AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

SPECIAL TYPES OF CHROMOSOMES

Polytene chromosomes

• Polytene chromosomes are giant chromosomes common to many dipteran (two-winged) flies.
• These were first discovered by E. G. Balbiani in 1882 in Dipteran salivary glands and hence commonly called salivary gland chromosomes.
• They begin as normal chromosomes, but through repeated rounds of DNA replication without any cell division (called endoreplication), they become large, banded chromosomes (see figure).
• For unknown reasons, the centromeric regions of the chromosomes do not endoreplicate very well.
• As a result, the centromeres of all the chromosomes bundle together in a mass called the chromocenter.
• Polytene chromosomes are usually found in the 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.
• The polytene chromosomes at the right are from the salivary glands of the fruit fly Drosophila melanogaster.
• The bands on each chromosome are like a road map, unique to each chromosome and well defined enough to allow high resolution mapping of each chromosome.
• The Drosophila Genome Project uses polytene chromosomes as a framework for the map.

Lampbrush chromosomes

• It was first observed by W. Flemming in 1882 and was described in detail in oocytes of sharks by Rukert in 1892.
• It consists of an axis from which paired loops extend in opposite directions, giving the appearance of a lamp brush. Hence the name Lamp Brush Chromosomes.
• It is found in the Oocytes of amphibians and in some insects.
• They are formed during the active synthesis of mRNA molecules for the future use by the egg during cleavage when no synthesis of mRNA molecules is possible due to active involvement of chromosomes in the mitotic cell division.
• It is larger in size. Hence it is called a giant chromosome.

B-chromosomes

• B-Chromosomes (also called supernumerary chromosomes, accessory chromosomes, accessory fragments, etc.) are without obvious genetic function and usually have a normal structure, are somewhat smaller than the autosomes.

Holokinetic chromosomes

• The chromosomes with a non-localized centromere are called as either holocentric or holokinetic chromosomes.