Replication in Drosophila chromosomes. I. Replication of intranucleolar DNA in polytene cells of D. nasuta.

The organization and replication of intranucleolar DNA in polytene cells of Drosophila nasuta have been examined. Normal Giemsa and fluorescent (Hoechst 33258 and quinacrine mustard) staining reveal that in D. nasuta polytene cells, the intranucleolar DNA is organized into condensed and disperse forms and very often shows connexion to the alpha heterochromatin of the chromocentre. Results of [3H]thymidine autoradiography indicate that the known underreplication of rDNA sequences in polytene cells is due to the slower replication of the intranucleolar DNA. Some aspects of the organization of condensed and disperse intranucleolar DNA are discussed.