A Levels Biology (9700)•9700/11/M/J/19
Explanation
Calculating missing base count from total strand length Steps:
- DNA is 19 base pairs long, so each strand has 19 nucleotides.
- Strand 1 has 7 A bases, 4 C bases, and 5 G bases.
- Sum of known bases: 7 + 4 + 5 = 16.
- T bases in strand 1: 19 - 16 = 3. Why B is correct:
- Total nucleotides per strand is 19; subtracting A + C + G counts gives T = 3. Why the others are wrong:
- A. 2: Incorrect subtraction; total known bases sum to 16, not 17.
- C. 5: Matches G count, not T.
- D. 7: Matches A count, not T.
Final answer: B
Topic: Structure of nucleic acids and replication of DNA
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