A chromosome is a physical structure that carries all of our genes. They are long, thin structures of protein and DNA, wrapped around a core of histones. Each chromosome is a double-stranded molecule with two long strands of DNA wrapped around a core of histones. The genetic material of an organism is carried within the chromosomes. The information is arranged in a specific order. 

Polytene chromosomes are a form of eukaryotic chromosomes where long fibres run in parallel from either the centromere or one of the ends of the chromosomes. The polytene chromosomes have unique features that make them visible under a microscope. They provide useful information about the DNA strands and the genes.

Polytene Chromosomes:

A polytene chromosome is a type of chromosome with numerous chromatids that are arranged in parallel; they are found in the salivary glands of the larval stage of the mayfly. Polytene chromosomes are found in Drosophila, the genus of fruit flies. Polytene chromosomes are considered to be the largest, most complex, and most interesting kind of chromosome.

Check out the  Tips for UPSC Preparation

Discovered Polytene Chromosomes:

The polytene chromosome is a type of eukaryotic chromosome that can rapidly change in size, shape, and number. The polytene chromosomes were first observed in the mid-1800s by Italian scientists who used a thin, transparent insect. They were named after the fact that the cells of their insect hosts changed shape and size during development.

Importance of Polytene Chromosomes:

The polytene chromosomes are the most important cellular components. This is because they determine the appearance of the cell nucleus and are the basis for the organisation of the cell, from the chromatin to the chromosomes to the nucleoli to the nucleosomes. They are the key to the normal development and functioning of the cells. In addition, they are the basis for determining the gender of the cell.

Read about UPSC syllabus pdf download

Features of Polytene Chromosomes:

• A polytene chromosome is a large and complex chromosome found in the nuclei of cells of the insect fat body.

• The polytene chromosomes in the salivary gland nuclei of larvae of dipteran flies are the sites of active gene regulation and replication which involves many kinds of RNA molecules and polypeptides.

• Polytene chromosomes are those which are found in the salivary glands of Drosophila. They are produced by the activity of specific Polymerase enzymes and it is these enzymes that are used to track the fate of any specific section of DNA.

• The chromosomes are seen as moving, like worms, through space as they divide.

• They are large, banded, and have a characteristic shape. The bands are arranged in a highly repetitive pattern that is generally conserved among species.

• The shape and arrangement of these chromosomes can provide important clues for the analysis of chromosomes in other cell types.

• The main features of the polytene chromosomes are their long and wavy shape, and their ability to expand and contract to a varying degree, thus allowing for the production of a large gamete that can undergo meiotic division several times. 

Composition of Polytene Chromosome:

A polytene chromosome is an X-shaped chromosome that is found in Drosophila melanogaster. These chromosomes are composed of euchromatin and heterochromatin. Euchromatin is the gene-rich portion of the chromosome and is responsible for the colour and shape of the chromosome. Heterochromatin is the gene-poor portion of the chromosome and is responsible for the colourless, wavy lines that run through the chromosome.

Polytene chromosomes have a repeated pattern of bands in the middle and are located in the cell nucleus and help to organise and regulate the DNA. The polytene chromosomes are made of layers that represent euchromatin and heterochromatin. The number of chromosomes that each layer represents is dependent on the stage of the polytene development of the tissue.

Also see UPSC question paper 2022

Polytene Chromosomes Structure:

The polytene appears as a parallel line of bands. These bands are called “puffs”; each puff represents a discrete site of transcription. The structure of Polytene Chromosomes is a form of mitosis in the cell cycle. Polytene chromosomes consist of a large number of short, homologous, tandemly arranged bands. There is evidence that polytene chromosomes are nonrandomly arranged in the cell. Therefore, there is a strong requirement for the cell to maintain the order of the bands via a mechanism that is distinct from kinetochore-microtubule attachment.

Formation of Polytene Chromosomes:

Polytene chromosomes are formed by the simultaneous transcription of more than one allele of a single gene, the result of which is the formation of a polytene chromosome that contains different gene products expressed at different times during development. The polytene chromosome is formed by the simultaneous transcription of different alleles of a single gene.

Conclusion:

The polytene chromosomes are located on the salivary gland chromosomes and consist of a series of individual segments or blocks that move in a characteristic banded pattern when the chromosomes are undergoing the process of active transcription. The specific pattern of movement and banding of the polytene chromosomes is diagnostic for the type of mutation in the DNA, and the affected genes.