6 m	=	Length of nucleus.
~2 m	=	DNA per nucleus.

To pack DNA into the tiny nucleus (the DNA packing problem), DNA is tightly wound around special proteins to form a nucleoprotein complex called chromatin. Chromatin proteins are predominantly histones, a family (H1, H2A, H2B, H3 and H4) of small proteins that are conserved in eukaryotes and contain many positively-charged basic amino acids that interact with negatively-charged DNA phosphate groups.

Primary	Two each of each histone (except H1) interlock to form a disc-shaped structure ~10 nm in diameter. In all eukaryotes, 147 bp of DNA wraps almost twice around the histone octamer to form the nucleosome. Nucleosome “beads” are each spaced by 15-55 bp (depending on species) “strings” of linker DNA. The basic and positively-charged histones bind tightly to DNA, protecting it from proteins. Careful nuclease treatment will digest linker DNA and release individual nucleosomes with their DNA still intact. However, linker DNA is somewhat protected by bound H1 and by inter-histone interactions. Newly replicated DNA in vivo assembles into nucleosomes shortly after the replication fork passes, nucleosomes do not spontaneously form in vitro at physiological salt concentration when histones are added to DNA. However, nuclear proteins have been characterized that bind histones and assemble them in vitro with DNA. These are thought to assemble new DNA into histones in vivo as well.
Secondary	Nucleosomes (the 1° chromatin structural unit) will stack on top of each other at physiological conditions (~0.15 M KCl), and the stacks will then intertwine into an irregular spiral (solenoid arrangement) that is ~30 nm and contains ~6 nucleosomes per turn. H1, the 5th major histone, is bound to DNA on inside of solenoid with one H1 molecule associated with each nucleosome. The 30-nm solenoid is less uniform than a perfect solenoid; condensed chromatin may actually be quite dynamic, with regions occasionally partially unfolding and then refolding into a solenoid structure.
Tertiary	At special scaffold attachment regions (SARs), the 30 nm fibers attach to a flexible protein scaffold; the unattached regions form chromatin loops. DNA can be released from the protein scaffold by treatment with detergent. In addition to this general structure, thousands of low-abundance regulatory proteins associate with specific DNA sequences. During meiosis, chromatin further folds and compacts into visible metaphase chromosomes.

Heterochromatin Heterochromatin is made up of dense, tightly packed, portions of the chromosome that are mostly inactive and often contain repetitive simple sequence DNA. This appears very dark in the electron microscope.Heterochromatin is highly condensed but can be converted to euchromatin by transcriptional activators targeted to that chromosomal region. Euchromatin Gene rich regions of the chromosome are much less densely packed and make up what is called Euchromatin. The decondensation of chromatin upon transcriptional activation can also be observed through its sensitivity to DNAse. For example, globin genes expressed in erythrocytes (in these cells, DNAse sensitive) but not expressed in other cells (DNAse resistant).