The double stranded helical structure of DNA is maintained by

Options

A. amide bonds
B. H-bonds
C. covalent bonds
D. phosphodiester bonds

Answer

To solve the question, "The double stranded helical structure of DNA is maintained by," we need to analyze the options provided: amide bonds, H-bonds, covalent bonds, and phosphodiester bonds.

### Step-by-Step Solution:

1. **Understand the Structure of DNA**:
- DNA (deoxyribonucleic acid) 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. **Identify the Bonds in DNA**:
- The strands of DNA are held together by different types of bonds:
- **Covalent Bonds**: These bonds form the backbone of each DNA strand, linking the sugar and phosphate groups of the nucleotides.
- **Hydrogen Bonds (H-bonds)**: These bonds occur between the nitrogenous bases of the two strands. Specifically, adenine (A) pairs with thymine (T) through two hydrogen bonds, and guanine (G) pairs with cytosine (C) through three hydrogen bonds.
- **Phosphodiester Bonds**: These are a type of covalent bond that connects the phosphate group of one nucleotide to the sugar of the next nucleotide in the same strand.

3. **Determine the Key Bonds for Helical Structure**:
- While covalent and phosphodiester bonds are crucial for the integrity of each individual strand, it is the hydrogen bonds between the complementary bases that hold the two strands together, allowing the double helix structure to form.

4. **Conclusion**:
- Therefore, the double-stranded helical structure of DNA is primarily maintained by hydrogen bonds (H-bonds) between the nitrogenous bases of the two strands.

### Final Answer:
The double-stranded helical structure of DNA is maintained by **H-bonds** (hydrogen bonds).

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