Cells passing through the more anterior portions of the primitive streak give rise to the endoderm, head mesoderm, and notochord.

#Id: 6880

#Unit 3. Fundamental Processes

Modification of H2A inhibits elongation by inhibiting a histone chaperone that removes histone octamers from DNA as Pol II transcribes through a nucleosome, then replaces them as the polymerase passes. 

#Id: 6881

#Unit 3. Fundamental Processes

This mechanism is a key feature of Polycomb repression, which is maintained through successive cell divisions for the life of an organism (~100 years for some vertebrates, 2000 years for a sugar cone pine!)

#Id: 6882

#Unit 3. Fundamental Processes

#Id: 6883

#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

#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.

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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.