Polytene Chromosomes

Polytene chromosomes are giant chromosomes common to many dipteran (two-winged) flies. They begin as normal chromosomes, but through repeated rounds of DNA replication without any cell division (called endoreplication), they become large, banded chromosomes (see figure). For unknown reasons, the centromeric regions of the chromosomes do not endoreplicate very well. As a result, the centromeres of all the chromosomes bundle together in a mass called the chromocenter.

Polytene chromosomes are usually found in the larvae, where it is believed these many-replicated chromosomes allow for much faster larval growth than if the cells remained diploid. Simply because each cell now has many copies of each gene, it can transcribe at a much higher rate than with only two copies in diploid cells.

The polytene chromosomes at the right are from the salivary glands of the fruit fly Drosophila melanogaster. the bands on each chromosome are like a road map, unique to each chromosome and well defined enough to allow high resolution mapping of each chromosome. The Drosophila Genome Project uses polyene chromosomes as a framework for the map.

*Polytene chromosomes alongside normal mitotic chromosomes. image blatantly stolen from a textbook somewhere.*

Polytene Chromosomes

*Drosophila* polytene chromosomes viewed on our microscope. the DNA is stained with DAPI (in blue) and a site on the left arm of chromosome 2 is labeled with a flourescent DNA probe (yellow).

*Drosophila* polytene chromosomes stained with Orescein and viewed under transmitted light. These chromosomes came from a fly strain called ltx13, which has most of the left arm of chromosome 2 on the right arm of chromosome 3.

Low magnification view of polytene chromosomes, stained the same as the first figure. I rendered the normally blue DAPI stain as cyan for your viewing pleasure.