Two strands of a molecule of DNA are linked sidewise by

Options

A. ester bonds
B. glycosidic bonds
C. purine-pyrimidine hydrogen bonds
D. all the above

Answer

To answer the question, "Two strands of a molecule of DNA are linked sidewise by," we can break down the solution into the following steps:

### Step-by-Step Solution:

1. **Understanding DNA Structure**:
- DNA is composed of two strands that form a double helix. Each strand is made up of a sugar-phosphate backbone and nitrogenous bases.

2. **Identifying the Bonds**:
- The strands are linked together through specific types of bonds. The sugar molecules are connected to phosphate groups, and the nitrogenous bases are attached to the sugars.

3. **Types of Bonds in DNA**:
- **Glycosidic Bonds**: These bonds connect the nitrogenous bases (purines and pyrimidines) to the sugar molecules.
- **Phosphodiester Bonds**: These bonds connect the sugar of one nucleotide to the phosphate group of the next nucleotide, forming the backbone of each DNA strand.
- **Hydrogen Bonds**: These bonds link the nitrogenous bases of the two strands together, holding the two strands in place.

4. **Linking Strands Sidewise**:
- The question specifically asks about how the two strands are linked sidewise. The sidewise connection refers to the connection between the nitrogenous bases of one strand to the nitrogenous bases of the other strand.

5. **Final Answer**:
- The two strands of a DNA molecule are linked sidewise by **hydrogen bonds** between the complementary nitrogenous bases (adenine with thymine, and guanine with cytosine).

### Summary:
The answer to the question is that the two strands of a molecule of DNA are linked sidewise by **hydrogen bonds**.

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