Polytene chromosomes: A general model for the eucaryotic interphase state

Abstract The euchromatic structures of insect polytene chromosomes represent an amplified version of the chromomeric organization in diploid chromatin. As such, they are an excellent model system for studying structure/function relationships of eucaryotic interphase chromosomes. Polytenization is accomplished by replication patterns that are different for euchromatic and heterochromatic chromatin and also seem to depend on the functional state of chromatin. In Diptera, polytene chromosomes are characterized by genetically determined discontinuities of DNA packing in bands and interbands that are modified by functional demands. The cytological visualization of proteins that are associated with the compaction and decondensation of chromatin, allows the analysis of the temporal and spatial dynamics of DNA/protein interactions in the context of structure, transcription, and the processing of RNA. Ribonucleoprotein particles may be followed on their way via the nuclear matrix through the pores of the nuclear envelope to the cytoplasmic compartment. Thus, polytene nuclei provide unique opportunities for studying the flow of genetic information from the site of storage to the site of action, i.e. from gene to phenotype.