THE AG-NOR PROTEINS PRESENT A CRESCENT-SHAPED DISTRIBUTION AT THE SECONDARY CONSTRICTIONS OF METAPHASE PTK1 CHROMOSOMES

The ultrastructural distribution of the proteins responsible for the s ilver staining of NORs was analyzed on serial thin sections of preembe dded silver-stained rat kangaroo PtK1 metaphase chromosomes. Our resul ts show that the Ag-NOR proteins present a crescent-shaped distributio n at the secondary constriction of each chromatid. Moreover, in some c ases the crescent-shaped structures of both chromatids are not symmetr ically arranged but show different orientations. These observations, t ogether with our own and previously reported light microscopy results obtained on silver-stained mammalian chromosomes, lead us to suggest a mechanism for the formation of apparent secondary constrictions at NO Rs.