By Levi Clancy for Student Reader on
Some specialized eukaryotic cells increase cell volume via endomitosis, where DNA synthesis is repeated without cell division and a normal chromosome develops into a giant polytene chromosome results.
Centromeres and telomeres endoreplicate poorly, leading to a bundle of duplicate chromatids (as many as 1,000) stuck at the chromocenter; thus, the ploidy of the cell remains constant. This was first observed in Drosophila melanogaster larval salivary glands.
Chromosome puffs are diffuse uncoiled regions of the polytene chromosomes where RNA transcription occurs; a large chromosome puff is a Balbiani ring. In addition to increased nucleic and cellular volume, polytene cells have metabolic advantages since multiple gene copies facilitate high levels of gene expression (which would be particularly useful in larval cells). Polytene chromosomes have very distinctive banding patterns that are useful for mapping the location of genes and observing their transcriptional activation.