Classify purines and pyrimidines
Adenine and guanine are double-ring purines; cytosine, thymine and uracil are single-ring pyrimidines.
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 two bases are purines?
What you will learn
- Classify adenine and guanine as purines
- Classify cytosine, thymine and uracil as pyrimidines
Two ring classes
Purines have a double-ring structure: adenine (A) and guanine (G) are purines. Pyrimidines have a single-ring structure: cytosine (C), thymine (T) and uracil (U) are pyrimidines. Structural formulae are not required.
A quick memory cue is that the two purines are A and G. The remaining three common nucleic-acid bases, C, T and U, are pyrimidines.
DNA and RNA use different sets
DNA uses A, T, C and G. RNA uses A, U, C and G; uracil replaces thymine. Both molecules therefore contain purines A and G, while the pyrimidine set differs by T or U.
The ring class is determined by the base, not by whether it is in a nucleotide, a DNA strand or an mRNA molecule.
Pause and explain
Which base is a pyrimidine found in RNA?
Worked example
Classify the bases in the DNA sequence A–C–G–T and the RNA base U.
Show the worked solution
- A and G are purines.
- C, T and U are pyrimidines.
Answer A and G are purines. C, T and U are pyrimidines.
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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.
Classify A, G, C, T and U as purines or pyrimidines.
Give me a hint
Group A and G together first.
Compare my reasoning
- A and G are purines.
- C, T and U are pyrimidines.
Adenine and guanine are purines. Cytosine, thymine and uracil are pyrimidines.
Look for these in your work
- Both purines correct
- All three pyrimidines correct
An RNA sequence contains A, U, G and C. State how many purines and how many pyrimidines it contains.
Give me a hint
Classify each of the four bases.
Compare my reasoning
- A and G are purines.
- U and C are pyrimidines.
The sequence contains two purines (A and G) and two pyrimidines (U and C).
Look for these in your work
- Purines counted
- Pyrimidines counted
A learner says “uracil is a purine because it replaces thymine in RNA.” Explain the error.
Give me a hint
Replacement in RNA does not change a base’s ring class.
Compare my reasoning
- Thymine and uracil are both pyrimidines.
- Purines are adenine and guanine.
The claim is incorrect: uracil and thymine are both pyrimidines. Adenine and guanine are the two purines.
Look for these in your work
- U correctly classified
- Purine pair stated
Common mistakes
- Classifying thymine as a purine because it occurs in DNA
- Forgetting that uracil is a pyrimidine
- Assuming RNA contains thymine instead of uracil
What can you explain now?
Which two bases are purines?
Compare with the explanation
Adenine and guanine
A and G are the double-ring purines.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which bases are purines, which are pyrimidines, and which base replaces thymine in RNA?
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.