Deoxyribonucleic Acid Structures in the Nucleolus

Abstract

PHASE-CONTRAST observation and electron microscopy indicate that the nucleolus is structurally heterogeneous consisting of a ground material in which is embedded a more compactly aggregated material generally described as filamentous (Estable and Sotelo¹ proposed for them the names of ‘pars amorpha’ and ‘nucleolonema’). Nucleoli of plant and animal cells are seen at mitosis and meiosis associated with a particular chromosome or chromosomes—the so-called ‘nucleolar organizer’ chromosomes—at the site of a heterochromatic region. If a chromosomal deletion includes such a region or if a mutation occurs², nucleolar material is then produced at other ‘loci’ (the latent organizers).