Radiation- Several species arise from a common ancestor over a short time

TLS Online TPP Program

#Id: 6879

#Unit 3. Fundamental Processes

The PRC1 complex contains a ubiquitin ligase that monoubiquitinylates histone H2A at lysine 119 in the H2A C-terminal tail

TLS Online TPP Program

#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. 

TLS Online TPP Program

#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!)

TLS Online TPP Program

#Id: 6882

#Unit 3. Fundamental Processes

TLS Online TPP Program

#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

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.