Mitotic cell divisions permit a fertilized ovum to develop into an organism. They are the preconditions for physiological renewal of cells and lead to regeneration of tissues after injury. With the exception of a fewcells (nerve cells, and cardiac and skeletal muscle cells), the ability to divide is maintained throughout the life cycle, though it varies among cells. As a rule, mitoses are less common in highly differentiated tissues.
Double helix of DNA and its duplication (replication). The double strand of DNA splits like a zipper and forms two new, completely identical DNA molecules. The original strands are shown in blue, the new ones in pink (the sugar−acid phosphate chain is shown as a band; A = adenine, C = cytosine, G = guanine, T = thymine; free nucleotide groups are marked by arrows) (After Hadorn and Wehner)