Q.35 Specialized chromosomes associated with larvae of some flies are called:
I. Lampbrush chromosome
2. Metacentric chromosome
3. Polytene chromosome
4. Somatic Chromosome
Specialized chromosomes associated with larvae of some flies are called polytene chromosomes.
Option Analysis
1. Lampbrush chromosome: These are giant chromosomes seen in oocytes of vertebrates like amphibians and birds during meiosis, with lateral loops for high transcription of RNAs like yolk proteins. Not specific to fly larvae.
2. Metacentric chromosome: A chromosome type with the centromere at the center, resulting in equal chromosome arms. This is a structural classification, not specialized or larva-specific.
3. Polytene chromosome: Correct answer—these form in salivary glands and other tissues of fly larvae (e.g., Drosophila) via repeated DNA replication without cell division (endoreduplication), creating giant, banded structures up to 1000+ DNA strands for massive gene expression.
4. Somatic chromosome: General term for chromosomes in body (non-reproductive) cells, not specialized or unique to fly larvae.
Introduction
Specialized chromosomes associated with larvae of some flies, particularly polytene chromosomes, are oversized structures in tissues like salivary glands of Drosophila larvae. Formed by endoreduplication, they enable high gene expression for larval growth and are key in cytogenetics studies.
Formation of Polytene Chromosomes
These arise from multiple DNA replications without mitosis or cytokinesis, aligning ~1000-2000 chromatids into cable-like chromosomes with distinct bands (condensed, gene-poor) and interbands (decondensed, active). Puffs (Balbiani rings) mark transcription sites.
Key Features and Functions
-
Banding Pattern: ~5000 bands visible under microscope, aiding gene mapping.
-
High Expression: Supports rapid synthesis of proteins like salivary glue before pupation.
-
Dynamic Puffs: Expand at active loci, correlating with developmental stages or hormones like ecdysone.
Comparison of Chromosome Types
| Type | Location | Key Feature | Fly Larvae Link |
|---|---|---|---|
| Lampbrush | Vertebrate oocytes | Lateral loops for RNA synthesis | None |
| Metacentric | Any cell | Centromere-centered arms | Structural only |
| Polytene | Fly larvae glands | Giant, banded, multi-stranded | Yes, specialized |
| Somatic | Body cells | Standard diploid set | General term |
Research Importance
Polytene chromosomes revolutionized gene regulation studies, revealing chromatin puffing and serving as models for eukaryotic genome organization in Diptera larvae.
