Morphology and Replication of Intranucleolar Dna in Polytene Nuclei

This report is concerned with filamentous intranucleolar structures in the polytene salivary gland cells of Drosophila melanogaster . A number of studies on a variety of cells have provided evidence for the chromosomal association of such structures (e .g. references 6, 7), fortifying the concept of Lettre et al . (11) that the nucleolonema, of historic controversy, is not an autonomous structure of the nucleolus, but instead is derived from a portion of the nucleolar chromosome . Support for that concept and, further, a basis for the proposition that all of the filamentous structures of the nucleolus have a common identity, may be found in this study of a single cell type in which intranucleolar structures of diverse morphological conformations ranging from a dense mass to a much extended, ramified filament have been observed . A series of micrographs are presented as evidence that all forms are homologous and are alternate forms of the laterally extruded nucleolus organizer locus of the chromosome complement . Radioautography of thymidine incorporation confirms that these structures contain replicating DNA and indicates that a negative correlation exists : the dynamic morphology is not related to replicative activity .