A fragment of DNA has 480 nucleotides out of which 110 are those of adenine. The number of cytosine nucleotides is

Options

A. 110
B. 130
C. 220
D. 240

Answer

To solve the problem of finding the number of cytosine nucleotides in a DNA fragment with a total of 480 nucleotides, where 110 of them are adenine, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Total Number of Nucleotides**:
- The total number of nucleotides in the DNA fragment is given as 480.

2. **Identify the Number of Adenine Nucleotides**:
- The number of adenine (A) nucleotides is provided as 110.

3. **Apply Chargaff's Rule**:
- According to Chargaff's rule, in double-stranded DNA, the amount of adenine equals the amount of thymine (T), and the amount of guanine (G) equals the amount of cytosine (C).
- Therefore, if A = 110, then T must also equal 110.

4. **Calculate the Total Number of A and T**:
- The total number of adenine and thymine nucleotides combined is:
\[ A + T = 110 + 110 = 220 \]

5. **Calculate the Number of G and C**:
- To find the total number of guanine (G) and cytosine (C), we subtract the total number of A and T from the total number of nucleotides:
\[ G + C = \text{Total Nucleotides} - (A + T) = 480 - 220 = 260 \]

6. **Determine the Number of G and C**:
- Since G and C must be equal (as per Chargaff's rule), we can set:
\[ G = C \]
- Therefore, we can express G + C as:
\[ G + C = 260 \implies 2C = 260 \implies C = \frac{260}{2} = 130 \]

### Conclusion:
The number of cytosine nucleotides (C) in the DNA fragment is **130**.