In a DNA molecule, nitrogenous base is linked to pentose sugar by 'A' linkage and the pentose sugar is linked to phosphate by 'B' linkage. Correctly identify A and B.

Options

A. A - phosphodiester linkage
B - N - glycosidic linkage
B. A- N - glycosidic linkage
B - phosphodiester linkage
C. A- covalent bond
B - phosphodiester linkage
D. A - hydrogen bond
B - ester linkage

Answer

To solve the question regarding the linkages in a DNA molecule, we will break down the components involved and identify the correct linkages step by step.

### Step 1: Understanding the Structure of DNA
DNA (deoxyribonucleic acid) is a polymer made up of nucleotides. Each nucleotide consists of three components:
- A nitrogenous base
- A pentose sugar (deoxyribose in DNA)
- A phosphate group

### Step 2: Identifying the Linkage between Nitrogenous Base and Pentose Sugar
The nitrogenous base in a nucleotide is attached to the pentose sugar. This attachment occurs at the 1' carbon of the sugar. The type of bond that links the nitrogenous base to the sugar is known as a **glycosidic bond**. In the context of DNA, this specific bond is referred to as **N-glycosidic linkage**.

### Step 3: Identifying the Linkage between Pentose Sugar and Phosphate Group
The phosphate group is attached to the 5' carbon of the pentose sugar. The bond that connects the sugar to the phosphate group is known as a **phosphodiester bond**. This bond is crucial for forming the backbone of the DNA strand.

### Step 4: Summarizing the Linkages
From the analysis:
- **A (linkage between nitrogenous base and pentose sugar)** is identified as **N-glycosidic linkage**.
- **B (linkage between pentose sugar and phosphate)** is identified as **phosphodiester linkage**.

### Conclusion
The correct answer to the question is:
- A is N-glycosidic linkage
- B is phosphodiester linkage

Thus, the correct option is **Option 2**.

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