Chromosomes are the physical carriers of genes, which are made up of DNA and associated proteins. The term chromosome was introduced by Waldeyer in 1888. Chromosomes occur in all the living organisms. The bacterial chromosomes are circular. It has closed circular DNA. Linear chromosomes are found in eukaryotes. Bridges in 1916 was the first to prove that the genes are carried on the chromosome.
The chromosomes are classified into different types based on shape and position of the centromere. According to the position of centromere, the eukaryotic chromosomes may be rod shaped (telocentric and acrocentric), L-shaped (sub-metacentric) and V-shaped (metacentric). There are two types of chromosomes based on their function. They are autosomes and sex chromosomes.
They are present in all the cells of the organisms. They control the somatic characteristics of an organism. In the human diploid cell, 44 chromosomes are autosomes whereas the rest two are sex chromosomes.
In the diploid cells of animals and certain plants, one or more special chromosomes are different from the autosomes in their morphological structures and behaviour. These chromosomes are involved in the determination of sex. They are called sex chromosomes. In human being, male has XY and female XX chromosomes.
These chromosomes are abnormal chromosomes. They differ from the basic structure of normal chromosomes. Eg. B-chromosomes and Double minutes. B-chromosomes are also called supernumerary and accessory chromosomes. They are additional chromosomes found in some individuals in a population. eg. maize. They are common in plants and they reduce viability.
Double minutes are unstable chromosome like structures. They have no centromere and formal telomeres. They occur in cancer cells which show resistance against drugs.
In Eukaryotic organisms certain chromosomes are found only in certain special tissues and are not seen in other tissues. These chromosomes are larger in size and are called giant chromosomes. In certain plants, they are found in the suspensors of the embryo. There are two types of giant chromosomes - polytene chromosome and lamp brush chromosome.
Polytene chromosomes were observed by C.G. Balbiani in 1881 in the salivary glands of Drosophila. The characteristic feature of polytene
chromosome is that along the length of the chromosome there is a series of dark bands alternate with clear zones called inter bands. The polytene chromosome has extremely large puff called Balbiani ring. It is also known as chromosomal puff. As this chromosome occurs in the salivary gland it is known as salivary gland chromosomes.
Lamp brush chromosomes were first observed by Flemming in 1882. It looked like brushes. They occur at the diplotene stage of meiotic prophase in oocytes of an animal Salamandor and in giant nucleus of the unicellular alga Acetabularia. The highly condensed chromosome forms the chromosomal axis, from which lateral loops of DNA extend as a result of intense RNA synthesis.