Chromosome are so called because they take up certain basic stains very readily (chromos= color, soma = body). But, they can also be observed by phase contrast microscopy in the unstained nuclei of dividing living cells. When the cell is not dividing, the chromosomes cannot be seen in the nucleus, even after staining. Chromosomes are the nuclear components composed of nucleo-protein. Chromosomes are made up of protein and nucleic acids called DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Chromosomes are also the bearer of hereditary units called genes. Chromosomes are able to reproduce themselves without change of morphology and physiological behavior.
 DNA is the principal constituent of chromosomes. DNA molecules determine the nature of cell and therefore of the whole organism.
 DNA and RNA
The number of chromosomes in the cells is constant for a species. Chromosomes always exist in pairs. In human cell, there are 46 chromosomes, i.e., there exist 23 pairs. Similarly, chromosomes are 12 pairs in frog cells, 20 pairs in rat cells, 7 pairs in rye grass and 8 pairs in onion cells etc.
All the cells of the body of an organism have the same number of chromosomes. In human beings, all the cells have 23pairs. Among these, 22 pairs of chromosome resemble each other and are called autosomes. One pair of chromosomes differs from the rest and called sex chromosome. In the sex chromosome of a male, one chromosome is big and the other is small whereas in a female, both the chromosomes are of the same size.
Autosomes are responsible for carrying on all the activities of the body, whereas sex chromosomes determine the sex of an organism and control other things related to it.

Functions of chromosome

 Each chromosome of a pair bears a gene. According to T.H. Morgan, these granuel like bodies called genes are the carriers of hereditary characters from one generation to another. Because of this, parents and offspring resemble each other in many aspects. Genes are made up of DNA and it is mainly DNA which carries the genetic information in the form of a code. Genes also have a special property of replicating without losing any information they hold.
Thomas Hunt Morgan

Thomas Hunt Morgan

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