As early as 1665, biologists have known that living organisms are composed of microscopic structures. At that time Robert Hooke looked at plant tissue--specifically cork-- under magnification and discovered that it was composed of small, rectangular shaped boxes. He called the boxes "cells" because they reminded him of the rooms that monks live in in their monastery. Those rooms are called "cells" too. By the mid 1800s Theodor Schwann, Matthias Jakob Schleiden, and Rudolf Virchow had contributed observations about cells which developed into the cell theory of life. The first postulate of the cell theory is that the fundamental (or basic) unit of life is the cell. All living organisms must have at least one cell.
Further research into the structure of cells themselves lead to the discovery of many organelles including the nucleus. By applying observations of Gregor Mendel and the structure of the nucleus lead to the discovery that it is chemical portions of DNA which actually define the hereditary properties of an organism. DNA has a large number of chemical bonds with a fairly complex structure. However, the basic structure of DNA which controls individual hereditary traits is called the Gene.