Base pairing and Chargaff's rule: Class 12 practice
In double-stranded DNA, A pairs with T and G pairs with C, so %A = %T and %G = %C, and A + G = 50%. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- DNA structure
The idea behind base pairing and chargaff's rule
In double-stranded DNA, A pairs with T and G pairs with C, so %A = %T and %G = %C, and A + G = 50%. The complementary strand is read by swapping A↔T and G↔C.
Step-by-step method
- Use A = T and G = C.
- A + T + G + C = 100%.
- Write the complementary strand base by base.
Worked example 1 (Basic)
A DNA sample has 16% adenine. Find the percentages of T, G and C.
- T = 16%
- G = C = (100 − 32)/2 = 34%
- Answer: T 16%, G 34%, C 34%
Worked example 2 (Standard)
Write the complementary strand of 5′-TCTACTACAAT-3′.
- Swap A↔T, G↔C
- Answer: 3′-AGATGATGTTA-5′
Worked example 3 (Challenge)
The template strand is 3′-ACAATACTTTCT-5′. Write the mRNA and the number of complete codons.
- mRNA copies the coding strand with U for T: 5′-UGUUAUGAAAGA-3′
- 4 codons
- Answer: 5′-UGUUAUGAAAGA-3′; 4 codons
Common mistakes and misconceptions
- Pairing A with G
- Forgetting the percentages total 100
Practice questions
- (Basic) A DNA sample has 14% adenine. Find the percentages of T, G and C.
- (Basic) A DNA sample has 31% adenine. Find the percentages of T, G and C.
- (Standard) Write the complementary strand of 5′-TCATAGCCGTA-3′.
- (Standard) Write the complementary strand of 5′-CAGTGAGGACG-3′.
- (Challenge) The template strand is 3′-AACCGAAGAAAC-5′. Write the mRNA and the number of complete codons.
- (Challenge) The template strand is 3′-CAGATCTAGTTT-5′. Write the mRNA and the number of complete codons.
Answer key with full working
- Q1: T = 14% → G = C = (100 − 28)/2 = 36% → Answer: T 14%, G 36%, C 36%
- Q2: T = 31% → G = C = (100 − 62)/2 = 19% → Answer: T 31%, G 19%, C 19%
- Q3: Swap A↔T, G↔C → Answer: 3′-AGTATCGGCAT-5′
- Q4: Swap A↔T, G↔C → Answer: 3′-GTCACTCCTGC-5′
- Q5: mRNA copies the coding strand with U for T: 5′-UUGGCUUCUUUG-3′ → 4 codons → Answer: 5′-UUGGCUUCUUUG-3′; 4 codons
- Q6: mRNA copies the coding strand with U for T: 5′-GUCUAGAUCAAA-3′ → 4 codons → Answer: 5′-GUCUAGAUCAAA-3′; 4 codons
Syllabus note
This skill is listed under "Molecular Basis of Inheritance" for Class 12 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is base pairing and chargaff's rule in?
"Molecular Basis of Inheritance", Class 12 Biology (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.