How many hydrogen bonds are present between complementary nitrogen bases of a DNA with 5 turns and 20 Adenines?

Options

A. 130
B. 90
C. 100
D. 80

Answer

To solve the question of how many hydrogen bonds are present between complementary nitrogen bases of a DNA with 5 turns and 20 adenines, we can follow these steps:

### Step-by-Step Solution:

1. **Determine the Number of Base Pairs**:
- Each turn of DNA contains 10 base pairs.
- Given that there are 5 turns, the total number of base pairs (BP) can be calculated as:
\[ \text{Total Base Pairs} = \text{Base Pairs per Turn} \times \text{Number of Turns} = 10 \times 5 = 50 \text{ BP} \]

2. **Identify the Number of Adenine and Thymine Base Pairs**:
- It is given that there are 20 adenines (A).
- Since adenine pairs with thymine (T) and forms 2 hydrogen bonds, the number of thymine bases will also be 20 (1 T for each A).
- Therefore, the number of A-T base pairs is:
\[ \text{A-T Base Pairs} = 20 \]

3. **Calculate the Number of Guanine and Cytosine Base Pairs**:
- The total number of base pairs is 50, and we have already established that 20 of these are A-T pairs.
- Thus, the number of G-C base pairs can be calculated as:
\[ \text{G-C Base Pairs} = \text{Total Base Pairs} - \text{A-T Base Pairs} = 50 - 20 = 30 \]

4. **Calculate the Number of Hydrogen Bonds**:
- Each A-T base pair has 2 hydrogen bonds. Therefore, the total number of hydrogen bonds from A-T pairs is:
\[ \text{Hydrogen Bonds from A-T} = \text{A-T Base Pairs} \times 2 = 20 \times 2 = 40 \]
- Each G-C base pair has 3 hydrogen bonds. Therefore, the total number of hydrogen bonds from G-C pairs is:
\[ \text{Hydrogen Bonds from G-C} = \text{G-C Base Pairs} \times 3 = 30 \times 3 = 90 \]

5. **Calculate the Total Number of Hydrogen Bonds**:
- The total number of hydrogen bonds in the DNA can be calculated by adding the hydrogen bonds from A-T and G-C pairs:
\[ \text{Total Hydrogen Bonds} = \text{Hydrogen Bonds from A-T} + \text{Hydrogen Bonds from G-C} = 40 + 90 = 130 \]

### Final Answer:
The total number of hydrogen bonds present between complementary nitrogen bases of the DNA is **130**.