5.15
Polytene Chromosomes
Polytene chromosomes are giant interphase chromosomes with several DNA strands placed side by side. They were discovered in the year 1881 by Balbiani…
Typically, when a diploid cell divides, it first duplicates each of its chromosomes. Then, the cell distributes a copy of each chromosome between the two daughter cells so that each daughter cell receives a complete set of chromosomes.
Certain cells undergo multiple cycles of chromosome duplication without undergoing any cell division. Such cells, called polyploid cells, contain multiple sets of each chromosome.
In the salivary gland cells of Drosophila larvae, this process is taken to an extreme. Here, hundreds to thousands of copies of each chromosome are generated without any cell division.
This results in the formation of unique, giant chromosomes, called polytene chromosomes.
As thousands of identical DNA sequences lay side by side, like crayons in a box, certain features in the chromatin become visible. These features exist in ordinary interphase chromosomes but are difficult to observe.
When viewed under a microscope, an alternating pattern of dark bands and lighter interbands can be seen in the polytene chromosomes.
The bands constitute 95% of the DNA, whereas the interbands constitute 5%.
Where the chromatin in the bands is more condensed and transcriptionally inactive, the chromatin in the interbands is less condensed and transcriptionally active.
As Drosophila larvae progress from one developmental stage to another, specific bands and interbands greatly increase in diameter, producing structures called ‘Puffs’ or ‘Balbiani’ rings.
These puffs arise from the de-condensation of chromatin and are sites of active DNA transcription. The chromatin in puffs is arranged in looped structures, much like those observed in ‘lampbrush’ chromosomes.
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Q1: What are polytene chromosomes and how do they form?
Polytene chromosomes are giant interphase chromosomes formed when cells undergo multiple rounds of DNA replication without cell division. In Drosophila salivary gland cells, hundreds to thousands of identical DNA copies remain aligned side by side instead of segregating, creating these massive structures. This process produces unique chromosomes visible under a microscope with distinctive banding patterns.
Q2: Why do polytene chromosomes show visible banding patterns?
Polytene chromosomes display alternating dark bands and light interbands because thousands of identical DNA sequences align side by side, magnifying chromatin compaction differences. Bands constitute 95% of the DNA and contain condensed, transcriptionally inactive chromatin. Interbands make up 5% and contain less condensed, transcriptionally active chromatin, making these features easily observable under a microscope.
Q3: What is the relationship between polytene chromosome bands and gene activity?
In polytene chromosomes, bands represent transcriptionally inactive regions with condensed chromatin, while interbands are transcriptionally active regions with loosely organized chromatin. This relationship demonstrates how chromatin compaction directly affects gene expression. The visible banding pattern thus reflects the underlying transcriptional state of different chromosome regions.
Q4: What are puffs and Balbiani rings in polytene chromosomes?
Puffs are expanded structures that form when interband regions de-condense to allow active RNA synthesis during development. Balbiani rings are exceptionally large puffs found in salivary glands of certain species with highly decondensed chromatin supporting very high transcription rates. Both structures represent sites of intense gene activity and serve as excellent models for studying transcription.
Q5: How do polytene chromosomes differ from typical mitotically dividing cells?
Polytene chromosomes form in cells that lack cell division after DNA replication, causing chromatids to accumulate without segregating. Unlike mitotically dividing cells, polytene cells maintain an intact nuclear membrane and nucleolus throughout consecutive replication cycles. This results in thousands of aligned chromatids arranged side by side, creating the characteristic giant chromosome structure.
Q6: Where are polytene chromosomes found in organisms?
Polytene chromosomes are found in salivary glands, intestines, muscles, and other tissues of dipteran insects like Drosophila larvae. They also occur in certain organs of mammals and in synergids and antipodes of flowering plants. These chromosomes were first discovered in 1881 in salivary glands of Chironomus plumosus larvae, earning them the alternate name salivary gland chromosomes.
Q7: How do chromomere patterns vary in polytene chromosomes?
Chromomere patterns are species-specific, representing multiple compact dark bands formed by identical chromatin compaction in aligned homologous chromosomes. The number and size of chromomeres can change during an organism's lifetime as developmental stages progress. These variable patterns reflect dynamic changes in chromatin organization and gene activity across different developmental periods.