What is Polytene Chromosome (Salivary Gland Chromosome)?

polytene chromosome or (Salivary Gland Chromosome)-compressed

An Italian cytologist E.G. Balbiani (1881) had observed peculiar 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).

Unfortunately, he did not recognize them as chromosomes, and his report remained buried in the literature. It was not until 1933 that Theophilus Painter, Ernst Heitz, and H. Bauer rediscovered them in Drosophila and recognized them as the chromosomes.

Since these chromosomes were discovered in the salivary gland cells, they were called salivary gland chromosomes. The present name polytene chromosomes were suggested by Kollar due to the occurrence of many chromonemata (DNA) in them.

Thus, some cells of the larvae of the dipteran insects such as flies (e.g., Drosophila), mosquitoes, and midges (Chironomus) become very large having high DNA content. These cells are unable to undergo mitosis and are destined to die during metamorphosis (Those cells of larva which are destined to produce the adult structures after metamorphosis, i.e., imaginal discs remain diploid).

Such polytenic cells are located most prominently in the salivary gland, but also occur in Malpighian tubules, rectum, gut, footpads, fat bodies, ovarian nurse cells, etc. Polyteny of giant chromosomes is achieved by replication of the chromosomal DNA several times without nuclear division (endomitosis), and the resulting daughter chromatids do not separate but remain aligned side by side.

In the process of endomitosis, the nuclear envelope does not rupture and no spindle formation takes place. In fact, polyteny differs from polyploidy, in which there is also an excess DNA per nucleus, but in which the new chromosomes are separate from each other.

A polytene chromosome of the Drosophila salivary gland has about 1000 DNA molecules that are arranged side by side and which arise from 10 rounds of DNA replication (210= 1024). Other dipteran species have more DNA, for example, Chironomus has 16000 DNA molecules in each polytene chromosome.

Further, the polytene chromosomes are visible during the interphase and prophase of mitosis. In them the chromomere (regions in which the chromatin is more tightly coiled) alternate with regions where the DNA fibers are folded more loosely.

The alignment of many chromomeres gives polytene chromosomes their characteristic morphology, in which a series of dark transverse bands alternates with clear zones called interband. About 85% of the DNA in polytene chromosomes is in bands and the rest 15% is in inter bands.

The crossbanding pattern of each polytene chromosome is a constant characteristic within a species and helps in chromosome mapping during cytogenetic studies. For example, in Drosophila melanogaster there are about 5000 bands and 5000 interband per genome, and each band and interband represent a set of 1024 identical DNA sequences arranged in the file.

Another peculiar characteristic of the polytene chromosomes is that the maternal and paternal homologous chromosomes remain associated side by side. This phenomenon is called somatic pairing. Consequently, in the salivary gland cells, the chromosome number always appears to be half of the normal somatic cells, e.g., D. melogaster, which has only 4 polytene chromosomes.

In Drosophila, pericentromeric heterochromatin of all polytene chromosomes also coalesces in a chromocenter. The preparation of a slide of the polytene chromosomes of dipterans for light microscopy is rather easy.

The larvae are taken at the third instar stage and the salivary glands are dissected out and squashed in aceto-carmine. In such preparations, these chromosomes in aggregate reach a length of as much as 2000 μm in D. melanogaster.

In female Drosophila, the polytene chromosomes are found in the form of five long and one short strand radiating from a single more or less amorphous chromocenter. One long strand corresponds to the X chromosome and the remaining four long strands are the left and right arms of II and III chromosomes.

The shortest strand represents the small dot-like IV chromosome. Each of these chromosomes contains maternal and paternal homologs in a somatic pairing that lacks in the sex chromosomes of male fruit flies.

Thus, in male Drosophila, the X chromosome remains single and thin and the Y chromosome exists indistinctly fused with the chromocenter. One-gene, one-band hypothesis. The fixed pattern of bands and interband in a Drosophila polytene chromosome suggested the early cytologists such as Painter (1933) and Bridges (1936) that each band might correspond to a single gene.

Accordingly, they concluded that Drosophila might contain only 5000 essential genes. It was also believed that bands were the sites of genes (DNA) and interband were relatively inert linker regions. Recent data, however, have contradicted this simple “one-band, one gene” hypothesis, now it is held that bands, as well as inter-bands, contain active genes and a band may even contain more than one gene.  At this juncture, a question arises,

why is the single long strand of chromatin in each chromosome subdivided into so many distinct regions?

The exact explanation of this question is still not known. However, Alberts et al., (1989) believed that this type of organization (i.e., banding and interbanding of chromosomes in general) may help to :

  • Keep the DNA organized;
  • Isolate genes from their neighbours and thereby preventing biological “crosstalk”, or
  • Regulate gene transcription for the cytodifferentiation,

Chromosome puffs or Balbiani rings

Constitutively expressed “housekeeping” genes could be located in interband, whereas cell-type-specific genes could be confined to the bands. Chromosome puffs or Balbiani rings. Chromosome puffs or Balbiani rings are the swellings of bands of the polytene chromosomes where DNA unfolds into open loops as a consequence of intense gene transcription (e.g., mRNA formation).

In 1954, W. Beerman compared the polytene chromosomes of different tissues of Chironomus larvae and showed that although the pattern of bands and interband was similar in all tissues, the distribution of puffs differed from one tissue to another.

Beerman and Bahr (1954) have studied the fine structure of these puffs. According to them, these puffs represent regions where the tightly coiled chromosomal fibers open out to form many loops. In fact, puffing is a cyclic and reversible phenomenon: at definite times and in different tissues of the larvae, puffs may appear, grow and disappear.

In salivary glands, the appearance of some puffs has been correlated with the production of specific proteins that are secreted in large amounts in the larval saliva. The process of puffing involves several processes such as the accumulation of acidic proteins, despiralization of DNA, accumulation of RNA polymerase II (an enzyme involved in the transcription of m RNA molecules), synthesis of mRNA, and release of newly synthesized mRNA in the cytoplasm.

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