DNA (Deoxyribonucleic Acid) is the hereditary material in humans and almost all other organisms. Nearly every cell in a person's body has the same DNA. Most DNA is located in the cell nucleus (nuclear DNA), but a small amount can also be found in the mitochondria (mitochondrial DNA).
DNA consists of two strands that wind around each other like a twisted ladder. This shape is known as a double helix. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases: adenine (A), thymine (T), guanine (G), or cytosine (C).
The two strands are held together by bonds between the bases: adenine pairs with thymine, and guanine pairs with cytosine. These base pairs form the "rungs" of the DNA ladder.
The information in DNA is stored as a code made up of four chemical bases (A, T, G, C). The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to how letters of the alphabet appear in a certain order to form words and sentences.
DNA contains the instructions needed for an organism to develop, survive and reproduce. To carry out these functions, DNA sequences must be converted into messages that can be used to produce proteins, which are the complex molecules that do most of the work in our bodies.
When cells divide, they must pass a complete copy of their genetic information to the daughter cells. This process, called DNA replication, occurs when the two strands of the DNA double helix separate and each strand serves as a template for a new complementary strand.
Genetics is the study of genes, genetic variation, and heredity in organisms. The human genome consists of approximately 3 billion base pairs, which reside in the 23 pairs of chromosomes within the nucleus of all our cells. Each chromosome contains hundreds to thousands of genes, which carry the instructions for making proteins.