Chromosomes and Their Organization
During division phase , the duplicated chromatin fibres become extremely shortened (= condensed), by coiling and folding ,into short think and microscopically visible rod like structures of various shapes and sizes, It is necessary to learn about these structures before dealing with the sequential changes of mitosis. These structures werer observed first by Naegeli 1842 and later by russow 1872 and Balbiani 1876 waldeyer 1888 named these, chromosomes gr.khroma =coloured + soma =body ) because these take a dark stain when treated with basis dyes.
Karyotype :Beneden 1888 discovered that the chromosomal pattern is a specific character .i.e. each species has its own chromosomal complement different from those of all other species ,This complement of a species is called its karyotype .It implies that in all cells membrane of a plant or animals species, not only the number of chromosomal is the same but even the shapes sizes, and structure of the various chromosomes in the set are the same .The chromosomal number varies from 10 to 50 in most of the species, The smallest number is however two found in a nematode subspecies ascaris megalocephala univalence, and the largest number is 1600 in aulacantha ( a radiolarian protozoan ) To quote a few examples the chromosomal number is 46 in man ( Homo sapiens ) 32 in Hydra vulgaris. 12 in housefly (Musca domestica ) 26 in frog (Rana pipiens ) in rabbit, (Oryctolagus cuniculus ) 38 in cat (Felish domesticus ) 78 in dog (cains familiaris) 80 in pigeon (Columba livia) 8 in fruitfly ( Drosophila melanogaster ) in mosquito ( Culex pipiens ) and so on.