Plastids in the cytoplasm of plants and one-celled animals synthesize and store food and other materials, such as starch or a working chemical. Many-celled animals store their food in special cells. Near the center of the cell is a roundish or oval-shaped nucleus. The nucleus controls the growth and division of the cell. A strand of DNA consists of a long series of genes, which are the units of heredity of plants and animals.
Genes determine the characteristics of a cell.
They do this by regulating the production of RNA, which in turn controls the manufacture of specific proteins. Human cells, for example, make only proteins unique to human beings. DNA strands are usually too thinly strung out to be seen with an optical microscope. Because the strands are readily stained with dyes, they are called chromatin. When a cell begins to divide, however, the chromatin thickens into the form of chromosomes.