In the rungs of the DNA ladder, why is it that A always bonds with T, and C always bonds with G?
The reason that A (adenine) always bonds to T (thymine) and that C (cytosine) always bonds to G (guanine) has to do with the way that different atoms are arranged in each of these four molecules. Picturing the four bases as puzzle pieces isn’t a bad way to think about it. The structure of thymine is actually very similar to cytosine, and the structure of adenine is similar to that of guanine. There are small differences that make A and T stick together better than A and C, or G and T.