Giant chromosomes (or polytene chromosomes) are found in the cells of larvae of flies and mosquitoes, in springtails and in some other invertebrates. In mosquitoes we find them, for example, in the cells of the bug Chironomus and in the cells of flies like Drosophila (fruit fly). The growth of these larvae occurs because the cells grow in size, not in number. The chromosomes also grow in size because the DNA multiplies many times without the cell or nucleus dividing. A giant chromosome therefore consists of hundreds to thousands of undivided chromatides. The chromatids have a sequence of light and dark bands, the so-called chromomers. Due to the dimensions of the giant chromosomes, these bands are very noticeable. On the basis of these bands it is easy to indicate where the genes are located on the giant chromosome (called ‘gene mapping’)
On the image, swellings, so called puffs, can be seen at particular sites along the length of the polytene chromosome. These are diffuse uncoiled regions where transcription is actively taking place.
Polytene = composed of many chromatides
Chromatides = the two identical parts of a chromosome
Gene = a piece of DNA containing certain hereditary characteristics© www.willemsmicroscope.com
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