The chromatin fibres get condensed into chromosomes during cell di-visions. They are capable of self-reproduction and they play an important rloe in heredity.
The nucleus was first observed and described by karl Nagli (1842) in the nuclei of plant cells. Chromosomes and their role in cell division was first explained by A. Schneider (1873). In 1887 Benden and Bovery reported that the number of chromosomes for each species is constant. T. H Morgan andH. Muller in 1922 revealed the occurrence of nearly 2000 genetic fac-tors on four chromosomes of Drosophila. In 1924, Robert Feulgen showed that chromosomes contain DNA.
The number of chromosomes is constant for a particular species. The reproductive cells such as sperm or ovum has one set of chromosomes and it is known as the haploid set(n). It is also known as the genome. The somatic or body cells contain two haploid set or genomes and are known as the diploid cells (2n). The diploid condition is arrived at by the union of the haploid male and female gametes in the sexual reproduction.
Common name Scientific name Chromosome Num-
Paramoecium P. Aurelia 30-40
Hydra H. vulgaris 32
Housefly Musca domestica 12
Fruit fly Drosophila sps 8
Pigeon Columba livia 80
Gorilla Gorilla gorilla 48
Man Homo sapiens 46
The shape of the chromosome changes from phase to phase. Each chromosome has a clear zone, known as centromere or kinetocore along their length. The centromere divides the chromosome into two parts. Each part is called the chromosome arm. Thus according to the position of the cen-tromere and nature of the chromosome arm, the chromosomes may be Telo-centric, Acrocentric, Submetacentric and Metacentric.