for BSc II students

Giant chromosomes

Some cells at particular stages contain large nucleus with giant or large sized chromosomes. The giant chromosomes are Polytene and lamp brush chromosomes.

Polytene chromosome

It occurs in the tissues of salivary gland, gut, trachea, fat body cells and Malpighian tubules of many insects of the order Diptera. It was first reported by E G Balbiani in 1881. The name polytene was suggested by Kollar due the occurrence of many chromonemata.

The polytene chromosomes are larger in size. The polytene chromosome of Drosophila melanogaster has total length of 200 microns in comparison to the 7.5 microns of somatic chromosome. The enormous size of the polytene chromosome is achieved by the duplication of chromonema for nine or ten times and duplicated chromonemata are not separated.

The polytene chromosome consists of closely coiled or associated homologus pairs of chromosomes. This association is called somatic pairing.

The polytene chromosome contains about 1000 times more DNA material than the somatic chromosomes.

Dark and light bands

Polytene chromosome bears alternating dark and light bands. The dark bands take deep stain with basic chromosomal stain and contains large amount of DNA and small amount of RNA. These are mainly composed of euchromatin. The light bands take light chromosomal stain and contains small amount of DNA and large amount of RNA. They are mainly composed of heterochromatin.

The polytene chromosome is caused by the process of endomitosis. In endomitosis, chromonemata become duplicated and resulted chromonemata do not separate to form new chromosome but remain closely associated. Also in endomitosis, nuclear membrane does not rupture and no spindle fibre formation takes place.

During development stages, polytene chromosome of larva of Dipterous insect, develop swelling at particular point of dark and light band. Such swelling is called puffs or bulbs. The puffing process involves accumulation of acidic protein, synthesis of RNA and storage of synthesized mRNA.

The chromonema of polytene chromosome gives out series of loops laterally. These rings are known as Balbiani rings. They are rich in DNA and mRNA. The formation of Balbiani rings are similar to puffs.

Lampbrush chromosome

The yolk rich oocytes of many vertebrates such as fishes, amphibians, reptiles and birds, contain exceptionally large sized chromosomes known as lamp brush chromosomes. They were discovered by Ruckert.

The lamp brush chromosomes are much longer. The length of lamp brush chromosome in Urodele oocyte may reach up to 5900 microns.

The lamp bush chromosomes are composed of the main axis and lateral loops. The main axis is composed of 4 chromatids or two bivalent chromosomes. The chromonemata of these chromaitd give out fine loops at lateral sides giving appearance of lamp brush or test tube brush.

The loops of lamp brush chromosomes are rich in RNA and protein. The RNA appears to be mRNA. The synthesis of protein and yolk take place near the loop.

Cytogenetical functions of chromosomes

Chromosomes are the most significant component of cell. They control most of the biological and genetical activities of cell. They contain genetical material, DNA which ultimately influences the biological phenomena atg molecular, physiological and gross morphological level.