Question

The two strands of DNA are held together by

• A. Peptide bonds
• B. Phosphodiester bonds
• C. S—S bonds
• D. Hydrogen bonds

Answer 1

The correct option is D Hydrogen bonds

Part 1: Peptide bonds
Peptide bonds link the adjacent amino acids in a polypeptide chain.

Part 2: Phosphodiester bonds
DNA is a double stranded helical molecule in which each strand is composed of repeating units of deoxyribonucleotides. Each deoxyribonucleotide is composed of a deoxyribose sugar with a phosphate attached to its $5$’ carbon and a nitrogenous base attached to its $1$’ carbon. The adjacent nucleotides are connected by phosphodiester bonds.

Part 3: Hydrogen bonds
The sugar and phosphodiester bonds form the backbone of each DNA strand and the nitrogenous bases of each strand are projected inwards and form complementary base pairings. The two strands of DNA are held together by the hydrogen bonds between the complementary bases.
Adenine pairs with thymine using $2$ hydrogen bonds and cytosine pairs with guanine using $3$ hydrogen bonds. The base pairs are stacked one upon the other like the rungs of a ladder.


Part 4: S-S bonds
S-S bonds or disulphide bonds are present between the sulphur containing amino acids in a protein. They help in stabilising the folded polypeptide chain to form secondary, tertiary or quaternary structures.

Final answer
(c) Hydrogen bonds