What you need to know (based on the AQA specification)
What you need to know (based on the AQA specification)
- The semi-conservative replication of DNA ensures genetic continuity between generations of cells.
- The process of semi-conservative replication of DNA in terms of:
- unwinding of the double helix
- breakage of hydrogen bonds between complementary bases in the polynucleotide strands
- the role of DNA helicase in unwinding DNA and breaking its hydrogen bonds
- attraction of new DNA nucleotides to exposed bases on template strands and base pairing
- the role of DNA polymerase in the condensation reaction that joins adjacent nucleotides.
When does DNA replication occur — and why?
Every time a cell divides, each daughter cell must receive a complete, identical copy of the DNA. To make this possible, DNA must be replicated before division begins. This happens during the S phase of interphase in the cell cycle.
If a cell divided without first copying its DNA, each daughter cell would receive only half the genetic information.
Semi-Conservative Replication
DNA replication is semi-conservative. This means that each new DNA molecule contains one original strand and one newly-made strand
- Semi = half; Conservative = kept the same
- Each daughter DNA molecule keeps half of the original parent DNA (one strand) and gains one new strand, built by complementary base pairing with the original strand (called the template strand)
- This is what makes the copy identical to the parent, and what preserves the genetic code across cell divisions
The Process of Semi-Conservative Replication
Before reading the steps, make sure you’re confident with nucleotide structure and base pairing. Replication is essentially un-pairing the original bases, then re-pairing them with new free nucleotides.
Step 1 — DNA helicase unwinds the double helix
- The enzyme DNA helicase breaks the hydrogen bonds between complementary base pairs (A–T, C–G)
- This unwinds the double helix and separates the two polynucleotide strands
- Each separated strand now acts as a template for a new strand
Step 2 — Free DNA nucleotides align by complementary base pairing
- Free DNA nucleotides (present in the nucleus) are attracted to the exposed bases on each template strand
- They line up by complementary base pairing:
- A pairs with T
- C pairs with G
- Hydrogen bonds form between the bases of the template and the incoming free nucleotides
Step 3 — DNA polymerase joins the nucleotides
- The enzyme DNA polymerase moves along the template strand
- It catalyses condensation reactions between adjacent nucleotides on the new strand
- These condensation reactions form phosphodiester bonds between the sugar of one nucleotide and the phosphate of the next, building the sugar–phosphate backbone
- The result is a new polynucleotide strand that is complementary to the template
Step 4 — Two identical DNA molecules
- There are now two complete DNA molecules, each made of one original strand + one new strand (semi-conservative)
- The two molecules are genetically identical to the original, and to each other
Summary of Enzymes
| Enzyme | Role |
|---|---|
| DNA helicase | Unwinds the double helix and breaks the hydrogen bonds between complementary bases |
| DNA polymerase | Catalyses condensation reactions that form phosphodiester bonds between adjacent nucleotides on the new strand |
Don't mix up the enzymes!
- DNA polymerase ≠ RNA polymerase. RNA polymerase does an equivalent job during transcription (covered in DNA & protein synthesis), but that’s a different process
- DNA polymerase only forms phosphodiester bonds (the sugar–phosphate backbone), NOT hydrogen bonds between complementary bases
- DNA helicase BREAKS hydrogen bonds — it does NOT hydrolyse them (no water added).
Meselson–Stahl experiment
What you need to know (based on the AQA specification)
What you need to know (based on the AQA specification)
Students should be able to evaluate the work of scientists in validating the Watson–Crick model of DNA replication.
When Watson and Crick proposed their double helix model in 1953, they suggested DNA replication was semi-conservative. But two other models were also plausible at the time, and scientists needed evidence to decide between them.
The three competing models
Each model makes a different prediction about what daughter DNA molecules look like after one round of replication:
The Meselson–Stahl experiment (1958)
Meselson and Stahl tested which model was correct using nitrogen isotopes:
- They grew bacteria in a medium containing ¹⁵N (heavy nitrogen) for many generations, so all the bacterial DNA was “heavy”
- They then transferred the bacteria to a medium containing only ¹⁴N (normal, lighter nitrogen)
- As the bacteria divided, any new DNA strands used the lighter ¹⁴N, while original strands kept their heavier ¹⁵N
- They extracted DNA after each generation and used centrifugation to separate it by density (heavier DNA sinks lower in the tube)
What semi-conservative predicts (and what was observed)
Generation 0 — all DNA is heavy (both strands ¹⁵N). One band at the bottom of the tube.
Generation 1 — every parent DNA molecule has been replicated once. Under semi-conservative replication:
- The two original heavy strands separate
- Each acts as a template for a new light strand built from ¹⁴N nucleotides
- Every daughter molecule = 1 heavy strand + 1 light strand → intermediate density → ONE intermediate band
Generation 2 — each Gen 1 molecule replicates again:
- Each Gen 1 molecule has 1 heavy strand (¹⁵N) + 1 light strand (¹⁴N). These separate — each acts as a template.
- The heavy strand acts as template: free ¹⁴N nucleotides pair with it. The newly-built strand is light.
- Daughter = 1 heavy + 1 new light = intermediate density
- The light strand acts as template: free ¹⁴N nucleotides pair with it. The newly-built strand is light.
- Daughter = 1 light + 1 new light = light density
- So every Gen 1 molecule produces one intermediate daughter + one fully-light daughter
Why this rules out the other two models
Why does Generation 1 rule out the conservative model?
Why does Generation 1 rule out the conservative model?
- Conservative model predicts the original heavy double helix stays completely intact — both old strands stay together
- So at Gen 1 you’d expect two bands — one heavy (the intact parent) and one light (the new molecule)
- But the actual result shows only one intermediate band — no heavy band, no light band
Why does Generation 2 rule out the dispersive model?
Why does Generation 2 rule out the dispersive model?
- Dispersive model predicts each new molecule is a random patchwork of old heavy fragments and new light fragments
- At Gen 1 this would also produce one intermediate band — so Gen 1 alone cannot distinguish dispersive from semi-conservative
- At Gen 2, the actual result shows two distinct bands — one intermediate, one fully light. Dispersive would predict more than two bands (or a broad smear) — because random fragmentation means different molecules end up with different proportions of ¹⁵N and ¹⁴N fragments, giving a range of densities. No molecule would be fully light this early.
Only semi-conservative replication matches both the Gen 1 result AND the Gen 2 result — which is why it’s the accepted model.
For a worked exam question using exactly this experiment, see the third question in the Exam Question Practice section below.
Why did Meselson and Stahl use nitrogen isotopes, not carbon or oxygen?
Why did Meselson and Stahl use nitrogen isotopes, not carbon or oxygen?
Nitrogen is a key component of the nitrogenous bases in DNA, and it’s much less common in other cell components than carbon or oxygen. Using ¹⁵N labels almost exclusively the DNA, making the density difference between old and new strands detectable. Carbon or oxygen would label many other molecules too, making the density difference between old and new DNA strands much harder to detect.
Exam Question Practice
Describe the role of DNA polymerase in the semi-conservative replication of DNA.
(2 marks)Hint
What does it JOIN? What kind of reaction does it catalyse? What kind of bond does it form?
Mark Scheme
- Joins (adjacent DNA) nucleotides (1 mark)
- By catalysing condensation reactions (1 mark)
- (Catalyses the formation of) phosphodiester bonds (between adjacent nucleotides) (1 mark)
Max 2 marks.
Tips from examiner reports
- DNA polymerase does NOT form hydrogen bonds — those form spontaneously between bases.
- DNA polymerase does NOT form complementary base pairs — those happen by attraction.
- DNA polymerase catalyses addition of nucleotides, forming phosphodiester bonds in the backbone (NOT between bases).
- Many students confuse “bases” and “nucleotides” — be precise.
Scientists determined the percentage of heart cells undergoing DNA replication using a chemical called BrdU. Cells use BrdU instead of nucleotides containing thymine during DNA replication.
Describe how BrdU would be incorporated into new DNA during semi-conservative replication.
(5 marks)Hint
Walk through the steps: how does the double helix open, how does BrdU pair up, and which enzyme joins it into the new strand?
Mark Scheme
- DNA helicase (1 mark)
- Breaks hydrogen bonds (between the two DNA strands) (1 mark)
- BrdU is complementary to adenine on the template strand (BrdU forms hydrogen bonds with adenine) (1 mark)
- DNA polymerase joins (adjacent) nucleotides to incorporate BrdU into the new DNA strand (1 mark)
- Phosphodiester bonds form (between nucleotides) (1 mark)
Comments from mark scheme
- Reject “hydrolyses hydrogen bonds” — helicase BREAKS them (no water added)
- Accept “H bonds” for hydrogen bonds
- Reject if DNA polymerase is said to catalyse complementary base pairing or to join nucleotides to the template strand directly
Tips from examiner reports
- Common remaining error: writing “A” (abbreviation) instead of “adenine” — write the full base name.
- DNA polymerase forms phosphodiester bonds between nucleotides — NOT between bases.
- Do NOT describe breaking of hydrogen bonds as hydrolysis — they are simply broken (no water added).
Scientists grew bacteria in ¹⁵N then transferred them to ¹⁴N. At intervals they removed samples, isolated the DNA, and measured its density. DNA made using ¹⁵N has a higher density than DNA made using ¹⁴N.
Figure A shows the three possible models (P, Q, R) predicting what density bands you’d see after each generation:
Figure B shows the scientists’ actual results:
Which of these models, P, Q or R, is supported by the results in Figure B? Give the letter and name of the model supported, and explain why the results do not support the other models.
(3 marks)Hint
For each rejected model, think about WHAT pattern of density peaks it would predict, and why the data shows something different.
Mark Scheme
- Model Q — Semi-conservative replication (1 mark)
- Model P (conservative) is unsupported because there should be two peaks in generation 1 (one heavy, one light) — but only one peak is shown (1 mark)
- Model R (dispersive) is unsupported because there should be MORE than 2 peaks in generation 2 / 3 OR one wide, overlapping peak in generation 3 — but the data shows two distinct peaks (1 mark)
Comments from mark scheme
- Accept answers in either order for marks 2 and 3
- Accept “density” or “distribution” for “peak”
- For “>2” accept “many” or “several”
Tips from examiner reports
- This question rewards careful interpretation of evidence: a good answer names the supported model AND explains what each rejected model would predict instead.
- Don’t just say “the data doesn’t fit” — say WHAT pattern would be expected and why the actual data differs.
Comments from mark scheme