Question

Which type of bond characterizes the primary structure of DNA?

• A. Hydrogen bond
• B. Ionic bond
• C. Phosphodiester bond
• D. Disulfide bond

Hint:

DNA backbone = Sugar + Phosphate
Nucleotides linked by → {Phosphodiester bonds}

Answer 1

The Correct Option is C

This question examines the chemical bonds that define the structure of DNA. The structure of DNA is hierarchical: the primary structure is the linear sequence of nucleotides in a single strand, while the secondary structure is the famous double helix formed by two complementary strands. Different chemical bonds are responsible for each level of structure.
Understanding the Question The question asks to identify the specific type of covalent bond that links nucleotides together to form the backbone of a single DNA strand, which constitutes its primary structure.
Key Concepts and Approach The key is to distinguish between the bonds *within* a single DNA strand (forming the backbone) and the bonds *between* the two strands. The approach is to identify the chemical linkage that creates the sugar-phosphate backbone.
Detailed Solution
Defining Primary Structure: The primary structure of DNA is the sequence of its nucleotides (A, C, G, T) linked together in a chain. This chain is formed by a repeating sugar-phosphate backbone.
The Backbone Bond: The nucleotides in this backbone are joined by strong covalent bonds called phosphodiester bonds. This bond connects the 5' carbon of one deoxyribose sugar to the 3' carbon of the next via a phosphate group. This creates the directionality (5' to 3') of the DNA strand.
Distinguishing Other Bonds:
Hydrogen bonds are weaker bonds that form between the nitrogenous bases of the two opposing strands (A with T, G with C). They are responsible for holding the double helix together (secondary structure), not forming the primary structure.
Disulfide bonds are found in proteins, not DNA.

Conclusion: The defining bond of DNA's primary structure is the phosphodiester bond.