NORs cluster as cells exit mitosis. (a) The short arms of acrocentric chromosomes 13, 14, 15, 21 and 22 contain NORs, which are separated during mitosis. (b) As cells exit mitosis and the nuclear membrane begins to reform, chromosomes begin to decondense. (c) Loops of chromatin may extend away from the core of the territory. (d) As G1 phase is established and nucleoli form, loops of NOR-containing chromatin co-associate with the other components of the nucleolus and ribosomal DNA gene transcription is initiated.