COP1 is found in the nucleus in the dark and in the cytoplasm in the light. COP1 movement into the nucleus in the dark requires the COP9 signalosome (CSN) complex

TLS Online TPP Program

#Id: 6884

#Unit 3. Fundamental Processes

The DNA of the inactive X also becomes methylated at most of its CpG island promoters. Specialized histone octamers in which histone H2A is replaced by a paralog of H2A called macroH2A also become associated with the inactive X.

TLS Online TPP Program

#Id: 6885

#Unit 3. Fundamental Processes

DNA methylation and macroH2A contribute to the stable repression of the inactive X chromosome  A. Wutz and J. Gribnau, Curr. Opin. Genet. Dev. 17 (2007): 387–393. inactive specific transcript), which is stably expressed only on the inactive X chromosome.

TLS Online TPP Program

#Id: 6886

#Unit 3. Fundamental Processes

Recruitment of Polycomb repressor complexes (PRC1 and PRC2), which trigger a series of chromosome-wide histone modifications (H2AK119 ubiquitination, H3K27 methylation, H4K20 methylation, and H4 deacetylation). Late in the process, an inactive X-specific histone variant, macroH2A

TLS Online TPP Program

#Id: 6887

#Unit 3. Fundamental Processes

Inactivation of the X chromosome in females is governed by the n–1 rule

TLS Online TPP Program

#Id: 6888

#Unit 3. Fundamental Processes

Regardless of how many X chromosomes are present, all but one will be inactivated. 

In normal females there are of course two X chromosomes, but in rare cases where nondisjunction has generated a 3X or greater genotype, only one X chromosome remains active

TLS Online TPP Program

#Id: 6889

#Unit 3. Fundamental Processes

Xic is a cis-acting locus that contains the information necessary to count X chromosomes and inactivate all copies but one. Inactivation spreads from Xic along the entire X chromosome.