Week 13 — Lab / Scientific Inquiry · "Strawberry DNA Extraction"

Course: Introduction to Biology — General Biology I (BIOL 101) · Silver Oak University (fictional sample) · Prof. Castellano
Objective: Objective 7 — observe real DNA and connect each extraction step to DNA structure; apply Chargaff's rule to base-percentage data · SLO A (observation & data interpretation) · SLO B (structure → function)
Worth 50 points · Labs group = 15% of the grade · Lab 13
Format: a hands-on at-home protocol (no special equipment) — you'll extract visible DNA from a strawberry, record careful observations, complete a short Chargaff calculation, and then catch the AI's mistakes when it interprets your results.

This is the course's signature weekly component. Every instructional week has one lab. This week's is a simple at-home extraction; other weeks use free virtual simulations (a virtual microscope, PhET, HHMI BioInteractive, LabXchange, Learn.Genetics). All lab resources are links to external sites — nothing to buy or download.

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Part 1 — The Big Picture

This week you learned that a gene is made of DNA — a double helix with a sugar-phosphate backbone, complementary base pairs (A–T, G–C), and the ability to copy itself. So far that's been a picture on a slide. Today you'll do something genuinely surprising: pull real DNA out of a strawberry with kitchen supplies and see it with your own eyes. A strawberry is ideal because most varieties are octoploid (they carry eight sets of chromosomes), so they're packed with DNA — you'll get a visible, stringy white clump.

The phenomenon: DNA is locked inside cells, inside membranes. To set it free and make it visible, each ingredient does one job:
- Dish soap dissolves the fatty cell membranes (and nuclear membranes), releasing the DNA into the liquid.
- Salt helps the DNA molecules clump together by neutralizing their charges (DNA's backbone is negatively charged).
- Cold rubbing alcohol is where DNA is not soluble, so the DNA comes out of solution (precipitates) and you can see it as a white, stringy mass at the alcohol layer.

Background (optional, ~8 min): Amoeba Sisters — "DNA Replication (Updated)" (for the structure + how DNA is copied): 🔗 https://www.youtube.com/watch?v=Qqe4thU-os8

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Part 2 — Your Scientific Question & Hypothesis

This is an observation-based lab (you're isolating and observing a molecule), so your "hypothesis" is a prediction about what you'll see and why.

The question: Can I extract and see the DNA from a strawberry using only household materials — and does each step do what the biology predicts?

Before you start, write your prediction (an "if… then…" statement is perfect):

If I mash the strawberry with soap and salt and then add cold alcohol, then I will see __, because the soap will _, the salt will , and the alcohol will ___.

Write it down now — you'll compare it to what you actually observe. (There's no "wrong" prediction; observing carefully is the point.)

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Part 3 — Materials & Procedure

You need (all common household items):
- 1 ripe strawberry (fresh or thawed) · a zip-top plastic bag · ~½ teaspoon dish soap · a pinch of salt (~¼ teaspoon) · ~3 tablespoons water · a coffee filter (or paper towel) + a cup · a clear glass · cold rubbing alcohol (isopropyl alcohol — keep it in the freezer beforehand) · a wooden skewer or toothpick.

Safety: rubbing alcohol is for external use only — do not drink it, keep it away from flames, and wash your hands after. Adult supervision if needed.

Procedure:
1. Put the strawberry (hull removed) in the zip-top bag, push out the air, seal it, and mash it with your fingers for about a minute until it's a smooth pulp.
2. In a cup, make the extraction liquid: mix ~3 Tbsp water, ½ tsp dish soap, and a pinch of salt. Stir gently (avoid making foam).
3. Pour the extraction liquid into the bag with the mashed strawberry. Reseal and gently mix for about a minute.
4. Place the coffee filter over a clean cup and pour the mixture through it to filter out the pulp. Let the liquid drip through; you want the clear-ish pink filtrate in the cup.
5. Pour the filtrate into a clear glass (about 2 cm deep). Tilt the glass and slowly pour an equal amount of cold rubbing alcohol down the side so it forms a layer on top — do not mix.
6. Wait 1–2 minutes and watch the boundary between the two layers. You should see white, stringy/cloudy DNA appear there. Gently lift some out with the skewer.

No strawberry handy? A banana, kiwi, or green peas work too (mash a small amount). If you can't run it at all, you can still complete the analysis using the observation prompts in Part 6 and the Chargaff calculation in Part 4b — but the extraction takes ten minutes and is worth doing.

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Part 4 — Data: Observations + a Chargaff Calculation

4a. Observation table (fill this in)

Step	What you observed
After mashing the strawberry	______
After mixing in the soap + salt liquid	______
Right after adding the cold alcohol layer	______
The extracted DNA (color, texture, how much)	______

4b. Chargaff calculation (show your work)

Scientists measured the bases in a sample of strawberry DNA and found it is 30% adenine (A). Using Chargaff's rule (%A = %T, %G = %C, and the four bases total 100%), fill in the rest:

Base	Percentage	How you got it
Adenine (A)	30%	given
Thymine (T)	______	______
Guanine (G)	______	______
Cytosine (C)	______	______
Total	______	should equal 100%

Show the arithmetic, e.g.: A + T = %, leaving % for G + C, split evenly → G = C = ___%.

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Part 5 — Identify Your Experiment's Parts

Answer in a sentence each:
1. What was the goal of this procedure (what were you trying to isolate)? __
2. What did the dish soap do at the molecular level? _
3. What did the cold alcohol do, and why does DNA appear there?
4. One controlled condition you kept the same that would matter if you compared two fruits (e.g., amount of fruit, same soap, same alcohol temperature): ___

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Part 6 — Analysis Questions

1. Describe the DNA you extracted — its color and texture. Were you surprised by how much there was? Why do strawberries give so much DNA (hint: octoploid — eight chromosome sets)?
2. Connect each step to the biology: soap → cell/nuclear membranes; salt → helps DNA clump; cold alcohol → DNA precipitates (comes out of solution). In your own words, why was each step necessary?
3. From your Chargaff calculation (4b): if A is 30%, why must T also be 30%, and why are G and C each 20% rather than 30%? Tie your answer to complementary base pairing (A–T, G–C).
4. The clump you extracted holds the entire instruction set for a strawberry, yet it fits in a drop of liquid. Using what you know about DNA's structure (a four-letter sequence, coiled tightly), explain how so much information fits in so little space.
5. Connect it forward: this week was about storing and copying DNA. Next week is about reading it (DNA → RNA → protein). Why does a cell need to copy its DNA accurately before it divides (hint: each daughter cell needs a complete, correct copy)?

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Part 7 — AI-Critique Moment (required — this is the BYOAI step)

Now bring in your approved chatbot (Gemini, Claude, or ChatGPT) and be the scientist who checks its work.

1. Paste your Chargaff data and ask it: "A strawberry DNA sample is 30% adenine. Using Chargaff's rule, what are the percentages of thymine, guanine, and cytosine? Also, which base pairs with adenine, and is DNA replication conservative or semiconservative?"
2. Check everything it says against your own work and this week's lecture:
   - Did it get the Chargaff math right — T = 30%, G = 20%, C = 20% — or did it slip (e.g., say C = 30%, or forget that the remainder splits evenly)? Re-derive it yourself.
   - Did it correctly say adenine pairs with thymine — or did it claim A pairs with guanine or cytosine? (Chatbots do this surprisingly often.)
   - Did it correctly call replication semiconservative — or did it say "conservative"?
3. Write 2–3 sentences reporting what the AI got right and at least one thing you had to correct or watch carefully. (If it happened to get everything right, say how you verified each claim — that's the skill.)

The habit all term: the tool drafts, you judge. A chatbot will confidently mis-pair bases, mis-add the Chargaff remainder, or call replication "conservative" — catching it is the point.

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Part 8 — What to Submit

Submit a single document (or text entry) with: your prediction, your completed observation table (4a), your Chargaff table with the arithmetic (4b), your Part 5 answers, your Part 6 analysis, and your Part 7 AI-critique paragraph. Due Sunday, Dec 6, 11:59 p.m. (50 points).

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Instructor answer key & model data — REMOVE BEFORE PUBLISHING TO STUDENTS

Students collect their own observations, so descriptions vary (DNA usually appears as a white, cloudy, stringy mass at the water–alcohol boundary; strawberries yield a lot because they're octoploid). The model values below are for grading the analysis and arithmetic; all numbers are pre-computed and independently verified by the week's Python quant-gate (PASS).

Model observations (illustrative):
- After mashing: a smooth red/pink pulp.
- After soap + salt: a looser, slightly foamy pink liquid (membranes breaking down).
- After cold alcohol: two layers form; a white, stringy, cloudy mass appears at the boundary.
- Extracted DNA: white/translucent, stringy or mucus-like ("snot-like"), liftable on a skewer.

Chargaff calculation (4b) — pre-computed and verified:
- Given %A = 30%. Because T pairs with A, %T = %A = 30%.
- A + T = 30 + 30 = 60%, leaving 100 − 60 = 40% for G + C.
- Because %G = %C, each = 40 ÷ 2 = 20%. So %G = 20%, %C = 20%.
- Total: 30 + 30 + 20 + 20 = 100% ✓.

Expected answers:
- Part 5: (1) isolate/see the strawberry's DNA; (2) soap dissolves the fatty cell and nuclear membranes, releasing DNA; (3) DNA is insoluble in cold alcohol, so it precipitates (comes out of solution) and collects at the alcohol layer; (4) any reasonable constant: same amount of fruit, same soap/salt amounts, same (cold) alcohol, same mashing time.
- Part 6: (1) white, stringy, surprisingly plentiful — strawberries are octoploid (8 chromosome sets), so lots of DNA. (2) soap frees DNA from membranes; salt neutralizes charge so DNA clumps; cold alcohol precipitates it for viewing. (3) %T = %A = 30% because every A is paired with a T; G and C are each 20% because the remaining 40% (after A + T take 60%) splits evenly between G and C (which are equal) — a direct consequence of complementary base pairing. (4) DNA stores information as a long sequence of four bases in a precise order, repeated billions of times and coiled tightly into chromosomes, so an enormous instruction set fits in a microscopic (or here, a small visible) amount of material. (5) before a cell divides, it must copy its DNA so each daughter cell gets a complete, accurate copy; semiconservative replication + base pairing make that copy faithful.
- Part 7 (AI-critique): full credit for a specific catch — most commonly the AI mis-deriving the Chargaff remainder (saying C = 30% instead of 20%), mis-pairing adenine (with G or C instead of T), or calling replication "conservative." Full credit also if the student verified each AI claim against their own arithmetic and reasoning.

Grading rubric — 50 points

Criterion	Full	Partial	None
Prediction — a clear "if…then…" prediction naming what each ingredient should do (8)	8	4–6	0–2
Observation table (4a) — all four steps observed and described (10)	10	5–8	0–4
Chargaff calculation (4b) — T 30%, G 20%, C 20%, arithmetic shown, total = 100% (12)	12	6–10	0–4
Step → biology (Part 5 + Part 6 #2–3) — soap/salt/alcohol roles + base-pairing reasoning correct (12)	12	6–10	0–4
AI-critique (Part 7) — names a specific thing checked/corrected in the AI's interpretation (8)	8	4–6	0–2

Quality gate (self-checked): every number in the Chargaff calculation is pre-computed and independently re-verified by the week's Python check (30% A → T 30%, G 20%, C 20%; total 100%) — quantitative gate: PASS; the biology of each extraction step is correct (soap → membranes, salt → clumping, cold alcohol → precipitation); base-pairing (A–T, G–C) and complementarity map correctly. No student-collected observation is asserted as "the" answer — the key grades the analysis and the arithmetic, not a specific clump.