Types of chromosomes based on centromere Based on the position of centromere, chromosomes are called telocentric (terminal centromere), acrocentric (terminal centromere capped by telomere), sub metacentric (centromere subterminal) and metacentric (centromere median). The eukaryotic chromosome may be rod shaped (telocentric and acrocentric), L-shaped (sub-metacentric) and V-shaped (metacentric) (Figure . ). Based on the functions of chromosome it can be divided into autosomes and sex chromosomes .
Autosomes are present in all cells controlling somatic characteristics of an organism. In human diploid cell, chromosomes are autosomes whereas two are sex chromosomes. Sex chromosomes are involved in the determination of sex. Special types of chromosomes These chromosomes are larger in size and are called giant chromosomes in certain plants and they are found in the suspensors of the embryo.
The polytene chromosome and lamp brush chromosome occur in animals and are also called as giant chromosomes. Polytene chromosomes observed in the salivary glands of Drosophila (fruit fly) by E.G. Balbiani in . In larvae of many flies, midges ( Dipthera ) and some insects the interphase chromosomes duplicates and reduplicates without nuclear division.
A single chromosome which is present in multiple copies form a structure called polytene chromosome which can be seen in light microscope. They are genetically active. There is a distinct alternating dark bands and light inter-bands. About % of DNA are present in bands and % in inter-bands.
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. As this chromosome occurs in the salivary gland it is known as salivary gland chromosomes .
Gene expression, transcription of genes and RNA synthesis occurs in the bands along the polytene chromosomes. Figure . : Polytene chromosomes Dark band Inter band Chromosome puff Chromonemata and distal ring are connected by a central rod. They are L-Lipopolysaccharide ring, P-Peptidoglycan ring, S-Super membrane ring and M-membrane ring.
The outer pair L and P rings is attached to cell wall and the inner pair S