Use base-pair totals to check a DNA model
Calculate paired-molecule counts without assuming single-strand equality.
Section B4.1 · Biological molecules
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-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.
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
How many bases occur in 15 base pairs?
What you will learn
- Calculate paired-molecule counts without assuming single-strand equality.
- Explain the conclusion from stated evidence and recognise its limits.
Count both strands
Each base pair contains two bases. A double-stranded segment with 24 base pairs therefore contains 48 bases in total. The total A count equals total T, and total C equals total G in the required paired model.
Use the stated whole-molecule scope. If total A is supplied, total T matches it; the remaining bases are divided equally between C and G. These calculations are supporting practice in pairing, not a separate syllabus demand for DNA percentage formulae.
Recognise unsupported claims
A and T do not each have to be 25% of every DNA molecule; different paired molecules can have different overall compositions. Equality within each complementary pair leaves many possible compositions.
Knowing total composition does not reveal the exact base sequence or prove a relationship between organisms. Counts ignore order. Classification comparisons require aligned sequence evidence, not just matching totals.
Sources for this lesson
Pause and explain
Do matching overall base counts prove two sequences are identical?
Worked example
A paired DNA segment contains 40 bases total, with 12 A. Find T, C and G totals.
Show the worked solution
- T = 12 because A pairs with T.
- Remaining bases = 40 − 24 = 16.
- C and G share those equally.
Answer T = 12, C = 8 and G = 8.
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.
How many bases are in 18 base pairs?
Give me a hint
Two per pair.
Compare my reasoning
- 18 × 2 = 36.
36 bases.
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 paired segment has 60 bases including 18 C. Find G, A and T.
Give me a hint
Match C and G then divide the remainder.
Compare my reasoning
- G = 18.
- 60 − 36 = 24 remain.
- A = T = 12.
G = 18, A = 12 and T = 12.
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.
Two segments have identical base totals. What extra evidence is needed to establish identical sequence?
Give me a hint
Restore the missing ordering information.
Compare my reasoning
- Compare aligned base order at each position.
- Equal counts alone are insufficient.
The aligned sequence of bases, not just totals.
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
- Different arrangements can have matching counts.
- Complementary equality does not fix those percentages.
What can you explain now?
Do matching overall base counts prove two sequences are identical?
Compare with the explanation
No, base order can differ.
Counts do not preserve sequence order.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Check a new paired-molecule count using equality and the total sum.
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.