The basic building blocks of the body are tiny structures called cells.The human body contains trillions of cells, such as nerve cells, muscle cells, and fat cells. The instructions on how each cell develops and functions are held in coded form by a chemical called deoxyribonucleic acid (DNA), which is found in tiny structures called chromosomes in the nucleus (control centre) of every cell. If a molecule of DNA could be unravelled, it would reveal a structure like a twisted ladder called a double helix.