Life Cycle of Antheraea mylitta

Giant Chromosomes: Polytene and Lampbrush Chromosomes

 

Introduction


Chromosomes are the structural units of heredity, made up of DNA and proteins. While most chromosomes in eukaryotic cells are microscopic, certain specialized cells exhibit giant chromosomes that can be seen under a light microscope. These enlarged chromosomes are Polytene Chromosomes and Lampbrush Chromosomes — remarkable for their size, structure, and role in active gene expression.

1. Polytene Chromosomes (Salivary Gland Chromosomes)


Polytene chromosomes are giant chromosomes formed by repeated rounds of DNA replication (endomitosis) without cell division, resulting in the alignment of thousands of identical chromatids side by side.

Discovered by E.G. Balbiani (1881) in the salivary glands of Chironomus larvae, and later studied by Painter (1933) in Drosophila melanogaster. Found in tissues like salivary glands, Malpighian tubules, fat bodies, and midgut epithelium of dipteran larvae.

Structure

Polytene chromosomes display a distinct banding pattern visible under a light microscope. The dark regions (chromomeres) are transcriptionally inactive, while the light regions (interbands) are active. Puffed regions or Balbiani rings represent active sites of RNA transcription.





Functions and Significance


1.  Indicate gene activity through puff formation.

2.  Help in cytogenetic mapping of genes.

3.  Serve as a model for studying gene regulation and chromatin organization.

2. Lampbrush Chromosomes

Lampbrush chromosomes are giant meiotic chromosomes characterized by extended lateral loops that resemble the bristles of a lampbrush. They are highly transcriptionally active and occur during the diplotene stage of meiosis I in oocytes.

Discovered by Walther Flemming (1882) and detailed by J. Rückert (1892). Found in growing oocytes of amphibians (Triturus, Rana), reptiles, and birds.

Structure

Lampbrush chromosomes consist of a central chromosomal axis with lateral loops extending outward. The loops are regions of active RNA synthesis. Chromomeres on the axis serve as attachment points for these loops. 

Functions and Significance


1. Each loop represents a gene being transcribed into RNA.
2. Provides RNA required for oocyte and early embryonic development.
3. Model for studying chromatin structure and transcriptional regulation.

Comparison between Polytene and Lampbrush Chromosomes

Feature

Polytene Chromosome

Lampbrush Chromosome

Cell type

Somatic cells (salivary glands)

Meiotic cells (oocytes)

Formation process

Endomitosis

Chromatin uncoiling forming loops

Appearance

Banded, multi-stranded

Looped, brush-like

Transcriptional activity

Localized (puffs)

Widespread (loops)

Function

Gene mapping, regulation

RNA synthesis during oogenesis

Organism examples

Drosophila, Chironomus

Amphibians, birds, reptiles

Conclusion


Both polytene and lampbrush chromosomes are giant, non-dividing, and transcriptionally active structures. They offer valuable insights into gene expression, chromatin organization, and cell differentiation. Their study bridges cytology and molecular genetics.

References


1. Alberts, B. et al. (2022). Molecular Biology of the Cell. Garland Science.
2. Lodish, H. et al. (2021). Molecular Cell Biology. W.H. Freeman.
3. De Robertis, E.D.P. & De Robertis Jr. (2001). Cell and Molecular Biology.
4. Griffiths, A.J.F. et al. (2020). Introduction to Genetic Analysis.
5. Snustad, D.P. & Simmons, M.J. (2021). Principles of Genetics. Wiley.
6. www.zoologys.co.in

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Giant Chromosomes: Polytene and Lampbrush Chromosomes