The sequence of N - bass in one strand DNA is ' 3 TAC CAC TCC ATA ATT 5 ' How many purines will be present in the Watson crick structure of this DNA ?

Options

A. 18
B. 5
C. 15
D. 10

Answer

To determine the number of purines present in the Watson-Crick structure of the given DNA sequence, we will follow these steps:

### Step 1: Identify the DNA Strand
The given DNA sequence is:
**3' TAC CAC TCC ATA ATT 5'**

### Step 2: Write the Complementary Strand
In the Watson-Crick model of DNA, the two strands are complementary and anti-parallel. The complementary bases pair as follows:
- Adenine (A) pairs with Thymine (T)
- Thymine (T) pairs with Adenine (A)
- Cytosine (C) pairs with Guanine (G)
- Guanine (G) pairs with Cytosine (C)

To find the complementary strand, we will replace each base in the original strand with its complementary base:
- T → A
- A → T
- C → G
- G → C

So, the complementary strand for the given sequence will be:
**5' ATG GTG AGG TAT TAA 3'**

### Step 3: Count Purines in Both Strands
Purines in DNA are Adenine (A) and Guanine (G). Now, we will count the number of purines in both strands.

**Original Strand (3' TAC CAC TCC ATA ATT 5'):**
- T: 3
- A: 4 (1 from TAC, 1 from CAC, 1 from ATA, 1 from ATT)
- C: 5
- G: 0

Total purines in the original strand = 4 (A) + 0 (G) = **4 purines**

**Complementary Strand (5' ATG GTG AGG TAT TAA 3'):**
- A: 5 (1 from ATG, 1 from GTG, 1 from AGG, 1 from TAT, 1 from TAA)
- T: 3
- G: 2
- C: 0

Total purines in the complementary strand = 5 (A) + 2 (G) = **7 purines**

### Step 4: Calculate Total Purines
Now, we add the number of purines from both strands:
- Total purines = 4 (from original strand) + 7 (from complementary strand) = **11 purines**

### Final Answer
The total number of purines present in the Watson-Crick structure of this DNA is **11**.

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