Eukaryotic Chromosome

Chromatin can be divided into two regions, euchromatin and heterochromatin, based on its state of condensation, that is, based on how tightly its constituent elements are packed together. Most of the cellular chromatin is euchromatin, which has a relatively dispersed appearance in the nucleus. It condenses significantly only during mitosis. Genes within euchromatin can be transcriptionally active or repressed at a given point in time.

Heterochromatin, on the other hand, is condensed in interphase, usually does not contain genes that are being expressed, and is among the last portions of the genome to be replicated prior to cell division. Heterochromatin frequently is localized at the periphery of the nucleus. It can be subdivided into constitutive and facultative heterochromatin. Constitutive heterochromatin is always inactive. It is often found adjacent to centromeres and telomeres. Facultative heterochromatin refers to DNA sequences that are specifically inactivated as the result of development or a regulatory event. One example of facultative heterochromatin is the mammalian X chromosome. The single X chromosome present in male cells is active. However, in female cells, one of the two copies present is directly and specifically inactivated.