​Polytene Chromosome​: Diagram, Structure & Functions

Polytene Chromosomes are specialized structures found in the nuclei of eukaryotic cells, characterized by their different banding pattern, and play a crucial role in understanding chromosomal organization, and cellular function. The unique structure of polytene chromosomes sets them different from the typical chromosomes found in most cells. They are also known as silvery gland chromosomes and unlike the typical chromosomes, polytene chromosomes undergo a process known as endoreplication, where DNA replication occurs without further cell division.

What is ​the Polytene Chromosome​?

Polytene chromosomes are specialized structures found in the nuclei of certain eukaryotic cells that show a unique banding pattern. Unlike typical chromosomes, polytene chromosomes result from a specific cellular process called endoreplication or endomitosis. The term “polytene” refers to the several threads or chromatids that form these chromosomes, and they are observed in certain tissues and organs of organisms like fruit flies and various plant species. Polytene chromosomes have features, such as alternating dark and light bands, which represent regions of condensed and decondensed chromatin. The banding pattern is a result of the unique structure of polytene chromosomes.

Polytene Chromosome Diagram

Polytene chromosomes are large chromosomes that look like a parallel line of bands. Each of these bands, referred to as “puffs”, represents a transcription site. Under a light microscope, the chromosomes’ alternating dark and light interbands are visible. Whereas the interbands have more RNA and less DNA, the dark bands have more DNA and less RNA.

How Polytene Chromosome​​ Formed?

Polytene chromosomes are formed through a specialized cellular process known as endoreplication or endomitosis. This process involves DNA replication without further cell division, accumulating multiple chromatids within the same nucleus. The formation of polytene chromosomes can be summarized in the following steps:

1. Initiation of Endoreplication

The process begins with the initiation of endoreplication, where a cell undergoes DNA replication without entering the usual stages of the cell cycle, such as mitosis.

2. Multiple Rounds of DNA Replication

Unlike regular cell cycles where DNA replication occurs once before cell division, endoreplication involves multiple rounds of DNA replication. This results in the formation of numerous identical DNA strands.

3. Accumulation of Chromatid

As each round of DNA replication progresses, the newly formed DNA strands, or chromatids, remain closely associated with the nucleus. The lack of cell division allows these chromatids to accumulate, forming the polytene structure.

4. Banding Pattern Formation

The closely aligned chromatids create a banding pattern within the polytene chromosomes. This pattern is a result of alternating regions of condensed chromatin (dark bands) and decondensed chromatin (light bands).

5. Specialization in Certain Tissues

Polytene chromosomes are often found in specific tissues and organs of organisms. For example, in Drosophila melanogaster (fruit fly), polytene chromosomes are observed in the salivary glands. The presence of polytene chromosomes in specific tissues helps in understanding the unique roles they play.

Structure of Polytene Chromosomes

Polytene chromosomes have a structure that sets them apart from regular chromosomes. The unique structure including their giant size, banding pattern, chromomeres, and endoreplication, help in performing genetic research.

• Giant Size: Polytene chromosomes are larger than regular chromosomes. Their giant size results from the accumulation of multiple chromatids within a single nucleus due to the process of endoreplication.
• Banding Pattern: One of the most distinctive features of polytene chromosomes is a characteristic banding pattern. The banding pattern consists of alternating dark and light bands, each representing different levels of chromatin condensation.
• Chromomeres: The dark bands in the banding pattern are called chromomeres. Chromomeres represent regions of tightly packed and condensed chromatin. These regions typically correspond to areas of low activity.
• Interbands: The light bands in the banding pattern are referred to as interbands. Interbands represent regions of decondensed and active chromatin. These areas are involved in cellular processes.
• Puffs: Certain regions of polytene chromosomes show swellings or puffs. Puffs are areas where the chromatin is decondensed. Puffs are associated with regions of active gene expression and they are visible as expanded portions of the chromosome.
• Replication Bridges: During DNA replication in polytene chromosomes, multiple replication forks are initiated, resulting in the synthesis of many copies of DNA strands. The synthesized DNA strands remain associated, forming structures known as replication bridges.

Functions of Polytene Chromosomes

The functions of Polytene Chromosomes are as follows:

• Gene Development: Polytene chromosomes undergo endoreplication, leading to the development of specific gene sequences. This development allows for the increased production of proteins, meeting the high metabolic demands of certain cells.
• Regulation of Gene Expression: The banding patterns on polytene chromosomes are associated with specific genes and help in the regulation of gene expression. The chromosomes show swellings called puffs, representing active transcription regions.
• Chromosomal Puffing: Puffs on polytene chromosomes indicate regions of active transcription and increased gene activity. The study of chromosomal puffing has contributed to the dynamics of transcription.
• Storage of Nutrients: In some cases, polytene chromosomes may function as a storage mechanism for essential nutrients needed during specific developmental stages or environmental conditions.

Importance of ​Polytene Chromosome​s

​Polytene Chromosome​ are important in the field of genetics and molecular biology due to their unique characteristics and help in understanding various aspects of genetic organization and cell regulation. Here are some key reasons highlighting the importance of polytene chromosomes:

• Gene Mapping: The visible banding pattern allows researchers to locate and identify specific genes along the chromosome precisely. This has proven valuable in understanding the arrangement of genes and their relative positions.
• Chromosomal Organization: The banding pattern of polytene chromosomes provides a detailed view of chromosomal organization, distinguishing between condensed regions (chromomeres) and decondensed regions. This helps in understanding the structure of chromosomes.
• Cellular Processes: The presence of dynamic regions known as puffs in polytene chromosomes indicates sites of active transcription. These puffs help understand cellular processes and the responsiveness of the genome to external factors.
• Understanding Endoreplication: The process of endoreplication leads to the formation of polytene chromosomes. This unique cellular mechanism helps in understanding cell cycle regulation and how cells can undergo DNA replication without division.

Conclusion

The unique structure of polytene chromosomes is different from the typical chromosomes. These oversized structures provide a unique glimpse into the organization and regulation of genetic information within the nucleus of certain cells. This unique banding pattern has proven to be instrumental in genetic studies, allowing researchers to map genes and understand the organization of genetic material within the chromosome. The banding pattern also aids in understanding gene expression, as the dark bands (chromomeres) indicate tightly packed DNA, while the light bands signify less densely packed, transcriptionally active regions. Their unique attributes contribute to a deeper understanding of genetic organization and regulation in specific tissues and developmental stages.

FAQs on ​Polytene Chromosome​

Why do ​Polytene Chromosome​ puff?

​Polytene Chromosome​ puff as a result of localized de-condensation of chromatin as puffing allows for a response to developmental changes or environmental changes in the regulation of gene expression.

What is the Cytological Significance of ​Polytene Chromosome​?

​Polytene Chromosome​ have cytological significance as they help in understanding the chromosomal organization, gene mapping, and deeper understanding of the spatial arrangement of genes.

What are Polytene and Lampbrush Chromosomes?

​Polytene Chromosome​ are oversized structures formed through endoreplication, crucial for gene mapping. Lampbrush Chromosomes, found in the nuclei of oocytes, have a brush-like appearance due to extended chromatin loops, playing a key role in meiotic prophase.

Where are ​Polytene Chromosome​ found?

​Polytene Chromosome​s are commonly found in the nuclei of specific tissues and organs in various organisms. In Drosophila melanogaster(fruit fly), they are present in the salivary glands.

Do Humans have ​Polytene Chromosome​s?

No, humans do not have ​Polytene Chromosome​. ​Polytene Chromosome​ are mostly observed in certain insects, like Drosophila melanogaster(fruit fly), and some plant species.

Why are Polytene chromosomes Useful?

Polytene chromosomes are useful to isolate and determine the DNA sequence of particular genes.

How many Arms does a Polytene Chromosome have?

It is a large chromosome with two short and five long arms that radiate from a central region known as the chromocentre.