The cell theory summarizes the main outcomes of the study of living organisms with microscopes:
1. The structural and functional unit of living organisms is the cell.
2. All living things are made up of cells.
3. Cells come exclusively by division of other cells.
It may be added that cells can be found isolated, as unicellular organisms, or as part of complex multicellular organisms, where cell populations associate to carry out the different functions of the body, each cell type being specialized in specific roles.
Strictly speaking, one of these statements is incomplete: "every cell comes from another cell". As we will see in the next page, the theory of the origin of life is the theory of the origin of the cell, and it is argued that the first cells appeared by physico-chemical mechanisms. Therefore, we could rephrase the statement by saying that every cell comes from another cell, except the first cells at the origin of life.
Moreover, there are nowadays new molecular techniques that may reformulate the same postulate 3. In the field of synthetic biology, a complete bacterial genome has been synthesized in vitro and introduced into other bacteria, with its own genome previously removed (Gibson et al., 2010). The result is a cell designed and produced in the laboratory, although only DNA has been chemically synthesized. However, it may be the first step towards the artificial synthesis of a whole cell from scratch, exclusively from organic molecules. Recently, a complete eukaryotic chromosome has been synthesized (Annaluru et al., 2014).
Annaluru N, et al. Total synthesis of a functional designer eukaryotic chromosome. Nature. 2014. 344(6179):55-58.
Gibson DG, et al. Creation of a bacterial cell controlled by a chemically synthesized genome. Nature. 2010. 329(5987):52-56.