Base Pair

A base pair refers to two bases which form a "rung of the DNA ladder." A DNA nucleotide is made of a molecule of sugar, a molecule of phosphoric acid, and a molecule called a base. The bases are the "letters" that spell out the genetic code. In DNA, the code letters are A, T, G, and C, which stand for the chemicals adenine, thymine, guanine, and cytosine, respectively. In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.

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

More:  DNA is 'read' in a specific direction, just like letters and words in the English language are read from left to right. Each end of DNA molecule has a number. One end is referred to as 5' (five prime) and the other end is referred to as 3' (three prime). The 5' and 3' designations refer to the number of carbon atom in a deoxyribose sugar molecule to which a phosphate group bonds.

This slide shows how the carbons in the sugars are numbered, to help you determine which ends is 5', and which is 3'. Once you figure out the direction in which one strand is read, you automatically know the direction in which to read the other strand. This is because the two strands are also antiparallel (they run in opposite directions), as mentioned in the previous slide.