question_answer 39)

  Briefly describe the cell theory.

Answer:

                  Schleiden and Schwann in 1838-39, formulated the cell' theory, which originally containedfollowing two statements (i) All living beings are made up of cells and products formed by the cells. (ii) Cells are the structural and functional units of life. The cell theory stated by Schleiden and Schwann failed to explain the question of origin ofcells. A major expansion of the cell theory was expressed by Virchow in 1855, in hisstatement 'Omnis cellula e cellula' (all cells arise from pre-existing cells) .This concept, wasthe actual idea of Nagelli (1846), which later on was elaborated by Virchow, along withconsiderable evidences in its support. The work of Nagelli and Virchow established celldivision as the central phenomenon in the continuity of life.. Thus, the modern cell theory is based on two facts (i) all living organisms are composed of cells and products of cell. (ii) cells are the basic structural and functional unites of life. (iii) all cells arise from pre-existing cells. Viruses are exception to cell theory as they are not composed of cell. They consist of anucleic acid (DNA or RNA) surrounded by a protein sheath and are incapable ofindependent existence, self regulation and self reproduction.