Demonstration of polytene and lampbrush chromosome from photograph Dr. Subhadra Roy Assistant Professor Department of Zoology Surendranath College

Polytene Chromosome

Polytene Chromosome

Polytene Chromosome

Characteristics They are giant chromosomes. The chromosomes show characteristic banding pattern representing chromatic and achromatic regions. Occasionally the bands form reversible puffs or Balbiani rings.

Comments Giant chromosomes were first observed by E. G. Balbiani in 1881 in salivary gland cells of Dipteran species. Polytene chromosomes are giant chromosomes found in the salivary glands, malpighian tubules, gut lining and adipose tissue cells of some Dipteran larvae. These chromosomes may reach a size upto 200 times (or more) the size of corresponding chromosomes at meiosis or in nuclei of normal mitotic cells. These giant chromosomes are formed by repeated cycles of endo- reduplication of single chromatids. This means that the chromatids replicates without cell division, as a result, the number of chromatids keeps increasing. This process is called Polyteny and therefore, these chromosomes are called polytene chromosomes. Polytene chromosomes show another characteristic feature i.e., somatic pairing. As a result of this, chromosomes always appear to be half of that in normal somatic cells.

Lampbrush Chromosome

Lampbrush Chromosome

Characteristics They are giant chromosomes. The chromosomes have a lampbrush-like appearance due to twin loops emerging from the chromomeres. Identical loops occur on both pairs of sister chromatids. Chromomeres have highly condensed chromatin.

Comments Lampbrush chromosomes were first seen in salamander oocytes by Fleming in 1882. The name lampbrush was given by Ruckert in 1892. The nomenclature comes from its similarity in appearance to the brushes used to clean lamp chimneys in old days. These chromosomes are transitory structures that exist during the extended Diplotene in oocytes of most invertebrates and vertebrates except mammals. The loops in these chromosomes are sites of RNA transcriptions. The condensed chromatin in chromomeres is generally not transcribed. The position of each loop is fixed. A given loop always contains the same DNA sequence. They have been widely used in studies of chromosome organization, gene expression, molecular morphology of RNA transcription, etc.

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