Use the helix model to explain base pairing and strand direction
Three steps, the way the exam actually works: work through the lab, write down your own observations, then answer a 6-point free response. What you recorded goes to the grader with your writing, so a conclusion that does not follow from your own evidence will cost you the point — exactly as it would with a real reader.
Predict before you look
- Adenine and guanine are purines; thymine and cytosine are pyrimidines. Why can a purine not pair with another purine inside the double helix?
- What does "antiparallel" mean for two strands, and which chemical feature of the backbone gives a single strand a direction at all?
Nothing to submit here — these are to think through, so the prediction below is an informed one rather than a guess.
Commit to an answer now. It is not graded and being wrong costs nothing — the point is to have something specific to reconcile against once you have the data.
Answer every prediction to unlock the lab. A sentence is enough.
Run the investigation
Predictions first
The procedure and the simulation unlock once you have committed above. Observing before predicting is how a wrong intuition survives a lab intact.
Record what you found
This is your reading of the source, not ours. The grader sees them, so your conclusions have to follow from what you actually recorded.
| Base paired with adenine | |
|---|---|
| Base paired with guanine | |
| What forms the outside backbone | |
| Relationship between strand directions |
Answer the free response
The DNA model makes structure visible. (a) Explain how complementary base pairing supports semiconservative replication. (b) State what “antiparallel” means in DNA and explain why the strands must run in opposite directions for replication enzymes to work correctly. (c) Explain why a change from G–C to A–T can matter even though the helix diameter stays nearly constant. (d) A student says the sugar–phosphate backbone is the information-bearing part of DNA because it is on the outside where enzymes can reach it. Evaluate that claim.
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