Lesson eLearning

A double‑stranded DNA fragment has the sequence 5'-AGCTTAGG-3' on one strand.

hard 3 marks Class 12 · Chemistry · Biomolecules
Answer
Molecular weight ≈ 4.91 × 10³ Da; total H‑bonds = 22

The correct answer is Molecular weight ≈ 4.91 × 10³ Da; total H‑bonds = 22 — Thus the fragment weighs about 4.91 kDa and contains 22 hydrogen bonds.

Step-by-step solution

  1. Write the complementary strand: 3'-TCGAATCC-5'.
  2. Count each base in the combined 16 nucleotides: A = 3, G = 5, C = 5, T = 3.
  3. Calculate the sum of base masses: 3×313 + 5×329 + 5×289 + 3×304 = 4 941 Da.
  4. Each strand of 8 nucleotides has 7 phosphodiester bonds; total bonds = 14.
  5. Subtract 2 Da per bond: mass correction = 14×2 = 28 Da.
  6. Molecular weight = 4 941 Da − 28 Da = 4 913 Da ≈ 4.91 × 10³ Da.
  7. Count hydrogen bonds: AT pairs (2) ×2 = 4, GC pairs (6) ×3 = 18; total = 22.
  8. Thus the fragment weighs about 4.91 kDa and contains 22 hydrogen bonds.
Common mistake: Forgetting to subtract the mass loss due to phosphodiester bond formation or mis‑pairing the complementary bases.

Concept tested: Molecular weight & H‑bonding in nucleic acids

Published · How this solution was prepared and checked

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