It is microscopic but has a huge presence in this world—it is a cell! Cells are the basic units that form all living things. All existing organisms are either cells themselves or made up of cells. Explore the cell’s composition, quantity, size, and characteristics. Learn all about cellular functions, chromosomes, how DNA chains are formed, the composition of viruses, the composition of cells, the nucleus, the cytoplasm, the membrane, the differences between animal and plant cells, cellular reproduction, mitosis, and meiosis.