Number of nucleotides per one turn of DNA double helix is

Options

A. 1)20
B. 2)40
C. 3)10
D. 4)60

Answer

To determine the number of nucleotides per one turn of the DNA double helix, we can follow these steps:

### Step 1: Understand the Structure of DNA
DNA (deoxyribonucleic acid) is structured as a double helix, which consists of two strands that wind around each other. Each strand is made up of nucleotides.

### Step 2: Identify Components of Nucleotides
Each nucleotide in DNA comprises three components:
- A phosphate group
- A five-carbon sugar (deoxyribose)
- A nitrogenous base (adenine, guanine, cytosine, or thymine)

### Step 3: Recognize the Base Pairing
In the DNA double helix, the nitrogenous bases pair specifically: adenine pairs with thymine (A-T) and guanine pairs with cytosine (G-C). Each pair of bases forms a base pair.

### Step 4: Determine the Number of Base Pairs per Turn
According to the Watson and Crick model of DNA, one complete turn of the DNA double helix consists of 10 base pairs.

### Step 5: Calculate the Number of Nucleotides
Since each base pair consists of two nucleotides (one from each strand), we can calculate the total number of nucleotides in one turn of the DNA helix:
- Number of base pairs per turn = 10
- Number of nucleotides per base pair = 2
- Therefore, total nucleotides per turn = 10 base pairs × 2 nucleotides/base pair = 20 nucleotides.

### Conclusion
The number of nucleotides per one turn of the DNA double helix is **20**.