Model Answer
0 min readIntroduction
Both polyteny and polyploidy represent deviations from the typical diploid chromosome number found in most organisms. However, they arise through fundamentally different processes. Polyploidy involves the multiplication of the entire genome, resulting in cells with three or more complete sets of chromosomes. Polyteny, on the other hand, involves repeated rounds of DNA replication *without* cell division, leading to the formation of giant chromosomes with many copies of the genome arranged side-by-side. Understanding these differences is crucial for comprehending chromosomal organization and its impact on gene expression and organismal characteristics.
Polyteny: A Detailed Explanation
Polyteny is a specific type of chromosomal organization observed in certain tissues of insects, particularly Diptera (flies) like Drosophila, and some plants. It arises due to repeated rounds of DNA replication during interphase, without subsequent cell division (endomitosis). This results in the formation of giant chromosomes, called polytene chromosomes, which are visible even during prophase. These chromosomes appear as thick, banded structures due to the alignment of numerous identical DNA strands.
- Mechanism: Endoreduplication – repeated DNA replication without cell division.
- Chromosome Number: The number of chromosome sets remains the same, but the amount of DNA per chromosome increases. For example, a diploid organism might have 2n = 8 chromosomes, but in polytene cells, each chromosome exists as hundreds or thousands of identical chromatids.
- Banding Pattern: The characteristic banding pattern observed in polytene chromosomes reflects the differential condensation of chromatin along the chromosome length. These bands are often used for cytogenetic mapping.
- Examples: Salivary glands of Drosophila melanogaster are a classic example. Polyteny is also found in the midgut and Malpighian tubules of insects.
Polyploidy: A Detailed Explanation
Polyploidy refers to the condition where an organism possesses more than two complete sets of chromosomes. This can arise through errors during meiosis (gamete formation) or through fertilization of gametes with an abnormal chromosome number. Polyploidy is common in plants and plays a significant role in plant evolution and speciation.
- Mechanism: Usually arises from non-disjunction during meiosis, resulting in gametes with an abnormal number of chromosomes. Autopolyploidy arises from duplication of chromosomes within a single species, while allopolyploidy arises from hybridization between two different species followed by chromosome doubling.
- Chromosome Number: The number of chromosome sets increases (e.g., triploid – 3n, tetraploid – 4n, etc.).
- Effects: Polyploidy can lead to increased cell size, altered gene expression, and often, increased vigor and adaptability. It can also lead to reproductive isolation and the formation of new species.
- Examples: Wheat (Triticum aestivum) is hexaploid (6n = 42). Many cultivated fruits, such as bananas and strawberries, are also polyploid.
Polyteny vs. Polyploidy: A Comparative Table
| Feature | Polyteny | Polyploidy |
|---|---|---|
| Mechanism | Repeated DNA replication without cell division (endomitosis) | Duplication of the entire genome (whole-genome duplication) |
| Chromosome Sets | Number of sets remains the same; DNA content per chromosome increases | Number of sets increases (3n, 4n, etc.) |
| Cell Division | No cell division occurs during the process | Typically arises during gamete formation or fertilization |
| Chromosome Appearance | Giant chromosomes with many aligned chromatids (banding pattern) | Chromosomes appear normal in number, but are present in multiple copies |
| Common Occurrence | Specific tissues in insects (e.g., salivary glands of Drosophila) and some plants | Common in plants; less frequent in animals |
Conclusion
In essence, polyteny and polyploidy are distinct phenomena that both result in increased genetic material within cells. Polyteny involves amplification of DNA within individual chromosomes, while polyploidy involves duplication of entire chromosome sets. While both can influence cell size and gene expression, their underlying mechanisms and evolutionary consequences differ significantly. Understanding these differences is vital for comprehending the diversity of chromosomal organization and its role in biological processes.
Answer Length
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