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Polytene Chromosome Structure and Function

    In this polytene chromosome structure and function post we have briefly explained about Describe the structure of polytene chromosome with a suitable diagram? history, formation, polytene chromosome diagram, structure, functions and significance of polytene chromosome.

    Polytene Chromosome Definition

    Polytene Chromosome History

    Formation of Polytene Chromosome

    Polytene Chromosome Structure and Function

    Structure of Polytene Chromosome

    Function of Polytene Chromosome

    Significance of Polytene Chromosome

    Giant chromosomes are special, enormously enlarged chromosomes about 100 times thicker than the ordinary mitotic chromosomes. These are seen in certain tissues of varied groups of animals and plants. They are easily visible under light microscope. The giant chromosomes are of two types: polytene and lampbrush.

    Polytene Chromosome Definition

    A giant chromosome produced by an endomitotic process in which, following synapsis of the two homologues, multiple rounds of replication produce chromatids that remain synapsed together in a haploid number of chromosomes. Large chromosome consisting of many chromatids formed by rounds of endomitosis following synapsis of the two homologues.

    Polytene Chromosome History

    Polytene chromosomes were first observed by Balbiani (1881) in Chironomus (a dipteran larva). Because of their large size showing numerous strands these are named as polytene chromosomes by Kollar. 

    These polytene chromosomes are found in in the larval salivary glands, midgut epithelium, and rectum and Malpighian tubules of various genera of dipterans. These are also known as salivary gland chromosomes because they have been best studied in the salivary gland cells of fly larvae.

    Formation of Polytene Chromosome

    The giant chromosome Polytene arises due to the multiple or repeated replication of the chromosomal DNA without nuclear division or endomitosis. The nuclear envelope remains intact. It does not rupture and spindles are not formed. So, continuous replication (endoreplication) of DNA is going on without any division.

    As a result, the daughter chromatids do not separate and remain joined side by side, which give rise to polytene chromosome. This type of giant chromosomes is visible during the interphase and prophase stage of mitosis.

    So, it is formed due to repeated replication of homologous chromosomes in which the replicated or newly formed sister chromatids do not get separated.

    Furthermore, there could be around 1000 identical DNA molecules aligned laterally in the polytene chromosome so multiple arms can be seen as chromatids are not separated.

    Polytene Chromosome Structure and Function

    Structure of Polytene Chromosome

    These chromosomes are about 100-200 times larger than those of somatic chromosomes. They are roughly cylindrical and exhibit a distinct pattern of transverse striated structures consisting of alternate darkly staining band and light staining interbands (Polytene Chromosome Diagram:1). Dark bands are rich in DNA along with a small amount of RNA and basic proteins.

    They are genetically active. The inter-bands contain less of DNA but more acidic proteins and hence they are less active (Diagram of Polytene chromosome:1). The polytene chromosomes are formed by repeated replication of DNA without division of chromosome into daughter chromosomes.

    This amplification without separation is called polytenization. As a result, a thick bundle of parallel DNA molecules all having the same banding pattern across them is produced. Thus, there can be as many as several thousands of chromonemata in a giant chromosome.

    Polytene Chromosomes

    Polytene Chromosome Diagram

    During the initial stages of development the bands or inter-bands of chromosomes exhibit swellings or puffs (Diagram of Polytene chromosome:1). During development the puffs appear and disappear in definite patterns in response to the needs of developing larvae for the RNAs.

    The puffs are genetic sites active in RNA synthesis. In some regions of polytene chromosomes the chromonemata may give out a number of loops at certain places. Such loops are known as the Balbiani rings. These rings are formed by the lateral stretching of loops. They are rich in mRNA like the chromosomal puffs.

    Function of Polytene Chromosome

    1. Polytene chromosomes carry genes which ultimately control physiology of an organism. These genes are formed of DNA molecules. These chromosomes also help in protein synthesis indirectly. The RNA present in the nucleolus serves as a means of transmission of genetic information to the cytoplasm, leading to the formation of specific protein.

    Significance of Polytene Chromosome

    1. They increase the cell volume and DNA content so it has multiple copies of genes that allow a high level of gene expression. 

    2. For example, in Drosophila melanogaster, the polytene chromosomes of the larval salivary glands help produce a large amount of adhesive mucoprotein, a type of glue before pupation. 

    3. The bar phenotype of kidney-shaped eyes in this fly is also produced due to the polytene chromosomes. The interbands interact with the active chromatin proteins and serve as binding sites for RNA polymerase II to begin replication and remodelling of nucleosome or short fragments of DNA.

    The answer to the question of describe the structure of polytene chromosome, polytene chromosome structure and function has been found. You can learn more about chromosomes and types at NotesHippo.com