Polytene chromosomes are large chromosomes formed through repeated DNA replication without cell division in specialized cells like the salivary glands of Drosophila melanogaster. This process produces many sister chromatids that remain attached, increasing cell volume. Polytene chromosomes were first observed in insect larvae and are characterized by distinctive light and dark banding patterns that can be used to identify genetic changes. They provide benefits like high gene expression levels and have been useful for studying chromatin structure and genomic equivalence in organisms.

2.  Polytene chromosomes are over-
sized chromosomes which have developed from
standard chromosomes and are commonly
found in the salivary glands of DROSOPHILA
MELANOGASTER Specialized cells undergo
repeated rounds of DNA REPLICATION
without CELL DIVISION, to increase CELL
volume, forming a giant polytene chromosome.
Polytene chromosomes form when multiple
rounds of replication produce many
sister chromatids that remain synapsed
together.

3.  Polytene chromosomes were originally observed in
the larval SALIVARY GLANDS of CHIROMOMUS
midges by Balbiani in 1881, but the hereditary nature
of these structures was not confirmed until they were
studied in DROSOPHILA MELANGOSTER in the early
1930s by EMIL HEITZ and HANS BAUER.They are
known to occur in secretory tissues of
other DIPTERAN insects such as the MALPHIGIAN
TUBULES of Sciara and also
in protists, plants, mammals, or in cells from
other INSECTS. Some of the largest polytene
chromosomes described thus far occur in larval
salivary gland cells of the Chironomid genus Axarus.

4.  In addition to increasing the volume of the cells' nuclei
and causing cell expansion, polytene cells may also
have a metabolic advantage as multiple copies of
genes permits a high level of gene expression.
In Drosophila melanogaster, for example, the
chromosomes of the larval salivary glands undergo
many rounds of endoreduplication, to produce large
amounts of glue before pupation. Another example,
within the organism itself is the tandem duplication of
various polytene bands located near the centromere
of the X chromosome which results in the Bar
phenotype of kidney-shaped eyes.

5.  Polytene chromosomes have characteristic light
and dark banding patterns that can be used to
identify chromosomal rearrangements and
deletions. Dark banding frequently corresponds
to inactive chromatin, whereas light banding is
usually found at areas with higher transcriptional
activity.The banding patterns of the polytene
chromosomes of Drosophila melanogaster were
sketched in 1935 by Calvin B. Bridges.

7.  The banding patterns of the chromosomes are
especially helpful in research, as they provide an
excellent visualization of transcriptionally
active chromatin and general chromatin structure. For
example, the polytene chromosomes
in Drosophila have been used to support the theory
of genomic equivalence, which states that all of the
cells in the body maintain the same genome. Polytene
chromosomes are about 200 µm in length.The
chromonema of these chromosomes divide but do
not separate.Therefore, they remain together to
become large in size.