Explain DNA double-helix structure and base pairing
Two antiparallel DNA strands form a double helix with complementary base pairs and phosphodiester-linked sugar–phosphate backbones.
Section 6.1 · Structure of nucleic acids and replication of DNA
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain DNA, RNA and protein synthesis. Written practice is self-checked; it does not replace teacher review or practical assessment.
Before you begin
- Recognise that DNA stores genetic information and that proteins are made from amino acids.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which bases pair in DNA?
What you will learn
- Describe complementary base pairing in antiparallel DNA strands
- Relate hydrogen-bond number to A–T and C–G pairs
- Describe phosphodiester bonds in the DNA backbone
Complementary strands run in opposite directions
DNA is a double helix made of two polynucleotide strands. The strands are antiparallel: one runs 5′ to 3′ and the other runs 3′ to 5′. A pairs with T, and C pairs with G, so each strand determines the complementary sequence of the other.
A–T pairs form two hydrogen bonds; C–G pairs form three. Hydrogen bonds join complementary bases across the strands. They are distinct from phosphodiester bonds in each strand’s sugar–phosphate backbone.
Phosphodiester bonds join nucleotides in one strand
A phosphodiester bond links the phosphate of one nucleotide to the sugar of the next, forming the covalent sugar–phosphate backbone. Bases project from this backbone and pair across the helix.
Check both sequence and direction when writing a complementary strand. If the given strand is 5′ to 3′, the aligned complement runs 3′ to 5′. Reverse it only if the question requests the complementary sequence written 5′ to 3′.
Pause and explain
Which bond links nucleotides along one DNA strand?
Worked example
Write the aligned complementary strand to 5′-A C G T-3′ and name the bonds across and along the molecule.
Show the worked solution
- Pair A with T and C with G.
- The complement is antiparallel.
- Hydrogen bonds cross the strands; phosphodiester bonds join nucleotides along a strand.
Answer The aligned complementary strand is 3′-T G C A-5′. Hydrogen bonds hold complementary bases across the strands; phosphodiester bonds link nucleotides along each sugar–phosphate backbone.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the aligned complementary DNA strand to 5′-G A T C-3′, including direction labels.
Give me a hint
Use A–T and C–G pairing, then show the antiparallel direction.
Compare my reasoning
- G pairs with C and A pairs with T.
- The complement is written 3′ to 5′ when aligned.
The aligned complementary strand is 3′-C T A G-5′.
Look for these in your work
- All base pairs correct
- Antiparallel direction labelled
Compare the bonds across DNA strands with those along one strand.
Give me a hint
Name the pair-linking and backbone-linking bonds.
Compare my reasoning
- Hydrogen bonds join complementary bases across the helix.
- Phosphodiester bonds join adjacent nucleotides along a backbone.
Hydrogen bonds join complementary base pairs across the two strands. Phosphodiester bonds link nucleotides along each sugar–phosphate backbone.
Look for these in your work
- Across-strand bond correct
- Backbone bond correct
A DNA fragment has six A–T pairs and four C–G pairs. Calculate the total number of hydrogen bonds between these base pairs.
Give me a hint
Use two per A–T pair and three per C–G pair.
Compare my reasoning
- A–T contributes 6 × 2 = 12 bonds.
- C–G contributes 4 × 3 = 12 bonds.
Total hydrogen bonds = (6 × 2) + (4 × 3) = 24.
Look for these in your work
- Both contributions calculated
- Total correct
Common mistakes
- Writing both strands in the same direction without changing the sequence
- Calling hydrogen bonds the covalent backbone bonds
- Pairing A with C or G with T
What can you explain now?
Which bases pair in DNA?
Compare with the explanation
A with T, and C with G
These are the complementary DNA pairs.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How do antiparallel direction, complementary pairing, hydrogen bonds and phosphodiester bonds organise DNA?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.