Cell

Cells are the basic units of life. It was recognized by Virchow in 1855 that they are the smallest units capable of maintaining the continuity of life or, as he expressed it, Omnis cellula e cellula (every cell derives from a cell). Organic matter on earth exists in the following forms: various types of cells, short sequences of stable proteins (Prions), extracellular genomes (viroids or naked RNA molecules), and protein capsules, or capsids, containing relatively short molecules of DNA or RNA often surrounded by an additional membrane (bacteriophages and viruses). Prions, viroids, phages, and viruses lack their own metabolism and the ability to reproduce independently, whereas all types of cells are true living systems with metabolic ability (metabolism) and the capacity for independent reproduction (cell multiplication). All cells share the following attributes:

• They are enclosed by a cell membrane (Fig. 1); some are motile (Fig. 2).

• Their systems of reproduction use DNA for...