Biochemistry Graduate Program

Nucleolar dominance is an epigenetic phenomenon that describes the formation of nucleoli, the sites of ribosome synthesis, at specific loci of chromosomes inherited from only one progenitor of a genetic hybrid. These loci, known as nucleolus organizer regions (NORs), consist of ribosomal RNA (rRNA) genes that are repeated in hundreds of copies. The molecular basis for nucleolar dominance is the silencing of rRNA genes at some NORs, but not others, a manifestation of a dosage control system that regulates the number of active rRNA genes depending on the cellular demand for ribosomes and protein synthesis. rRNA gene silencing involves cytosine methylation, histone post‐translational modifications and noncoding RNAs, but the precise mechanisms by which specific sets of rRNA genes are chosen for silencing are unclear.