Polytene chromosomes are giant chromosomes found in dipteran flies that form through repeated DNA replication without cell division. They begin as normal chromosomes but become large structures with many aligned DNA molecules. Specifically, in Drosophila melanogaster the salivary gland chromosomes undergo multiple rounds of replication to contain around 1000 DNA molecules each. This gives polytene chromosomes their characteristic banded morphology with dark and light regions. Their size allows for high levels of gene expression and production of materials like glue before flies undergo metamorphosis.
3. Introduction
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.
Polytene chromosomes are oversized chromosomes
which have develop from standard chromosomes somes
and are commonly found in the salivary glands of
Drosophila melanogaster.
4. History
An Italian cytologist E.G.Balbini (1881) had observed
peliculiar structures in the nuclei of certain secretory cells.
(e.g.,of salivary glands) of midge, Chironomus (Diptera).
These structures were long and sausage-shaped and
marked by swellings and cross striations (transverse bands)
Since these chromosomes were discovered in the salivary
gland cells ,they were called salivary gland chromosomes.
The present name polytene chromosomes was suggested
by Kollar due to the occurrence of many chromonemata
(DNA) in them.
5. Structure
A polytene chromosomes of Drosophila salivary gland
has about 1000 DNA molecules which are arranged side
by side and which arise from 10 rounds of DNA
replication .
Other dipteran species have more DNA ,for example,
Chironomus has 16000 DNA molecules in their each
polytene chromosomes.
The polytene chromosomes are visible during
interphase and prophase of mitosis.
The alignment of many chromosomes gives polytene
chromosomes their characteristic morphology , in which
a series of dark transverse bands alternates which clear
zones called interbands.
6. About 85 per cent of the DNA in polytene
chromosomes is in bands and rest 15 per cent is in
interbands.
Another peculiar characteristics of the polytene
chromosomes is that maternal and paternal
homologous Chromosomes remain associated side
by side .
Thus phenomenon is called somatic pairing .
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9. Function
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 endo reduplication, to
produce large amounts of glue before pupation.
10. Reference
Cell Biology and Genetics by:-
P. S. Varma
V. K. Agraval
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