Calculate the number of weak hydrogen bonds in a DNA strand `204Å` long with 15% thymine bases.

Options

A. 154
B. 162
C. 138
D. 120

Answer

To calculate the number of weak hydrogen bonds in a DNA strand that is 204 Å long with 15% thymine bases, we can follow these steps:

### Step 1: Determine the total number of base pairs in the DNA strand.
The distance between two base pairs in DNA is approximately 3.4 Å.

To find the total number of base pairs, we divide the total length of the DNA by the distance between the base pairs:

\[ \text{Number of base pairs} = \frac{\text{Total length of DNA}}{\text{Distance between base pairs}} = \frac{204 \, \text{Å}}{3.4 \, \text{Å}} \approx 60 \]

### Step 2: Calculate the percentage of adenine (A) and thymine (T) bases.
Given that thymine (T) makes up 15% of the bases, according to Chargaff's rule, adenine (A) will also make up 15% of the bases. Therefore:

\[ \text{Percentage of A} = 15\% \]
\[ \text{Percentage of T} = 15\% \]

### Step 3: Calculate the number of A-T base pairs.
Since A pairs with T, the total percentage of A-T pairs is:

\[ \text{Total A-T pairs} = 15\% + 15\% = 30\% \]

To find the number of A-T base pairs:

\[ \text{Number of A-T pairs} = 30\% \times 60 \approx 18 \]

### Step 4: Calculate the number of hydrogen bonds from A-T pairs.
Each A-T pair forms 2 hydrogen bonds:

\[ \text{Hydrogen bonds from A-T} = 18 \times 2 = 36 \]

### Step 5: Calculate the percentage of guanine (G) and cytosine (C) bases.
Since A and T account for 30% of the bases, the remaining bases (G and C) account for:

\[ \text{Percentage of G and C} = 100\% - 30\% = 70\% \]

According to Chargaff's rule, G and C are in equal amounts, so:

\[ \text{Percentage of G} = 35\% \]
\[ \text{Percentage of C} = 35\% \]

### Step 6: Calculate the number of G-C base pairs.
To find the number of G-C base pairs:

\[ \text{Number of G-C pairs} = 35\% \times 60 \approx 21 \]

### Step 7: Calculate the number of hydrogen bonds from G-C pairs.
Each G-C pair forms 3 hydrogen bonds:

\[ \text{Hydrogen bonds from G-C} = 21 \times 3 = 63 \]

### Step 8: Calculate the total number of hydrogen bonds.
Now, we can find the total number of weak hydrogen bonds in the DNA strand:

\[ \text{Total hydrogen bonds} = \text{Hydrogen bonds from A-T} + \text{Hydrogen bonds from G-C} = 36 + 63 = 99 \]

### Final Answer
The total number of weak hydrogen bonds in the DNA strand is **99**.

---