**Question:** If the 4% of thymine residues in DNA is 28%, what is the % of cytosine?

**Core Concept:** DNA molecule structure and composition

**Why the Correct Answer is Right:**
1. DNA is a double-stranded molecule composed of four nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C).
2. Thymine (T) makes up 4% of DNA, meaning it is present in 4 out of every 100 nucleotides.
3. The question states that 4% of thymine residues is equal to 28%. This implies that the total percentage of DNA is 100%.
4. To find the percentage of cytosine (C), we can use the formula: (number of cytosine residues / total number of nucleotides) * 100%

**Why Each Wrong Option is Incorrect:**
1. Option A: 16% (correct but does not account for thymine residues)
2. Option B: 24% (does not account for the fact that 4% is equal to 28%)
3. Option C: 20% (does not account for the presence of thymine)
4. Option D: 24% (does not account for the fact that 4% is equal to 28%)

**Why the Correct Answer is Right:**
1. To find the percentage of cytosine (C), we know that 4% of nucleotides is equal to 28%.
2. We can set up the equation: (number of cytosine residues / total number of nucleotides) * 100%
3. Substitute the known values: (number of cytosine residues / 100) * 100% = 28%
4. Solve for the number of cytosine residues: number of cytosine residues = 28 * 100 / (100 - 4) = 28 * 11 = 304
5. Since there are 100 nucleotides in total, the number of cytosine residues is 304.
6. The percentage of cytosine residues is then: (304 / 100) * 100% = 30.4%

**Why Each Wrong Option is Incorrect:**
1. Option A (16%) is too low because it ignores the presence of thymine residues.
2. Option B (24%) is too high because it does not account for the presence of thymine residues.
3. Option C (20%) is too low because it ignores the presence of thymine residues.
4. Option D (24%) is too high because it does not account for the fact that 4% of nucleotides is equal to 28%.

**Core Concept:** Percentages in molecular biology and genetics refer to the ratio of specific molecules or components to the total number of molecules or components.

**Why the Correct Answer is Right:**
1.