Parity rules were given by Erwin Chargaff

I. Rule of complementarity 

II. Rule of Equal amount applies more closely to introns than to exons

III. Cluster rule – Purines in nontemplate strand applies more closely to Exons

TLS Online TPP Program

#Id: 6875

#Unit 3. Fundamental Processes

Polycomb or Pc-G proteins do not initiate repression, but they are responsible for maintaining it.

TLS Online TPP Program

#Id: 6876

#Unit 3. Fundamental Processes

Most Polycomb proteins are subunits of one of two classes of multiprotein Polycomb repressive complexes: PRC1 and PRC2.

TLS Online TPP Program

#Id: 6877

#Unit 3. Fundamental Processes

The PRC2 complexes contain histone deacetylases that inhibit transcription. They also contain a subunit [E(z) in Drosophila, EZH2 in mammals] with a SET domain, which is the catalytic domain of several histone methyl transferases.

TLS Online TPP Program

#Id: 6878

#Unit 3. Fundamental Processes

PRC1 complex binds the methylated nucleosomes through dimeric Pc subunits (CBXs in mammals), each containing a methyl lysine–binding domain (called a chromodomain) specific for methylated H3 lysine 27. 

Binding of the dimeric Pc to neighboring nucleosomes is proposed to condense the chromatin into a structure that inhibits transcription.

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.