Polytene Chromosomes | Botany Optional for UPSC PDF Download

Polytene Chromosomes

Polytene chromosomes are exceptionally large chromosomes found in various tissues, notably in the salivary glands of insects. These chromosomes have a unique structure and serve important functions.

Structure

• In insects, especially in salivary glands, they are often referred to as "salivary gland chromosomes" due to their prevalence.
• Polytene chromosomes are characterized by their substantial size, which results from the presence of numerous longitudinal strands called chromonemata, hence the name "polytene" (many-stranded).
• They typically measure about 0.5 mm in length and have a diameter of approximately 20 μm.
• These chromosomal strands are formed through repeated chromosome division without corresponding cytoplasmic division, a process known as endomitosis.
• Polytene chromosomes contain two distinct types of bands: dark bands and interbands. Dark bands are deeply stained, while interbands are lightly stained with nuclear dyes.
• Dark bands contain a higher proportion of DNA and less RNA, whereas interbands have more RNA and less DNA, with DNA content in interbands ranging from 0.8% to 25%.
• Polytene chromosomes exhibit enlargements at certain points, forming swellings known as "puffs." This process is called "puffing."
• Puffs are characterized by the uncoiling of individual chromomeres within a band, and they mark the locations of active genes where messenger RNA (mRNA) synthesis occurs.
• Chromonemata in puffs uncoil and spread out, forming numerous lateral loops that resemble rings, often referred to as "Balbiani rings."
• These Balbiani rings consist of DNA, RNA, and a few proteins, and they are crucial sites for transcription, housing transcription-related machinery like RNA polymerase and ribonucleoproteins.
• In protozoans, where transcription doesn't take place, puffs primarily comprise DNA.

Function

• Polytene cells, with their multiple copies of genes, contribute to an elevated level of gene expression.
• In certain insects like Drosophila melanogaster, larval salivary gland chromosomes undergo multiple rounds of endoreduplication, producing substantial quantities of adhesive mucoprotein (commonly referred to as "glue") before pupation.
• In Drosophila, there are instances of tandem duplication of specific polytene bands near the X chromosome's centromere, resulting in the kidney-shaped eyes observed in the Bar phenotype.
• Interbands within polytene chromosomes play crucial roles in interacting with active chromatin proteins, nucleosome remodeling, and origin recognition complexes.
• They serve functions such as acting as binding sites for RNA polymerase II, initiating replication, and initiating nucleosome remodeling of short DNA fragments.