Figure 2.5 shows the organic structural formulas for the bases found in DNA and RNA.
The numbering system used in biochemistry is shown at the right. So, for instance, we say that adenine has an amino group attached to the 6 position of the ring and cytosine has an amino group on the 4 position of the ring.
Adenine (A), guanine (G), and cytosine (C) are found in both DNA and RNA. Thymine (T) is found almost exclusively in DNA (rare exceptions will be noted) while uracil (U) is found only in RNA.
Why do you suppose this is?
Here is my answer: 