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Chromosome Number

DNA is the main genetic constituent of the cells. Within the cells DNA is not free but is complexed with proteins in structure called chromatin. At the time of cell division, the chromatin becomes condensed into chromosomes.

An electron micrograph of a human chromosome
Chromosomes
Karl Nagli (1842) observed rod-like chromosomes in nuclei of plant cells. Walter Flemming (1878) introduced the term chromatin to describe the thread-like material of the nucleus that became intensely coloured after staining. E. Strasburger (1875) discovered thread like strictures which appeared during cell division. W. Roux (1883) suspected the involvement of chromosomes in the mechanism of inheritance. The present name chromosome (Gr., chrom = colour, soma= body) was introduced by W. Waldeyer in 1888.

Chromosome Number: The number of chromosomes in the cell nucleus varies considerably among different plants and animals; however each species has a specific chromosome number. 

In the somatic cell of a plant or an animal, the chromosomes are paired. Each member of a pair of chromosomes is called homologue. The original or diploid number (2n) of a somatic cell is derived from two gametes, each of which possesses the haploid number(n). 

Man has 46 chromosomes or 23 pairs (n=23, 2n= 46). {'n' normally signifies the gametic or haploid chromosome number, 2n is the somatic or diploid chromosome number in an individual.}

Group
Common Name
(Scientific Name)
Chromosome Number
Plants:


Algae
Chalamydomonas (Chalamydomonas reinhardii)
2
Fungi
Yeast (Saccharomyces cerivisiae)
32
Bryophytes
Riccia (Riccia arvensis)
8
Pteridophytes
Selaginella (Selaginella exigua )
72
Gymnospems
Yellow pine (Pinus ponderosa)
24
Angiospems



Arabidopsis (Arabidopsis thaliana)
5

Garden pea (Pisum sativum)
14

Tobacco (Nicotiana tabacum)
48

Wheat (Triticum aestivum)
42

Coffee (Coffea arabica)
44

Potato (Solanum tuberosum)
48

Sugarcane (Saccharum officinarum)
80
Animals:


Protozoa
Paramecium (Paramecium aurelia)
30-40
Cnidaria
Hydra( Hydra vulgaris)
32
Nematode
Round worm (Ascaris lumbricoides)
24
Arthropoda
Mosquito (Culex pipiens)
6
Chordata
Frog (Rana esculenta)
26

Pigeon (Columba livia)
80

Gorilla (Gorilla gorilla)
48

Chimpanzee (Pan troglodytes)
48

Mouse (Mus musculus)
40
The organism with the lowest number of the chromosomes is the nematode, Ascaris megalocephalus univalens which has only two chromosomes in the somatic cells (2n=2). In the radiolarian protozoan Aulacantha is found a diploid number of approximately 1600 chromosomes.
Among plants, the lowest chromosome number has been reported in Haplopappus gracilensis (n=2) and the highest in Ophioglossum reticulatum (Adders-tongue) (n=630).

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