Deoxyribonucleic acid or DNA contains the biological instructions that make each species unique. It is the genetic material that an organism (like us humans or dogs or plants) inherits from their parents. Chemically, DNA is a very long molecule (a polymer) made out of sugar and phosphate groups and four different types of bases: adenine (A), thymine (T), guanine (G) and cytosine (C). The order, or sequence, of these bases determines what biological instructions are contained in a strand of DNA. For example, the sequence ATCGTT might instruct for brown hair, while ATCGCT might instruct for black hair.
DNA segments that carry this genetic information are called genes. The genes dictate how we are made and what our bodies look like. In other words the genes carry all the instructions for making up our body. So there is a gene that tells the body to have brown hair and so on.
Other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information.
DNA is found inside a special area of the cell called the nucleus. Because thecell is very small, and because organisms have many DNA molecules per cell,each DNA molecule must be tightly packaged. This packaged form of the DNA iscalled a chromosome. During cell division, DNA unwinds so it can be copied and the copies transferred to new cells. DNA also unwinds so that its instructions can be used to make proteins and for other biological processes