Special types of chromosome

 Some of the chromosomes in some special tissues are found to be different from the normal chromosomes, they are called special type chromosomes.

Polytene or giant chromosome

Polytene chromosomes are giant chromosomes common to many dipteran (two-winged) flies. These chromosomes were first observed by E. G. Balbiani (1881) in salivary glands of dipteran species. They begin as normal chromosomes, but through repeated rounds of DNA replication without any cell division (called endoreplication), the number of chromonemata keeps on increasing leading to large, banded chromosomes. Along the linear axis, polytene chromosomes have variations in the concentration of the chromatin. Regions of high concentrations are known as chromomeres (bands), and regions with low concentrations are known as inter-chromomeres (inter-bands). For unknown reasons, the centromeric regions of the chromosomes do not endoreplication very well. As a result, the centromeres of all the chromosomes bundle together in a mass called the chromocenter. These chromosomes may reach a size up to 200 times (or more) the size of corresponding chromosomes at meiosis or in nuclei of ordinary mitotic cells.

This is why these chromosomes are also popularly known as polytene chromosomes and the condition is described as polyteny. The number of chromonemata (fibrils) per chromosome may reach up to 2000 in extreme cases.

Another characteristic of these giant chromosomes is that they are somatically paired. Each parental chromosome is tightly paired with its homolog (somatic synapsis). There are regions where two homologous chromosomes are separated (asynapsis). All the chromosomes are linked together by the pericentromeric regions to create a single chromocenter. In Drosophila melanogaster, the giant chromosomes are found in the form of five long and one short strand radiating from a single more or less amorphous mass known as chromocenter. One long strand corresponds to the X chromosome and the remaining four long strands are the arms of II and III chromosomes. The centromeres of all these chromosomes fuse to form the chromocenter.

In the male flies, the Y chromosome is also found fused within the chromocenter and is therefore not seen as a separate strand.


Another characteristic of these giant chromosomes is that they are somatically paired. Each parental chromosome is tightly paired with its homolog (somatic synapsis). There are regions where two homologous chromosomes are separated (asynapsis). All the chromosomes are linked together by the pericentromeric regions to create a single chromocenter. In Drosophila melanogaster, the giant chromosomes are found in the form of five long and one short strand radiating from a single more or less amorphous mass known as chromocenter. One long strand corresponds to the X chromosome and the remaining four long strands are the arms of II and III chromosomes. The centromeres of all these chromosomes fuse to form the chromocenter.

In the male flies, the Y chromosome is also found fused within the chromocenter and is therefore not seen as a separate strand.



The bands of polytene chromosomes become enlarged at certain times to form swellings called puffs. The formation of puffs is called puffing. The puffing is caused by the uncoiling of individual chromomeres in a band. The chromonemata of puffs give out a series of many loops laterally. As these loops appear as rings, they are called Balbiani rings after the name of the researcher who discovered them. They are the site for transcription and are formed of DNA, RNA, and a few proteins which are essential for transcription.

Polytene chromosomes are usually found in salivary glands of larvae, where it is believed these many-replicated chromosomes allow for much faster larval growth than if the cells remained diploid. Simply because each cell now has many copies of each gene, it can transcribe at a much higher rate than with only two copies in diploid cells. It should, however, be emphasized that these giant chromosomes have also been found in malpighian tubules, fat bodies, ovarian nurse cells, gut epithelia, and some other tissues.





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