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Lampbrush chromosomes. Coloured transmission electron micrograph (TEM) of lampbrush chromosomes (LBCs). Magnification: x300 when printed at 10 centimetres wide.

Variation in Chromosome Types: The Lampbrush Chromosome

If we observe the nucleus of each cell, we’ll find that the DNA molecule in it is packaged into thread-like structures. These are called chromosomes that carry hereditary information for everything, i.e., height to eye color. These chromosomes are made up of DNA that are tightly coiled many times around proteins called histones that support the structure of chromosomes.

Normally, chromosomes are not visible in the cell nucleus, not even under a microscope if the cell is not dividing. But during cell division, the DNA becomes more tightly packed and then the chromosomes become visible under microscope.

Normal chromosomes are usually seen during mitotic & meiotic cell divisions. However, in specialized tissues or in certain species or species groups, non-standared or unusual chromosomes exist. These chromosomes serve as valuable tools for cytogenetic research and distinguish themselves by their unusual size. Lampbrush chromosome is one of them.

What is lampbrush chromosome?

Lampbrush chromosomes are one type of giant chromosomes that occur in the diplotene stage of primary oocyte nuclei in vertebrates & invertebrates and also in the Y chromosomes of Drosophila spermatocytes. Lampbrush chromosomes can even be larger than the polytene giant chromosomes of Diptera.

Discovery

  • Flemming (1882)- He found a large chromosome in egg mother cell of amphibian animal.
  • Ruckett (1882)- He named this chromosome as Lampbrush Chromosomes, because this chromosome looks like the lamp brush or test-tube brush.
Figure: Smilarities between lampbrush chromosome (left) and a typical lamp/test-tube brush (right)
  • Beermann (1952)- He first realized that the lampbrush chromosome loops are similar in function to the puffs of the polytene chromosomes.
  • Gall & Callan (1962)- They applied the most improved techniques in studying lampbrush chromosomes.

Distribution

  • These chromosomes have been most commonly observed in the nuclei of certain animal oocytes/egg mother cells during the prophase of meiosis.
  • They are in the diplotene stage of meiosis, and may remain in this stage for a year or more while the egg is maturing.
  • They are found in fish, birds, reptiles and amphibians.

Nature  

  • These chromosomes are characterized by a typical diplotene bivalent appearance showing chiasmata.
  • From the darkly stained chromosomes, thousands of thin chromatin loops are extending at rectangles.

Size

Figure: Lampbrush chromosome under electron microscope
  • Lampbrush chromosomes is the largest among all type of chromosomes.
  • 600 to 800 µm in length.
  • It can be even 1000 µm (=1 mm) in Triturus spp. and Rana spp.

Structure

The following part describes the structure of lampbrush chromosome in two headings: Physical and Chemical structure.

Physical Structure

Figure: Diagrammatic representation of the lampbrush chromosomes of the Triturus, a bivalent with two chiasmata and with the paired arrangement of the loops extending out from each homologue.

Figure:Diagrammatic representation of the lampbrush chromosomes of the Triturus, a bivalent with two chiasmata and with the paired arrangement of the loops extending out from each homologue.

Chemical Structure

Herbert MacGregor and Harold Garnet Callan (1962) published a research article on lampbrush chromosome in the Journal of Microscopical Science.

They worked at The University of St. Andrews.

Chromosome Enzyme Function
Protease
  • No effect
Lampbrush chromosome RNase
  • Matrix destroyed
  • Loop axis and main axis of chromosome stable
DNase
  • Loop axis destroyed very quickly
  • Then main axis dissolved
  • Then chromosome dissolved

 

  •   Lampbrush chromosome is not composed of proteins.
  •   Matrix have many ribonucleo-proteins.
  •   Loop axis and main axis composed of DNA.

Ultra-structure of Lampbrush chromosome

  1. Callan and Loyed by using phase-contrast microscopy, they strongly proved that lampbrush chromosome is four stranded.
  2. Lampbrush chromosome consists of longitudinal axis formed by a single DNA molecule along which hundreds of bead like chromomeres are distributed.
  3. Two symmetrical lateral loops (one for each chromatid) emerge from each chromomere.
  4. About 5 to 10% of the DNA is in the lateral loops. The loops consist of uncompacted  DNA and proteins, but have a good amount of RNA and they are transcriptionally active.
  5. The DNA of loops are the sites of intensive RNA synthesis.
  6. The axis having compacted DNA is transcriptionally inactive.
  7. Loop is 90-200 Å in diameter. The size of loop and chromomere are inversely proportional.

Function

  • These chromosomes are involved in the synthesis of RNA and proteins by their loops.
  • Lampbrush chromosomes probably help in the formation of certain amount of yolk materials for the egg.

Differences between lampbrush & polytene chromosomes

Lampbrush Chromosomes Polytene Chromosomes
Replication Not at all Many times
Diameter Large Less
Type Meiotic Mitotic
Nature Vertically expanded Transversely divided

Similarity between lampbrush and polytene chromosome

  1. Polytene and lampbrush chromosomes both are bivalent.
  2. The strucure and function of puff in polytene chromosome and loop in lampbrush chromosomes are more or less same.
  3. The main objective of both the chromosomes is to increase metabolic activities without increasing the number of chromosomes. Polytene chromosome does it by transverse division whereas lampbrush chromosome perform it by expanding loops vertically.
  4. The extra metabolism takes place within the puff in polytene chromosomes and loops in lampbrush chromosomes.

Reference

  • Class lecture of Chandan Kumar Dash, Department of Botany, University of Dhaka.
  • Cytogenetics (Plants, Animals, Humans) by Jurgen Schulz-Schaeffer.

 

 

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About Khaleda Akter Shompa

Khaleda Akter Shompa
Currently studying in 2nd year at Department of Botany, University of Dhaka. Wants to learn multiple things for the development of skill. A good listener also.

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Abida Latif Tamanna
Abida Latif Tamanna
2 months ago

Great organisation and quite an informative article. Thank you.

Khaleda Akter Shompa
Khaleda Akter Shompa
Reply to  Abida Latif Tamanna
2 months ago

Thank you so much for your appreciation & support.

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