DNA strands are termed antiparallel because of

Options

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

Answer

**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. **Directionality of DNA Strands**: Each DNA strand has a directionality, which is indicated by the 5' (five prime) and 3' (three prime) ends. The 5' end has a phosphate group attached to the fifth carbon of the sugar, while the 3' end has a hydroxyl (-OH) group attached to the third carbon of the sugar.

3. **Antiparallel Orientation**: In a double-stranded DNA molecule, the two strands run in opposite directions. This means that if one strand runs from 5' to 3', the complementary strand runs from 3' to 5'. This orientation is referred to as "antiparallel."

4. **Role of Hydrogen Bonds**: The two strands of DNA are held together by hydrogen bonds that form between complementary nitrogenous bases. Adenine (A) pairs with thymine (T) through two hydrogen bonds, and guanine (G) pairs with cytosine (C) through three hydrogen bonds.

5. **Conclusion**: The term "antiparallel" refers to the orientation of the two strands of DNA running in opposite directions (5' to 3' and 3' to 5'). The presence of hydrogen bonds between the complementary bases stabilizes this structure.

6. **Final Answer**: Therefore, the correct answer to the question is option A: hydrogen bonds.

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