Question

Estimate the relative expansion of your prepared polytene chromosomes by comparing its packing ratio to a reference of 2x104 base pairs/nm for an interphase human chromosome (this reference was calculated based on data obtained from Am J Hum Genet, 2002). To estimate this ratio, you will first need to establish the average chromosome length (# DNA base pairs) in Drosophila virilis. This can be done by referring to the Genome Data Viewer database of Drosophila melanogaster to calculate the mean length of its chromosomes – one can reasonably assume that both ‘sister’ fruit fly species have chromosomes of comparable sizes. You can then estimate the ratio of DNA base pairs/nm in your preparation of polytene chromosomes by dividing the mean number of base pairs per chromosomes over the linear chromosomal length measured in the previous question. Finally compare the ratio value of your preparation of polytene chromosomes to the one of interphase human chromosomes to deduce the fold expansion of chromosomes in the salivary glands of D. virilis. Can you now explain why one can directly observe polytene chromosomes under a conventional light microscope? Insert your full calculation details and reasoning.

Answer 1

It is clear from the comparison that the polytene chromosomes at interphase basically arise as result of endomitosis. This amplication leads to the formation of many threads called chromonemas in each chromosome of Drosophila virility which leads to appearances of puffs. Since, these chromosomes are highly amplified, they stain densely at certain regions called Dark bands and lightly at some regions called interbands. All these facts account for the reason that such chromosomes are easily observed under light microscope