Polytene chromosomes

Polytene chromosomes are oversized 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 (endomitosis), to increase cell volume, forming a giant polytene chromosome. Polytene chromosomes form when multiple rounds of replication produce many sister chromatids that remain fused together.

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.

Polytene chromosomes were discovered by Balbiani in 1881. They are commonly found in the salivary glands of insects. Hence it is also called salivary gland chromosome. It is larger in size. Hence it is a giant chromosome. The larger size of the chromosome is due to the presence of many longitudinal strands called chromonemata. Hence named as polytene chromosomes (many stranded). The many strands of the giant chromosome is due to repeated division of the chromosome without the cytoplasmic division. This type of division is called endomitosis. The polytene chromosome contains two types of bands namely dark bands and inters bands. The dark bands are darkly stained and the inter bands are lightly stained with nuclear stains. The dark bands contain more DNA and less RNA. The inter bands contain more RNA and less DNA. The bands of polytene chromosomes become enlarged at certain times to form swellings called puffs. The formation of puffs is called puffing. In the regions of puffs, the chromonemata uncoil and open out to form many loops. The puffing is caused by the uncoiling of individual chromomeres in a band. The puffs indicate the site of active genes when mRNA synthesis takes place. The chromonemata of puff give out many series of loops laterally. As these loops appear as rings, they are called Balbiani rings after the name of the inventor. They are formed of DNA, RNA and a few proteins. Lamp brush Chromosome was discovered by Ruckert in 1892. It is a large chromosome. It contains lateral loops and appears like a brush. Hence the name lamp brush chromosome. It is found in the oocytes of Sagitta, Sepia, Echinaster (Echinoderm), insects, sharks, amphibians, reptiles and birds. It is 5900 µm in size. Hence, it is called a giant chromosome. Each lamp brush chromosome consists of main axis and many lateral loops. The main axis of each chromosome is formed of 4 chromatids. The main axis contains a series of thickenings called chromomeres. From each chromomere a pair of lateral loops one on each side arises. Each loop has an axial fibre. The axial fibre is the continuation of the chromonema of the main axis. It is rich in DNA. The axial fibre of the loop is surrounded by a matrix. The matrix is formed of RNA and proteins. The matrix gives a fuzzy appearance. The synthesis of protein and yolk takes place in the lateral loops. These proteins are being used in the early development of embryos. <Arumugam, N.2011. Cell & Molecular Biology, Saras Publication 268-270 >

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