Across from each base is it's mirror image. G is the mirror image of C, C is the mirror image of G, A is the mirror image of T, and T is the mirror image of A. Think of it like your hand being held up to a mirror. If you hold your right hand up to a mirror it isn't an identical copy of your hand, but is instead a mirror image that more resembles your left hand. Your left and right hands are mirror images of each other.
Chirality!