Explain how the antiparallel orientation of the two strands in a DNA double helix arises from the structure of nucleotides and the bonds formed between them.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Each nucleotide has a 5' phosphate group and a 3' hydroxyl group on the deoxyribose, giving the strand a directionality.
Phosphodiester bonds link the 3'–OH of one nucleotide to the 5' phosphate of the next, so a polynucleotide runs 5'→3'.
Hydrogen bonds between complementary bases can only be satisfied when the two strands run in opposite directions, because the geometry of base pairing requires the sugars to face opposite ways.
Thus one strand is 5'→3' while the complementary strand is 3'→5', giving the double helix an antiparallel orientation.
Phosphodiester bonds link the 3'–OH of one nucleotide to the 5' phosphate of the next, so a polynucleotide runs 5'→3'.
Hydrogen bonds between complementary bases can only be satisfied when the two strands run in opposite directions, because the geometry of base pairing requires the sugars to face opposite ways.
Thus one strand is 5'→3' while the complementary strand is 3'→5', giving the double helix an antiparallel orientation.
Examiner tips
- Use the term ‘polarity’ and ‘phosphodiester bond’. Show the 5'→3' direction of each strand. Explain that base‑pair geometry forces opposite orientation. Finish with the conclusion that strands are antiparallel.
Common mistakes
- Forgetting to mention the 5' and 3' ends. Saying strands are parallel instead of antiparallel. Not linking the phosphodiester bond to strand directionality.
Mark scheme (4 marks)
- Each nucleotide has a 5' phosphate group and a 3' hydroxyl group on the deoxyribose sugar, giving each strand a directionality (polarity).
- Phosphodiester bonds form between the 3'–OH of one nucleotide and the 5' phosphate of the next, so each polynucleotide strand runs in a 5'→3' direction.
- Hydrogen bonds between complementary bases can only form when the two strands run in opposite (antiparallel) directions, because the geometry of base pairing requires the sugars to face opposite ways.
- Therefore one strand runs 5'→3' while the complementary strand runs 3'→5', making the two strands antiparallel.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Nucleic acids questions
- Explain how the structure of a DNA nucleotide differs from that of an RNA nucleo…
- Explain how the antiparallel orientation of the two strands in a DNA double heli…
- Explain how the structure of a nucleotide monomer enables DNA to store genetic i…
- Explain how the structure of RNA differs from that of DNA in ways that make RNA …
- Explain how the structure of the DNA double helix allows it to be both stable un…
- Explain how differences in the base composition of RNA compared with DNA relate …