Polytene chromosomes

p chr..

1. Polytene Chromosomes Presented by : Prajapati Dimple B M.sc sem-1 CBO-403 Department of Life sciences, H.N.G.U.,Patan

2. Content Introduction History Structure Function

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 .

7. 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.

8. Reference Cell Biology and Genetics by:- P. S. Varma V. K. Agraval https://en.m.wikipedia.org https://www.slideshare.net

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