Polytene chromosomes.

Formation of Polytene Chromosomes

Polytene chromosomes are formed through a process called endomitosis, where DNA replication occurs repeatedly without subsequent cell division. This is particularly prominent in rapidly growing and metabolically active tissues. The replicated DNA strands remain tightly paired, leading to a dramatic increase in chromosome size. The number of DNA strands can vary depending on the tissue and developmental stage, sometimes reaching over 1000 copies.

Structure of Polytene Chromosomes

The structure of polytene chromosomes is characterized by distinct alternating dark bands and light interbands. These bands represent regions of condensed chromatin, while interbands represent regions of decondensed chromatin. Several key structural features are observed:

• Bands: These are tightly packed regions of DNA, generally representing heterochromatic regions and often containing repetitive DNA sequences.
• Interbands: These are less condensed regions, typically containing euchromatic DNA and actively transcribed genes.
• Puffs: These are localized swellings of the chromosomes, representing regions of active gene transcription. They appear as balloon-like structures and are sites where the DNA is uncoiled, allowing access for RNA polymerase and other transcription factors. Puffs are dynamic and can change in size and number depending on the developmental stage and environmental conditions.
• Chromocenter: This is a darkly stained region at the base of the chromosome, representing the centromeric region and often containing highly repetitive DNA.
• Fibrils: Fine strands extending from the chromosome, representing the individual DNA strands.

Significance of Polytene Chromosomes

Polytene chromosomes have been instrumental in several areas of genetic research:

• Gene Mapping: The distinct banding patterns allow for precise mapping of genes to specific chromosomal locations. Researchers can correlate phenotypic traits with the presence or absence of bands, aiding in gene localization.
• Study of Gene Expression: Puffs indicate active genes, providing a visual representation of gene expression patterns. Changes in puff size and number can be correlated with developmental stages or environmental stimuli.
• Detection of Chromosomal Aberrations: Deletions, duplications, inversions, and translocations can be easily visualized as alterations in the banding patterns.
• Understanding Chromatin Organization: Polytene chromosomes provide insights into the organization of chromatin and the relationship between chromosome structure and gene function.

Organisms Exhibiting Polytene Chromosomes

While most prominently observed in Diptera, polytene chromosomes are also found in:

• Salivary glands of *Drosophila melanogaster* (fruit fly): This is the classic example and most extensively studied system.
• Midgut cells of *Chironomus* (non-biting midge): Used extensively for studying environmental effects on gene expression.
• Some plant tissues: Polytene chromosomes have been reported in certain plant tissues, though they are less common and less well-studied than in insects.
• Protozoa: Certain protozoan species also exhibit polytene chromosomes.

The presence of polytene chromosomes is often linked to tissues with high metabolic activity and a need for rapid protein synthesis, such as salivary glands responsible for producing large amounts of saliva.