|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.