Special Chromosomes- Lampbrush, Polytene, B-Chromosome
There are some chromosomes which are very different in their morphology. These chromosomes are known as special types of chromosomes. There are three types of special chromosomes:
- Polytene Chromosome
- Lampbrush Chromosome
- The hereditary chromosomes that occur inside the nucleus and play phenotypic role are called A-chromosome.
- All the other chromosomes which are genetically unnecessary are called B-Chromosome.
- B-chromosomes are also called supernumerary chromosome or accessory chromosome.
Properties of B-Chromosome
- They are dispensable that is; it is not necessary that B-chromosome may occur in all organisms or in all the constituent cell of an organism.
- They are genetically unnecessary.
- They do not show Mendalian inheritance.
- B-chromosomes are predominantly heterochromatic.
- They adversely affect the fertility and vigour of the carrier organism because they impair the meiotic pairing of homologous chromosome.
- They are remnants of inter-specific hybridization produced due to non-dysjunction of chromosomes.
B-chromosomes in occurs in Bryophytes, Gymnosperm, Angiosperm (but most of the species of family Poaceae contains B-chromosome) such as Avena, Agrastis, Deschampsia.
Polytene chromosome was first discovered by E. P. Balbiani in Dipterian insect Chironomous.
Characteristics of Polytene Chromosome
- It consists of cable of DNA fibres running parallel to each other.
- It is produced due to endo-reduplication of DNA or chromosomes without separation and cytokinesis.
- Somatic pairing– The polytene chromosomes contains homologous pair of maternal and paternal chromosomes occurring in somatic cell.
- It consists of bands and interbands. The darkly stained region is called bands and the lightly stained region between two bands is called interband.
- Balbiani puff- Polytene chromosome contains local swelling due to extrusion of DNA fibres and these extruded DNA fibres are transcriptionally active and they transcribe into nascent RNA molecules needed for sythesising proteins and enzymes in the carrier cells.
- They are inter-phasic visible chromosomes.
- Recently polytene chromosomes have been reported in plant
- They generally occur in secretory cells.
- The centromere of all the polytene chromosomes forms a mega centromeric region called chromo-centre.
They are called lampbrush chromosome because they resemble the brush used to clean the chimney of kerosene lamp.
It was discovered by Flemming (1882) from the oocyte of Xenopus but the detailed structure was provided by J. Ruckert.
- It is meiotic bivalent occurs during the diplotene stage of meiosis of cell division.
- Each lampbrush chromosome consist of maternal and paternal bivalent held together at chiasmata.
- Each chromosome of bivalent consist of two sister chromatids that forms loops opposite to each other.
- The loops are the active site of transcription that occurs in the dividing cells.
- Lampbrush chromosome occurs in yolk rich oocyte of amphibians.
- It has been reported from the giant nucleus of Acetabularia.
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