IB Biology D1.1 DNA replication: practice questions

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D1.1-01 · Paper 1A style · multiple choice · 1 mark

Heavy nitrogen and replication

Bacteria were grown for many generations in a medium in which the only source of nitrogen was the heavy isotope ¹⁵N. They were then moved to a medium containing only the light isotope ¹⁴N, where each cell replicated its DNA twice.

After the two rounds of replication, what percentage of the DNA molecules would contain both ¹⁵N and ¹⁴N? [1]

1. 0 %
2. 25 %
3. 50 %
4. 100 %

Show the answer and the reasoning

Answer: C. C is correct. After the first round every molecule is hybrid: one heavy parental strand and one new light strand. In the second round each hybrid molecule separates; its heavy strand gives one hybrid molecule and its light strand gives one all-light molecule, so half the molecules are hybrid. A is what a conservative model would predict (parental molecule stays heavy, new molecules are all light). B confuses strands with molecules: 25 % of all the strands are heavy, but each heavy strand sits in a different molecule. D is the answer after only one round of replication, and also what a dispersive model (old and new DNA mixed along every strand) would predict after two.

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The 5 questions in D1.1

1. D1.1-01 Heavy nitrogen and replicationPaper 1A style · multiple choice · 1 mark
2. D1.1-02 Leading and lagging strandsPaper 1A style · multiple choice · 1 mark · HL
3. D1.1-03 Separating DNA fragmentsPaper 1A style · multiple choice · 1 mark
4. D1.1-04 Copying the base sequencePaper 2 style · short answer · 7 marks
5. D1.1-05 Amplifying crime-scene DNAPaper 1B style · data-based · 10 marks

More of D Continuity and change

• D1.2 Protein synthesis
• D1.3 Mutation and gene editing
• D2.1 Cell and nuclear division
• D2.2 Gene expression
• D2.3 Water potential
• D3.1 Reproduction
• D3.2 Inheritance
• D3.3 Homeostasis
• D4.1 Natural selection
• D4.2 Stability and change
• D4.3 Climate change

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Every Biology and Chemistry question here is original, written for practice in the style of the papers set since the 2025 syllabus, with invented data. Developed independently from the International Baccalaureate Organization, which has not endorsed it. Found a mistake? Email vytis@limeadmissions.com with the question id.