Polytene chromosomes are giant, specialized chromosomes found in the tissues of certain dipteran insects, particularly in salivary glands, Malpighian tubules, trachea, gut, fat bodies, and ovarian nurse cells. Their distinct banding pattern and somatic pairing make them a valuable model for studying gene regulation and chromosome structure.
They are formed when the chromosomes remain tightly associated after multiple rounds of S-phase and become one supersized chromosome. Here are the polytene chromosome notes, including its structure and functions.
Discovery and Nomenclature
First observed (1881) by the Italian cytologist E.G. Balbiani in the salivary gland cells of Chironomus (a dipteran insect).
He described them as long, sausage-shaped structures.
Recognized as chromosomes (1933) by cytologists Theophilus Painter, Ernst Heitz, and H. Bauer.
They identified them in Drosophila and confirmed their chromosomal nature.
Polytene chromosomes were originally called salivary gland chromosomes, later named polytene chromosomes by Kollar, referring to the presence of multiple chromonemata (DNA strands).
Occurrence and Biological Significance
Found in the larval cells of dipteran insects (Drosophila, mosquitoes, and midges).
These cells contain a high DNA content but do not perform mitosis.
Cells with polytene chromosomes die during metamorphosis, except for imaginal discs, which remain diploid and form adult structures.
They are present in various tissues, including:
Salivary glands
Malpighian tubules
Rectum and gut
Fat bodies
Ovarian nurse cells
Formation: Endomitosis Process
The polytene chromosomes are said to be developed as a result of the duplication of chromosomes. The duplicated chromonemae do not separate to form new chromosomes but remain closely associated with each other. This process of duplication of chromosomes without separation is called endomitosis or endoploidy.
Endomitosis (Endoreduplication)
Polytene chromosomes undergo multiple rounds (9–10 cycles) of DNA replication without cell division, which leads to the enlargement of the chromosome.
Unlike mitosis, no spindle formation or nuclear envelope rupture occurs in endomitosis.
Daughter chromatids remain aligned side by side to form thick, multi-stranded structures.
Unlike polyploidy, the daughter chromatids of endometriosis remain together instead of separating.
Polytene Chromosome Notes: Structure and Composition
DNA Content and Chromosomal Arrangement
Drosophila polytene chromosomes contain approximately 1000 DNA molecules, resulting from 10 rounds of DNA replication (2¹⁰ = 1024 copies).
Chironomus polytene chromosomes can contain around 16,000 DNA molecules.
The number of chromonemata per chromosome ranges from 512 to over 4000, depending on the species.
Somatic Pairing
Polytene chromosomes contain some closely coiled homologous pairs of chromosomes as well. This association is called somatic pairing.
Polytene chromosomes are larger than other somatic chromosomes due to their multiple duplication of 9-10 times. The duplicated chromosomes are not separated and remain connected, causing the larger size.
Maternal and paternal homologous chromosomes remain aligned side by side, which reduces the visible chromosome count.
In Drosophila melanogaster, instead of 8 somatic chromosomes, only 4 polytene chromosomes are seen.
In males:
The X chromosome remains single and thin.
The Y chromosome fuses indistinctly with the chromocenter.
The pericentromeric heterochromatin of all polytene chromosomes merges into a chromocenter.
Banding Pattern and Genetic Activity
Chromomeres and Banding
Polytene chromosomes exhibit alternating dark and light bands. The cytological staining of chromosomes shows a series of dark-coloured bands that appear along the length, alternating with clear zones.
Dark bands
The dark bands contain highly coiled DNA, rich in RNA and basic proteins associated with gene expression. Moreover, these dark bands have more tightly coiled chromonemata than the lighter regions.
In the regions of bands, the individual chromonemata is in a linear order perpendicular to the linear axis of the chromosome. The bands are more active genetically.
Light bands (interbands)
The interbands consist of DNA and acidic proteins.
These light-coloured or clear zones have fibrillar bands while the dark areas are fulgent positive.
Studies have shown that the absence of specific bands can be correlated with changes in the genetic behavior of an organism.
Drosophila melanogaster has approximately 5000 bands and 5000 interbands.
85% of DNA is concentrated in bands, while 15% is in interbands.
One-Gene, One-Band Hypothesis
Early cytologists Painter (1933) and Bridges (1936) suggested that each band in a polytene chromosome corresponds to a single gene.
It was estimated that Drosophila has around 5000 essential genes.
However, later studies revealed that:
Bands and interbands both contain active genes.
A single band may contain multiple genes.
Function of Bands and Interbands
In addition to the coiling differentiation, there are chemical and functional differences as well.
Alberts et al. (1989) proposed that banding patterns help in:
Organizing DNA structure.
Isolating genes to prevent transcriptional interference.
Regulating gene expression for cell differentiation.
Housekeeping genes might be located in interbands.
Tissue-specific genes may reside in bands.
Chromosomal Puffs and Balbiani Rings
Chromosomal Puffs
During the development of polytene chromosomes of the larva of Dipterous insects, they develop swellings at a particular point of a dark band or inter-band.
Such chromosomal swellings are known as chromosomal puffs.
Chromosomal puffs are swollen regions (puffs) in polytene chromosomes that appear during larval development.
They represent active sites of gene transcription (mRNA synthesis).
In 1954, Beermann studied it in Chironomus larvae.
He found that the bands and interbands in the puffs are similar in all tissues but differ in their distribution, form, grow, and disappear cyclically, responding to developmental signals.
Balbiani Rings
The puffs may be of different sizes, and the larger ones are called Balbiani rings. The chromonemata of polytene chromosomes give out many series of lateral loops. The lateral loops are interrelated. Because of these lateral loops, these particular regions of chromosomes start looking fuzzy. These are the Balbiani rings.
They appear as lateral loop-like structures extending from chromonemata.
They are extremely rich in DNA and RNA, indicating high transcriptional activity.
Balbiani rings are associated with specific protein production in salivary glands.
Formation of Puffs
The appearance of chromosomal puffs depends on the equilibrium of protein metabolism in the nucleus and cytoplasm. Usually, they appear when secretory activity is most pronounced. Temperature, shock, and several other factors may also induce puffing.
The puffing process involves,
DNA despiralization (unfolding into loops).
Accumulation of acidic proteins.
RNA polymerase II activation, leading to mRNA transcription.
Newly synthesized mRNA is released into the cytoplasm.
Conclusion
Polytene chromosomes serve as a powerful model in cytogenetics, helping scientists study gene regulation, chromosomal behavior, and developmental biology. Their banding patterns, somatic pairing, and puff formation provide valuable insights into transcriptional activity and chromosomal structure. The study of polytene chromosomes continues to be essential for understanding genetic mechanisms in insects and broader applications in molecular biology.
References
Agarwal, P. V. |. V. (2004). Cell biology, Genetics, Molecular Biology, Evolution, and Ecology: Evolution and Ecology. S. Chand Publishing.
Stormo, B. M., & Fox, D. T. (2017). Polyteny: Still a giant player in chromosome research. Chromosome Research: An International Journal on the Molecular, Supramolecular and Evolutionary Aspects of Chromosome Biology, 25(3-4), 201. https://doi.org/10.1007/s10577-017-9562-z
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