DNA strands are antiparallel because of the presence of `:`

Options

A. H-bonds
B. Phosphate diester-bonds
C. Di-sulphide-bonds
D. Phosphate-bonds

Answer

**Step-by-Step Solution:**

1. **Understanding DNA Structure**: DNA is composed of two strands that run in opposite directions. Each strand consists of a backbone made of sugar and phosphate groups, with nitrogenous bases attached to the sugar.

2. **Identifying the Directionality**: Each DNA strand has a directionality indicated by the 5' (five prime) and 3' (three prime) ends. The 5' end has a phosphate group, while the 3' end has a hydroxyl group.

3. **Defining Antiparallel Orientation**: The two strands of DNA are said to be antiparallel because one strand runs from 5' to 3', while the other runs from 3' to 5'. This orientation is crucial for the formation of base pairs.

4. **Role of Bonds in DNA Structure**: There are three main types of bonds in DNA:
- **N-glycosidic bonds**: Connect nitrogenous bases to the sugar.
- **Phosphodiester bonds**: Link the sugar-phosphate backbone of adjacent nucleotides.
- **Hydrogen bonds**: Form between complementary nitrogenous bases (adenine with thymine, and guanine with cytosine).

5. **Significance of Hydrogen Bonds**: The hydrogen bonds between the nitrogenous bases stabilize the DNA structure. They allow for the base pairing that is essential for the double helix formation.

6. **Conclusion**: The antiparallel nature of DNA strands is primarily due to the presence of hydrogen bonds between the complementary bases, which ensures proper base pairing and stability of the DNA molecule.

**Final Answer**: The DNA strands are antiparallel because of the presence of hydrogen bonds.

---