BASE PAIR: A structural unit in DNA consisting of either an Adenine- Thymine or a Guanine-Cytosine link across the double strands of the molecule. Adenine always links with thymine, guanine with cytosine due to the arrangement of the hydrogen donors and receptors between these bases. In the diagram at the right, a portion of the lac enzyme promotor region on E. coli is shown. The
"TATA" box is in bold and is thought to serve as a firm binding site for RNA polymerase. There are 13 base pairs in this diagram. Click on the button to see one of them.