DNA, also known as deoxyribonucleic acid, is a fundamental molecule found in all living things. It serves as the basis for heredity, specifying which traits are passed on from parents to children through the generations. It also contains instructions for our body cells to perform their specific functions.

Structure
In humans, most of the DNA is in the form of tightly coiled strands called chromosomes, found inside the cell nucleus. There are 46 chromosomes in a human cell. If you unwind each chromosome and place them end-to-end, you will have a long, double-stranded helix that is about 3 meters in length—all from one microscopic human cell.

The DNA helix looks like a twisted ladder. The two sides are composed of the four bases: adenine (A), thymine (T), guanine (G), and cytosine (C), and the rungs of the ladder represent hydrogen bonds that connect specific pairs of these molecules together: A–T and G–C.

The arrangement of these molecules, called the DNA sequence, spell out the instructions for our physical characteristics and body functions. These instructions are found in units called genes.

Replication
The specific pairing of DNA molecules presents a simple mechanism for their replication. Replication occurs whenever a cell divides in two during growth and development.

During replication, the helix unwinds and an enzyme separates the two strands. Another enzyme, DNA polymerase, adds the four molecules A, T, C, and G to each strand based on the strands sequences: A is added to a T on the strand, and C is paired with a G.

The products of replication are two sets of double-stranded DNA molecules that have exactly the same sequence as the original. During cell division, each cell receives one set of DNA. In this way, all the cells of the body have the same DNA molecules.

In the laboratory, scientists use an artificial method for replication to examine specific portions of DNA. To learn more, please visit the Polymerase Chain Reaction page.