DNA contains the base pairs adenine, thymine, cytosine, and guanine. Adenine and guanine are purines, while thymine and cytosine are pyrimidines. The DNA molecules possess a double-helix structure. The base pairing occurs at the interior of the helix. Based on the properties of how the bases pair with one another, it is possible to infer the percentage of one type of base according to the percentages of the others. This process is useful, because it will help you to better understand the characteristics of DNA structure and composition.
Know how the bases in DNA pair with one another. In the DNA strand, the purines always pair with the pyrimidines. Adenine pairs with thymine, and cytosine pairs with guanine.
Understand Chargaff's rules. Chargaff's rules state that, as a result of how the DNA bases pair, the amount of adenine and thymine are equal, and the amount of cytosine and guanine are equal.
Write down the information that you already know regarding the percentages in the DNA strand. Often, a percentage problem will reveal the percentages of some or one of the other bases. For example, there may be 30 percent cytosine. From this information, you can determine that there is 30 percent guanine, because the amount of cytosine and guanine is equal.
Add the percentages of the cytosine and guanine together, subtract this from 100 percent, and divide this by two. Adding 30 percent and 30 percent gives you 60 percent. Subtracting 100 percent from 60 percent yields 40 percent. Dividing 40 percent by two gives you 20 percent. Therefore, there is 20 percent thymine. Since the amounts of thymine and adenine are equal, there is 20 percent adenine in the DNA strand.