If the percentage of thymine residues in DNA is 28%. What is the percentage of cytosine?
**Core Concept**
The question involves the base composition of DNA, specifically the relationship between thymine and cytosine. In DNA, thymine (T) pairs with adenine (A) through hydrogen bonding, while cytosine (C) pairs with guanine (G). The base composition of DNA is not random and follows specific rules.
**Why the Correct Answer is Right**
Given that the percentage of thymine residues in DNA is 28%, we can calculate the percentage of adenine (A) since T and A are complementary bases. Since T = A, and we know T's percentage is 28%, the percentage of A is also 28%. Now, we need to find the percentage of C. The percentage of G is not directly given, but we know that the total percentage of the four bases (A, T, C, G) must add up to 100%. The sum of A and T is 28% + 28% = 56%. Therefore, the remaining percentage (100% - 56%) is the combined percentage of C and G. To find the percentage of C, we can use the fact that C = G. Let's call the combined percentage of C and G "x". Then, x = x + x (since C = G), which simplifies to 2x = 44 (100% - 56%). Solving for x, we get x = 22%.
**Why Each Wrong Option is Incorrect**
**Option A:** This option would be incorrect if the question stated the percentage of adenine instead of thymine, since A = T.
**Option B:** This option would be incorrect if the question gave the percentage of guanine instead of thymine.
**Option C:** This option would be incorrect if the question stated the percentage of cytosine equals the percentage of thymine, which is not the case.
**Option D:** This option would be incorrect if the question gave the percentage of a different base, such as adenine or guanine.
**Clinical Pearl / High-Yield Fact**
The base composition of DNA is not random and follows specific rules, including the complementarity of base pairs (A-T and G-C). Understanding these rules is essential for various medical applications, including molecular biology and genetics.
**Correct Answer:** C. 22%