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Chapter: BIOLOGY (ZOOLOGY) Standard XI first year 11th text book Assignment topics question and answer Explanation Definition

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Chromosomes of Animal Nucleus or Human Nucleus

Chromosomes of Animal Nucleus or Human Nucleus
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.

Chromosomes

 

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-

Ber                 

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.


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