Describe the Special types of Chromosomes.

Explanation

Special types of chromosomes :
1. Special types of chromosomes are found only in certain special tissues. These chromosomes are larger in size and are called giant chromosomes in certain plants and they are found in the suspensors of the embryo.
2. The polytene chromosome and lamp brush chromosome occur in animals and are also called as Giant chromosomes.
polytene chromosomes :
1. It is observed in the salivary glands of Drosophila (fruit fly ) by C.G Balbiani in 1881. in larvae of many flies, and some insects the interphase chromosomes duplicates and reduplicates without nuclear division.
2. A single chromosome which is present in multiple copies form a structure called polytene chromosome which can be seen in light microscope.
3. They are genetically active. these is a distinct alternating dark bands and light inter- bands. About 95% of DNA are present in bands and 5% in inter-bands.
4. The polytene chromosome has extremely large puff called balbiani rings which is seen in chironomous larvae. It is also known as chromosomal puff. puffing of bands are the sites of intense RNA synthesis.
5. As this chromosome occurs in the salivary gland it is known as salivary gland chromosomes. polyteny is achieved by repeated replication of chromosomal DNA several times (endomitosis).
6. Gene expression, transcription of genes and RNA synthesis occurs in the bands along the polytene chromosomes.
Lampbrush chromosomes:
1. It occurs at the diplotene stage of first meiotic prophase in oocytes an animal salamandar and in giant nucleus of the unicellular alga acetabularia.
2. It was first observed by Flemming in 1882. The highly condensed chromosome forms the chromosomal axis, from which lateral loops of DNA extend as a result of intense RNA synthesis.