In a 3.2 Kbp long piece of DNA , 820 adenine bases were found. What would be number of cyosine bases

Options

A. 1560
B. 1480
C. 780
D. 740

Answer

To solve the problem of finding the number of cytosine bases in a 3.2 Kbp long piece of DNA with 820 adenine bases, we can follow these steps:

### Step-by-Step Solution:

1. **Convert Kbp to Base Pairs**:
- The length of the DNA is given as 3.2 Kbp (kilobase pairs).
- Since 1 Kbp = 1000 base pairs, we calculate:
\[ 3.2 \text{ Kbp} = 3.2 \times 1000 = 3200 \text{ base pairs} \]

2. **Identify the Number of Adenine and Thymine Bases**:
- According to Chargaff's rule, in double-stranded DNA, the amount of adenine (A) is equal to the amount of thymine (T).
- Given that there are 820 adenine bases, there will also be:
\[ \text{Number of thymine bases (T)} = 820 \]

3. **Calculate the Total Number of A and T Bases**:
- The total number of adenine and thymine bases combined is:
\[ A + T = 820 + 820 = 1640 \]

4. **Calculate the Total Number of Cytosine and Guanine Bases**:
- The total number of base pairs in the DNA is 3200. Therefore, the number of cytosine (C) and guanine (G) bases combined can be calculated as:
\[ C + G = \text{Total base pairs} - (A + T) = 3200 - 1640 = 1560 \]

5. **Determine the Number of Cytosine and Guanine Bases**:
- According to Chargaff's rule, the amount of cytosine (C) is equal to the amount of guanine (G). Therefore:
\[ C = G \]
- Since \( C + G = 1560 \), we can express it as:
\[ C + C = 1560 \implies 2C = 1560 \implies C = \frac{1560}{2} = 780 \]

### Final Answer:
Thus, the number of cytosine bases in the DNA is **780**.

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