For a gene to be transcribed, RNA polymerase must first find the gene. This is made more difficult by the tight packing required to fit the entire genome within the nucleus. The cell uses this packing to its advantage, though, to prevent access to and expression of genes in some chromosomal regions.
Regions that are tightly condensed are called heterochromatin and can be distinguished from more open regions (euchromatin) by their dense staining, when viewed under a microscope. In females, an entire X chromosome in each cell is kept condensed throughout life, to avoid a "double dose" of these genes (recall that females have two X chromosomes, while males have only one). This random X inactivation leads to mosaicism, in which some female cells express genes from one chromosome, while others express genes from the other.
Even within an active chromosome, some regions may be temporarily inactivated. Inactive heterochromatin can be converted to active euchro-matin, and vice versa, by chemical modification of the histone proteins to which the DNA is attached in the chromosome. Negatively charged DNA is chemically attracted to the positively charged histones. By adding or removing chemical groups to the histones, this attraction can be modulated. A weaker attraction, as would occur by adding negatively charged groups to the histones, tends to open up the chromatin, favoring gene expression. A stronger attraction keeps it more condensed.
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