From Grade 9 biology: Cell theory: all cells come from previously existing cells.

From wikipedia:

The generally accepted parts of modern cell theory include:

All known living things are made up of one or more cells.
All living cells arise from pre-existing cells by division.
The cell is the fundamental unit of structure and function in all living organisms.
The activity of an organism depends on the total activity of independent cells.
Energy flow (metabolism and biochemistry) occurs within cells.
Cells contain hereditary information (DNA) which is passed from cell to cell during cell division.
All cells are basically the same in chemical composition in organisms of similar species.