DNA

DNA:  the chemical inside the nucleus of a cell that carries the genetic instructions for making living organisms.

DNA sequencing:  Determining the exact order of the base pairs in a segment of DNA.

Sources:  image from the National Human Genome Research Institute (NHGRI) Genetic Illustrations; definitions from the National Human Genome Research Institute (NHGRI) Glossary of Genetic Terms

More:  Double-stranded DNA takes the shape of a double helix, like a ladder that has been twisted.

A always pairs with T (or in RNA, it pairs with U) G always pairs with C. So if you know the sequence of bases on one side of the 'ladder,' you can automatically determine the sequence of bases on the other side. The strands are therefore complementary to each other.

Sometimes the two strands of DNA are referred to as the plus and minus strand, positive and negative, sense and antisense, or Watson and Crick.