11. Of these, Sir2p, Sir3p and Sir4p form the core of heterochromatin . 12. Also in yeast, RNA silencing has been shown to maintain heterochromatin structure. 13. Constitutive heterochromatin can affect the genes near itself ( position-effect variegation ). 14. The constitutive heterochromatin stains darker because of the highly condensed nature of the DNA. 15. Histone modifications are one of the main ways that the cell condenses constitutive heterochromatin . 16. Sir3p binding follows, further condensing the heterochromatin . 17. PRC2 recruits MACROH2A is a replacement for histone H2A that also supports heterochromatin formation. 18. Reasons might be the presence of repetitive sequences, extensive DNA methylation, or heterochromatin . 19. This heterochromatin greatly reduces, if not completely silences gene expression of the Barr body. 20. Yeast, " Drosophila " and mammals all have heterochromatin flanking their centromeres.
