If the nitrogen-based molecules present in the DNA of an animal are found to be 13.2% adenine molecules, what percentage of guanine would be present in the animal’s DNA?
a. 13.2%
b. 36.8%
c. 26.4%
d. 73.6%
The correct answer and explanation is:
The correct answer is b. 36.8%.
In DNA, nitrogenous bases follow a specific pairing rule known as Chargaff’s rules. According to these rules, adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G). The percentage of adenine should be equal to the percentage of thymine, and the percentage of cytosine should be equal to the percentage of guanine.
Given that adenine (A) is 13.2%, this means thymine (T) is also 13.2%. Together, adenine and thymine account for:
13.2% + 13.2% = 26.4%.
The remaining percentage (100% – 26.4% = 73.6%) must be divided equally between cytosine (C) and guanine (G), since they pair together as well. Therefore, each of these two bases will account for half of the remaining percentage:
73.6% ÷ 2 = 36.8%.
Thus, the percentage of guanine in the DNA is 36.8%.
To summarize:
- Adenine (A) = 13.2%
- Thymine (T) = 13.2%
- Cytosine (C) = 36.8%
- Guanine (G) = 36.8%
This relationship between the bases ensures the double-stranded structure of DNA, with complementary base pairing.