 Polytene chromosomes are over-
sized chromosomes which have developed from
standard chromosomes and are commonly
found i...
 Polytene chromosomes were originally observed in
the larval SALIVARY GLANDS of CHIROMOMUS
midges by Balbiani in 1881, bu...
 In addition to increasing the volume of the cells' nuclei
and causing cell expansion, polytene cells may also
have a met...
 Polytene chromosomes have characteristic light
and dark banding patterns that can be used to
identify chromosomal rearra...
 The banding patterns of the chromosomes are
especially helpful in research, as they provide an
excellent visualization o...
Polytene chromosomes
Polytene chromosomes
of 7

Polytene chromosomes

polytene chromosome
Published on: Mar 4, 2016
Published in: Education      
Source: www.slideshare.net


Transcripts - Polytene chromosomes

  • 1.  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.
  • 2.  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.
  • 3.  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.
  • 4.  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.
  • 5.  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.