Nearly every cell contains an organism's entire genome. Yet not every gene is expressed all the time. To express only the genes necessary, the cell must regulate gene expression. The packing of chromosomes and existence of nucleosomes, the structure formed by DNA winding around nucleosomes, allows for this regulation.
For a gene to be read by the enzyme RNA polymerase, and thus transcribed into a message that can leave the nucleus, that gene must be accessible to the enzyme. Genes deeply packed into chromosomes are not accessible. The tightly packed form of chromatin is called heterochromatin. To alter accessibility of genes, cells use a chromatin remodeling complex, a group of proteins bound together that adjusts the binding of nucleosomes to make any region of the DNA more or less accessible. These complexes can bind to the nucleosome, the DNA strand, or both, making them either more or less accessible as needed. The less tightly packed form of chromatin is called euchromatin. By adjusting between heterochromatin and euchromatin, chromatin remodeling complexes can alter gene expression.
Alternatively, the cell can chemically modify the histones to alter gene expression. Two common modifications are methylation—the addition of a methyl group (–CH3)—and acetylation—the addition of an acetyl group (–C2H3O). These modifications can change the way histones bind DNA and each other, either condensing or relaxing the chromatin structure. They can also recruit certain other proteins to the site. These proteins may, in turn, condense or relax chromatin, or they may express or silence nearby genes. The mechanisms that modify histones in this manner can be inherited, leading to hereditary changes in gene expression that do not rely on changes to the genes themselves. Epigenetics, the study of heritable changes in the expression of genes that are caused by the environment, further analyzes the mechanisms that modify histones and the resultant effects of these changes.