Notes on the structure of a polytene chromosome

Polytene Chromosomes:

These multithreaded chromosomes were first re­ported by Balbiani in 1881 while observing the squash of salivary cells of Chironomous. Subsequently they have been seen in the larvae of dipterans (Drosophila etc). The name polytene was suggested by Kollar due to the appearance of many threads.

Besides salivary glands the polytene chromo­somes also occur in rectal epithelium and malphigian tubules. The nuclei in the salivary gland cells are much larger than the normal ones and are about 25 µ in diametre with the giant chromosomes larger by 50-200 times than the normal chromosomes. The polytene chromosome in the sali­vary cells of Drosophila melanogaster (Painter 1933) are about 200|i in length.

The enormous size is due to the duplication of chromonema which do not separate. According to an estimate, the polytene chromosomes have 1000 times more DNA than the normal somatic chromosomes.

Structure:

Each polytene chromosome is transversely striated by alternat­ing bands and interband regions. In Drosophila, over 5000 bands have been found in the four chromosomes of the salivary gland cells. The bands take an intense stain (Feulgen) while the interband regions stain less intensely.

The bands have chromomeres arranged at right angles to the long axis of the chromosome. The band actually represents regions of tight coils of the chromonema. Van Herwerden (1911) interpreted these bands as gyres of a continuous spiral. This view was later supported by Kostoff (1930). The bands have a high DNA content and absorb U.V. light at 2600 A⁰.

The interbands stain lightly and they are negative to feulgen stain indicating a low concentration of DNA. The chromonema in the interband region are less coiled and absorb very little U.V. light.

Painter (1933) and Bridges (1936) have shown that in Drosophila the bands are clearly associated with the genes. Association of certain defects due to mutated genes has also been associated with the bands. Certain recent studies however have shown that together with the bands, the interband re­gions also have genes.

Chromosome puffs:

There are certain interesting structures associated with the bands in the giant chromosomes called chromosome puffs or Balbiani rings. The swelling in bands at certain stages are given the name chromo­some puffs. It is quite possible that these puffs are associated with the meta­bolic activities and represent areas of active RNA synthesis.

According to Beerman and Bahr (1954), the coils in the puff region open out to form many loops. The larger chromosome puffs are called Balbiani rings and they show a high DNA and mRNA content. 14 Explain the structure of a lamp brush chromosome?

Lamp brush chromosomes:

The giant chromosomes found in the oocytes of fish, amphibians, reptiles and aves look like the brush used to clean the chimney of kerosene lamp-hence the name lamp brush chromosomes. These are very large and are characterised by loops arising out of the main axis laterally.

Discovered first by Flemming is 1882, in amphibian oocytes, they were studied in great detail by J. Rucket (1892) in the oocytes of shark. Rarely lamp brush chromosomes have also been found in some invertebrates (Se­pia, Echinaster, insects etc) but they are not as large as those of vertebrates. It seems they are present in certain plants also (Gran 1958).

In some organisms, lamp brush chromosomes as long as 1000µ and 20µ wide have been reported during early prophase I. Towards the end of the prophase, they get reduced in size. There are reports of lamp brush chromo­somes as long as 5900 microns in some salamander oocytes. These chromo­somes are elastic and can be stretched.

Structure:

There are two main parts viz., the main axis and lateral loops. The main axis has two chromatids (the homologues have four) and is rich in DNA. The axis is in continuity with the loop axis. The tips of the axis are without loops and are the regions of telomeres.

The centromeric region is also without loops. At certain regions along their length the chromonema of the axis gets coiled and forms the chromomeres. The chromomeres are found in pairs. Electron microscopic studies of the lamp brush chromosomes of the salamander, Triturus (Miller and Beatty 1969) have shown the axial fibre containing dense granules which are probably the molecules of the enzyme RNA polymerase.

The loops are of two main types viz. typical and special. Most of the loops which are typical consist of a bent central axis from which is given out RNA fibrils. These RNA fibrils are of progressively increasing length as a result one side of the loop is pronouncedly thick. The special loops are symmetrical and have granules at the end of RNA fibrils. Each loop axis varies from 30-50A. Axial fibre is made up of DNA, while the matrix has RNA and proteins.

Lamp brush chromosomes are involved in the synthesis of RNA and pro­teins. Each loop is believed to represent one long operon consisting of repetitive cistrons (structural genes). Each gene locus codes for RNA. The loop is supposed to synthesise at a high rate because of repetitive gene se­quence. There are also reports that the lamp brush chromosomes help in the formation of yolk material in the egg.