Mitosis and Growth and Repair

Mitosis produces two daughter cells that are genetically identical to each other and to the parent cell, with the same number of chromosomes (46 in human body cells). This makes mitosis the type of cell division used for growth and repair in multicellular organisms, as well as for asexual reproduction.Growth: a multicellular organism grows by increasing its number of cells. A human begins as one cell, the zygote, and an adult body contains around 3 × 10¹³ cells, all produced by mitosis. Because every cell has the same genetic information, cells can then differentiate into specialised types by switching particular genes on or off.Repair: damaged or worn-out cells are replaced by new, identical cells, for example when a skin wound heals, a broken bone mends or the lining of the gut is renewed. The new cells must be identical so that they carry out the same function as the cells they replace.In animals, mitosis happens throughout the body but at different rates. Cells of the skin, gut lining and bone marrow divide often, whereas most nerve cells and heart muscle cells rarely divide in adults, so these tissues repair poorly.Mitosis is tightly controlled by checkpoints in the cell cycle. If this control is lost, cells divide uncontrollably and form a tumour, which may become cancer.Worked example: the cell cycle of a cell in the gut lining lasts 24 hours. Starting from one cell, after 3 days (3 cycles) there would be 2³ = 8 cells.