IB Biology · Theme D: Continuity and change · SL and HL
D1.1 DNA replication
A one-page summary of D1.1 DNA replication, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How is new DNA produced?
How has knowledge of DNA replication enabled applications in biotechnology?
What D1.1 covers
Copying the genome accurately
- D1.1.1Every division needs an exact copy
- D1.1.2Half old, half new
- D1.1.2Meselson and Stahl: one band, then two
- D1.1.2 · Data bookletThe bases and their pairs are in the data booklet
- D1.1.3Unwinding before copying begins
PCR and gel electrophoresis — DNA in the lab
- D1.1.4Copying DNA outside a cell
- D1.1.4Sorting DNA by size
- D1.1.5One technique, many uses
HL — the replication fork in detail
- D1.1.6 · HLDNA polymerase only builds one way
- D1.1.7 · HLOne strand smooth, one strand in pieces
- D1.1.8 · HLStarting and extending each strand
- D1.1.8 · HLRemoving primers, sealing the gaps
- D1.1.9 · HLCatching mistakes as they happen
D1.1 DNA replication: summary
Why replicate
- DNA replication produces two exact copies — needed for growth, tissue replacement and reproduction.
- It copies rather than mixes alleles; rare errors that are not corrected become mutations (D1.3).
Semi-conservative model
- Each new molecule = one original strand + one new strand; complementary base pairing gives high accuracy.
- Meselson–Stahl's density-gradient experiment provided the classic evidence for this model.
Helicase & DNA polymerase
- Helicase unwinds the double helix, breaking hydrogen bonds between strands.
- DNA polymerase builds a new strand from a template (SL: general function only).
PCR & gel electrophoresis
- PCR amplifies DNA using primers, Taq polymerase and repeated heating/cooling cycles.
- Gel electrophoresis separates fragments by size using an electric current; more markers tested = lower chance of a false match.
HL · Leading & lagging strands
- DNA polymerase only builds 5'→3': the leading strand is continuous, the lagging strand is discontinuous (Okazaki fragments).
- Primase starts each fragment; polymerase III extends it; polymerase I replaces primers; ligase seals the gaps.
HL · Proofreading
- DNA polymerase III proofreads and replaces mismatched nucleotides as it goes, boosting accuracy further.
- Errors that escape proofreading can become permanent mutations (D1.3).
Key terms
- Semi-conservative replication
- A model of DNA replication in which each new molecule keeps one original strand and one newly synthesized strand.
- Primer
- A short nucleotide sequence that provides the free 3' end DNA polymerase needs before it can begin extending a strand.
- Taq polymerase
- A heat-stable DNA polymerase, originally from a hot-spring bacterium, used to extend new strands during PCR.
- DNA profiling
- Comparing PCR-amplified, individually variable DNA regions to identify or match a person, e.g. in forensics or paternity testing.
- Okazaki fragment HL
- One of the short DNA sections making up the discontinuously synthesized lagging strand.
- DNA proofreading HL
- DNA polymerase III's removal and correction of a mismatched nucleotide immediately after it is added.
Sample exam questions
Three of the 39 multiple-choice questions for D1.1. Try each one before opening the answer.
Question 1. Which statement about the two DNA molecules produced by replication is correct?
- Each has a different base sequence from the original.
- Each is half the length of the original molecule.
- Each has one DNA strand and one RNA strand.
- Both have the same base sequence as the original.
Show the answer
Answer: D. Replication produces exact copies: both new molecules have base sequences identical to the original and to each other.
Question 2. Why is DNA replication needed in a multicellular organism?
- To produce variation between body cells
- For growth and the replacement of damaged tissue
- To supply the energy needed for cell division
- To convert RNA into DNA before mitosis
Show the answer
Answer: B. Growth (more cells) and the replacement of damaged or worn-out tissue both need every new cell to receive a complete, identical copy of the genome.
Question 3. Which statement about DNA replication and genetic variation is correct?
- Replication mixes alleles to give new combinations.
- Replication produces new alleles in every division.
- Replication copies alleles; errors not corrected become mutations.
- Replication takes place only in the cells that go on to make gametes.
Show the answer
Answer: C. Replication copies; it does not mix alleles. New combinations come from meiosis and fertilization, and the rare copying errors that are not corrected become mutations.
Linking questions
Questions that connect D1.1 to other parts of the course, the kind that come up in Paper 2.
- DNA replication requires a supply of nucleotides. Explain how the deoxyribose sugar (B1.1) differs from ribose and why this is essential for DNA stability. (see B1.1)
- The two strands of DNA are held together by hydrogen bonds between complementary bases. How do the properties of hydrogen bonds (A1.1) enable both stability and the separation required for replication? (see A1.1)
- DNA replication occurs before cell division (D2.1). Explain why replication must be completed before mitosis begins, and what would happen if a cell divided with partially replicated DNA. (see D2.1)
- Mutations (D1.3) can arise from errors in DNA replication that escape proofreading. Explain how a single base substitution during replication can lead to a change in the polypeptide sequence. (see D1.3)
Practise D1.1
Study notes, every question and full markschemes for D1.1 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.