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 best explains why DNA replication is essential in multicellular organisms?
- It produces genetic variation among the organism's cells
- It allows growth (increase in cell number) and the replacement of damaged or worn-out tissue
- It supplies the energy multicellular organisms need for movement
- It converts RNA back into DNA in every cell
Show the answer
Answer: B. Every new cell produced by mitosis needs a complete, identical copy of the genome. DNA replication supplies that copy before each division, which is what makes growth and tissue replacement possible in a multicellular body.
Question 2. A cell enters mitosis without having fully replicated its DNA. What is the most likely direct consequence?
- The daughter cells will each receive a complete, extra copy of the genome
- The cell will produce more ATP than normal
- The daughter cells will not each receive a complete copy of the genetic material
- The rate of transcription will increase to compensate
Show the answer
Answer: C. Mitosis distributes one full set of chromosomes to each daughter cell on the assumption that replication is already complete. If replication is incomplete, at least one daughter cell will end up with missing genetic information, which is generally lethal or severely damaging to the cell.
Question 3. A mutation eliminates the 3′→5′ exonuclease (proofreading) activity of DNA polymerase III, while leaving its ability to add nucleotides unaffected. What is the most likely consequence?
- DNA replication would stop completely, since synthesis could no longer occur
- Okazaki fragments would no longer be joined together
- RNA primers would no longer be removed from the lagging strand
- The rate of nucleotide misincorporation (errors) during replication would increase significantly
Show the answer
Answer: D. Without the ability to detect and excise mismatched nucleotides, errors that would normally be caught and corrected during synthesis are instead left in place, so the overall error (mutation) rate of replication rises. Synthesis itself can still proceed, since the polymerase activity that adds nucleotides is unaffected.
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.