DNA strands are termed antilparallel be cause of

Options

A. H-bonds
B. phospho-diester bonds
C. disulphide (S-S bonds)
D. none of the above

Answer

To understand why DNA strands are termed "anti-parallel," let's break down the concept step by step:

### 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 nucleotides, which consist of a sugar, a phosphate group, and a nitrogenous base.

2. **Orientation of DNA Strands**:
- Each DNA strand has a directionality, which is defined by the orientation of the sugar molecules in the backbone. One end of the strand has a free phosphate group (5' end), while the other end has a free hydroxyl group (3' end).

3. **Definition of Anti-parallel**:
- The two strands of DNA run in opposite directions. This means that if one strand runs from the 5' to the 3' direction, the other strand runs from the 3' to the 5' direction. This arrangement is what we refer to as "anti-parallel."

4. **Importance of Anti-parallel Orientation**:
- The anti-parallel arrangement is crucial for the formation of hydrogen bonds between the nitrogenous bases of the two strands. For example, adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C) through hydrogen bonds.

5. **Symmetry and Bonding**:
- The anti-parallel orientation allows for proper alignment and symmetry between the base pairs. If the strands were parallel, the bases would not align correctly, leading to improper bonding and instability.

6. **Conclusion**:
- Therefore, the term "anti-parallel" refers to the opposite directional orientation of the two DNA strands, which is essential for the correct formation of hydrogen bonds between the nitrogenous bases.

### Final Answer:
DNA strands are termed anti-parallel because one strand runs in the 5' to 3' direction, while the other runs in the 3' to 5' direction, allowing for proper hydrogen bonding between complementary bases.