The process of X inactivation

The expression of a specific gene within the X inactivation centre is required for compaction of the X chromosome into a Barr body. This gene, discovered in 1991, is named Xist (for X inactive specific transcript). The Xist gene product is a long RNA molecule that does not encode a protein. Instead, the role of Xist RNA is to coat one of the two X chromosomes during the process of X inactivation. The Xist gene on the inactivated X chromosome continues to be expressed after other genes on this chromosome have been silenced.

The process of X inactivation can be divided into three phases: initiation, spreading, and maintenance. During initiation, one of the X chromosomes is targeted for inactivation. This chromosome is inactivated during the spreading phase, so called because inactivation begins near the X inactivation center and spreads in both directions along the chromosome. Spreading requires the transcription of the Xist gene and coating of the X chromosome with Xist RNA. After coating, proteins associate with the Xist RNA and promote compaction of the chromosome into a Barr body. Maintenance refers to replication of the compacted chromosome during subsequent cell divisions. Although initiation and spreading occur only during embryonic development, maintenance occurs throughout the individual's life. Continued activity of the Xist gene on an inactivated X chromosome maintains this chromosome as a Barr body during cell division. Whenever a somatic cell divides in a female mammal, the Barr body is replicated to produce two Barr bodies.