Correct answer: (B) Phosphate group

Explanation:

The negative charge on the DNA backbone is due to the phosphate groups in the sugar-phosphate backbone.

• Each phosphate group (PO₄³⁻) carries a negative charge, making DNA a negatively charged molecule.
• During electrophoresis, the negatively charged DNA moves toward the anode (+) under the influence of an electric field.
• The force required to accelerate the DNA molecules is directly proportional to the number of phosphate groups, because each nucleotide contributes one phosphate group to the backbone.

Why Not the Other Options?

• (A) Sugar molecules ❌

  • The deoxyribose sugar in DNA does not contribute to the charge; it only provides structural support.

• (C) Nitrogenous base ❌

  • The bases (A, T, G, C) are uncharged at neutral pH, so they do not influence DNA’s migration in electrophoresis.

• (D) Both phosphate and sugar molecules ❌

  • While the sugar is part of the backbone, it does not carry a charge; only phosphate groups contribute to the negative charge.

Final Answer:

✅ (B) Phosphate group