Apply complementary DNA base pairing
Use A with T and C with G in aligned paired strands.
Section B4.1 · Biological molecules
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall the distinction between an element, a molecule and a cell structure.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which base pairs with C?
What you will learn
- Use A with T and C with G in aligned paired strands.
- Explain the conclusion from stated evidence and recognise its limits.
Pair one position at a time
DNA bases pair in fixed combinations: A with T, and C with G. The same rule works in reverse, so T pairs with A and G with C. The full chemical base names are not required in this syllabus statement.
For an aligned schematic, write the complement directly opposite each supplied base. Do not copy the same sequence unchanged or pair A with C. A pairing exercise here does not claim to calculate ancestry or a whole genetic trait.
Use the stated direction convention
In a drawing where opposing positions are supplied, preserve the alignment. If a question imposes a particular strand-reading direction, follow it; a directly aligned complement and a reverse-reading string need not be written identically.
One double-stranded molecule has matching total A and T counts and matching total C and G counts. An individual single strand need not contain equal counts of complementary bases.
Sources for this lesson
Pause and explain
Must A and T counts be equal on one isolated strand?
Worked example
Give the directly aligned complement of A C T G A.
Show the worked solution
- A pairs with T; C with G.
- T pairs with A; G with C.
- The last A pairs with T.
Answer T G A C T.
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.
Give the aligned complement of G T A C.
Give me a hint
Apply each pair.
Compare my reasoning
- G→C, T→A, A→T, C→G.
C A T G.
Look for these in your work
- Include the named molecules or components in your answer.
- Explain the relationship using the stated building blocks or paired-base evidence.
A strand segment is A A C. Give its aligned complement.
Give me a hint
Keep both A positions.
Compare my reasoning
- A→T twice; C→G.
T T G.
Look for these in your work
- Include the named molecules or components in your answer.
- Explain the relationship using the stated building blocks or paired-base evidence.
Why can a strand with three A bases and one T still form valid double-stranded DNA?
Give me a hint
Consider its opposite strand.
Compare my reasoning
- The opposite strand contains three T and one A at matching positions.
- Combined A and T counts balance.
Single-strand counts can differ; complementary pairing balances the combined molecule.
Look for these in your work
- Include the named molecules or components in your answer.
- Explain the relationship using the stated building blocks or paired-base evidence.
Common mistakes
- The opposing strand supplies the matching counts.
- T is one of the required bases.
What can you explain now?
Must A and T counts be equal on one isolated strand?
Compare with the explanation
No; equality applies to totals across the paired molecule.
Opposing bases balance the combined counts.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Write a new five-base segment and its aligned complement, then check every pair.
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.