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Lampbrush chromosome (Giant Chromosome)

The document discusses Lampbrush chromosomes, which were first observed in salamander oocytes in 1882. Lampbrush chromosomes occur during prolonged diplotene stage of meiosis in oocytes of many animal species. They have an elongated structure with loops projecting from the chromosome axis, and each loop performs intense transcription of hnRNA and mRNA. This transcription allows oocytes to build up RNA and proteins required for future embryonic development.

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Presented to- Dr. G KUMAR University of Allahabad Presented by- Salman Khan MSc-3rd Sem Topic-Lampbrush Chromosome
History:-  The Lampbrush chromosome were first observed in Salamander (amphibians) oocyte by FLEMING (1882).  Described by RUCKERT (1892) in shark oocyte.  He coined Lampbrush, because they appear as lamp cleaning brush.  BRUN and BOTTINO (1989) preferred the term “test tube brush chromosome”.  GALL and CALLAN interpreted structure in functional term.
Occurrence:-  The Lampbrush chromosome occur at the diplotene stage in meiosis prophase-1 in the primary oocyte in all animal species both vertebrate and invertebrates.  They have been described in Molluscans, insects, sharks, amphibians, reptiles, birds and also mammals.  They also occur in spermatocytes in several species.  Generally they are smaller in invertebrate then vertebrate.
Reason for Lampbrush chromosome structure:-  The Lampbrush chromosome are characterised by a remarkable change in structure.  They have been extensively studied in amphibian oocytes since, these chromosome are observed during prolonged diplotene stage  These are in the form of bivalents  Such a Lampbrush stage may persist for months or years as the primary oocyte build up a supply of mRNA molecule and and other require for the ultimate development into a new individual.  In oocytes of amphibians and other lower vertebrate’ the early stage of development occurs very rapidly so that the stages of independent locomotion and feeding is attained as soon as possible.
Structural organisation:-  Each RNA polymerase molecule is attached to nascent RNA and associated proteins generating a visible “brush like” appearance.  During diplotene stage the Lampbrush chromosome are held in a stretched outform which can be visualized in a light microscope.  The axis of Lampbrush chromosome from which the loops project, consist visually of linear arrays of compacted beads series of ellipsoidal chromosome.  They are meiotic bivalents and each bivalent consist of two homologous chromosome held together by chiasmata.
 The length of the individual Lampbrush chromosome may sometime become even larger than giant salivary gland chromosomes.  The largest length is 1mm observed in few amphibians.  The total length of the entire Lampbrush chromosome is 5-6mm and is organized into 5000 chromosomes. Conti….
Loops:-  The loops are always symmetrical each chromosomes having two of them one for each chromatid.  The centromere of the chromosome bears no loops.  About 10000 loops per chromosome set or haploid set.  The size of loops varies with an average of 4-5 micron in inter chromomeric fibres.  About 5-10% of DNA exists in the lateral loops the rest being tightly condensate in the chromomeres which are transcriptionally inactive.  The size of loop increases with the genome size.  Each loop has an axis made up of single DNA molecule that is unfolded form the chromosome during RNA synthesis.
Transcription in loop:-  Electron microscope of the loops has shown that RNA polymerase molecule are attached to the principle axis (DNA) of the loop from which RNA fibrils of increasing length extends.  Each loop of Lampbrush chromosome is found to perform intense transcripts with hnRNA that is precursor of mRNA molecules for various ribosomal protein or cytoplasmic ribosomes histone protein.  Each lateral loop is covered by a matrix that consists of RNA transcript with hnRNA binding protein attached to them.  RNA synthesis starts at the thinner end and progresses towards the thicker end.  Generally then matrix is asymmetrical being synthesis starts at one end of the loop than at the other.
 As transcription continues the DNA strand loop the fibril of RNA that is hnRNA lengthens.  Protein gets associated with these RNA fibrils as they are formed and ultimately ribonucleoprotein product is released.  Actin filaments may be involved in extending the Lampbrush chromosome loop away from the chromomeric axis.  As meiosis proceeds further the number of loops gradually decreases and loops ultimately disappear due to reabsorption back into the chromosomes.  The loop can be categorized by size thickness and other morphological characteristics Conti……
Biological significance:-  Loops are used in chromosome mapping especially each loop which appears at a constant point in the chromosome.  Lampbrush chromosome would be extremely useful to visualized gene expression in its natural state and enables to observe changes are associated with transcription.  Lampbrush chromosome provide evidence for eukaryotic gene amplification which is required during the growth phase of oocytes.  They are very good material for hybridization analysis.