Potent form of benzo(a)pyrene induces inactivating mutations at codons 175, 248, & 273 of the p53 gene

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#Unit 3. Fundamental Processes

Inactive X chromosome also becomes associated with PRC2 complexes, which catalyze the trimethylation of histone H3 lysine 27. This methylation results in association of the PRC1 complex and transcriptional repression

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#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.

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#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.

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#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

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#Unit 3. Fundamental Processes

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

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#Unit 3. Fundamental Processes