Practice question

Polytene chromosomes arise from repeated rounds of endoreplication without chromatid segregation or cytokinesis, producing thousands of aligned chromatids remaining synapsed, visible as giant banded structures with interbands and transcriptionally active puffs. Best described in dipteran salivary glands, especially Drosophila melanogaster third instar larvae where tissue-specific gene amplification supports massive glue secretion and high metabolic demand. Banding pattern provides physical cytological map for gene localization and transcription mapping. Unlike mammalian liver polyploidy forming binucleate cells, true polytene with visible Balbiani rings occurs primarily in flies and some plants.