Cell Doctrine (Cell Principle)

The cell doctrine (sometimes called the cell principle) is essentially a checklist that gathers together the core statements that, taken as a set, make up the modern cell theory. First, every organism, without exception, is made up of cells. Second, every new cell comes only from a pre-existing cell through the process of cell division - the addition Rudolf Virchow made in 1858, which rules out spontaneous generation. Third, the cell is both the structural unit (the physical building block) and the functional unit (the smallest thing that is actually 'alive') of every living organism. Fourth, a cell carries hereditary (genetic) information that gets passed on from one cell to the next whenever the cell divides. Fifth, despite their outward diversity, all cells are broadly similar in their basic chemical composition and metabolic activities - the biochemistry of a bacterium and the biochemistry of a human liver cell share the same fundamental logic. Sixth, it is specifically the cell's DNA that controls both its structure and its function. And finally, the doctrine makes room for an interesting exception within cells themselves: sometimes a cell that has t …