Week 13 · Biology Lab & Inquiry

Week 13 Biology Lab & Inquiry: Strawberry DNA Extraction

Introduction to Biology Free sample edition

Part 1 — The Big Picture

This week you learned that DNA is a twisted ladder: sugar-phosphate rails, base-pair rungs that join A with T and G with C, and two strands that are complementary, not identical. In this lab you pull that molecule out of a strawberry at your kitchen counter. Most cultivated strawberries carry eight sets of chromosomes, which is one reason they give up so much DNA. You will run the extraction three ways, leaving out one ingredient at a time, so you can test what each ingredient does: soap breaks open the cell membranes, salt helps the DNA strands clump together, and cold alcohol brings the DNA out of the liquid. Then you work Chargaff's rules on an illustrative base composition, and last you check a chatbot's account of your lab and catch where it scrambles an order, confuses mitosis with meiosis, garbles a ratio, mislabels a structure, oversimplifies a mechanism, or invents a figure.

Time budget: Plan for about 120 minutes.

Part 2 — The Task

Guiding question: When you pull DNA out of a strawberry, what does each ingredient (soap, salt, and cold alcohol) contribute to the white strands you see?

Before you begin — hypothesis: Before you mash any fruit, write one hypothesis in the if… then… because… form about one ingredient: if the soap (or the salt) is left out of the extraction liquid, then the white cloud will change in a way you predict, because of the job that ingredient does. Then predict which of the three cups will show the thickest cloud.

Workshop data: Your own three-cup strawberry extraction scores, or the illustrative fallback scores

Source: Cloud scores you record yourself from the three cups in the at-home protocol in Part 3, plus a base composition constructed for this lab (not a measured strawberry value); if you cannot run the protocol, use the illustrative fallback scores printed in the fallback, which were constructed for this lab and are not real data.

Rows: one row per cup (full recipe, no soap, no salt), so the observation table has 3 cup rows; the hypothesis table has 4 rows that name the parts of the experiment, the composition table has 5 rows, and the reasoning table has 3 rows
Columns: cup, what went into the extraction liquid, the cloud score from 0 to 3, and a one-line description of what you saw
Unit: a cloud score on a 0-to-3 scale, where 0 means no white strands and 3 means a thick white cloud

Part 3 — Materials & Procedure

Tools and materials

Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet)
Three ripe strawberries, three zip-top bags, dish soap, table salt, water, and rubbing alcohol chilled in the freezer
Three clear cups, a strainer or coffee filter, a spoon, and a toothpick
A document or notebook for your written answers

Ways of working

virtual-simulation-inquiry
at-home-protocol-and-data-table
figure-and-diagram-interpretation

Procedure

Open the Learn.Genetics How To Extract DNA From Anything Living page from this week's Readings & Resources page and read why each step works. Gather the materials, label three cups A, B, and C, and keep the rubbing alcohol away from any flame; do not taste anything in this lab. Tool: Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet). Check: Three labeled cups, three bags, and the alcohol chilling in the freezer.
Before you mash any fruit, fill in the first table in Part 4: your if… then… because… hypothesis, your prediction of the thickest cloud, the one ingredient that changes between cups, and what you will hold the same. Tool: Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet). Check: The first table holds a hypothesis, a prediction, the changed ingredient, and the things held constant.
Make three extraction liquids, one per bag, each with the same amount of water: bag A gets a squirt of dish soap and a pinch of salt; bag B gets the same pinch of salt and no soap; bag C gets the same squirt of soap and no salt. Add one strawberry to each bag, seal it, and mash it for the same length of time. Tool: Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet). Check: Three bags of mashed fruit that differ in one ingredient only.
Strain each bag into its matching cup. Tilt the cup and pour cold rubbing alcohol slowly down the side until it forms a layer on top about as deep as the liquid. Wait a few minutes without stirring. Tool: Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet). Check: A clear alcohol layer sits on top of the fruit liquid in each cup.
Score the white cloud where the layers meet in each cup from 0 to 3 (0 no white strands, 1 faint wisps, 2 a thin cloud, 3 a thick cloud), write one line on what you see, and try lifting the strands with a toothpick. Tool: Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet). Check: Each cup has a score and a one-line description in the observation table.
Open a blank sheet and work the illustrative base composition. Type 22 in cell B2 for cytosine. In B3 type =B2 for guanine, in B4 type =(100-2*B2)/2 for adenine, in B5 type =B4 for thymine, and in B6 type =SUM(B2:B5). Tool: Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet). Check: B3 through B5 hold the other three percents, and B6 returns 100, matching your hand arithmetic.
Complete the composition and reasoning tables, answer the analysis questions in your document, then paste your completed tables into an approved chatbot with the prompt in Part 6 and check its answer against the scripted checks. Tool: Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet). Check: Completed tables, written answers, and a list of the chatbot's errors with corrections.

Fallback route

When to use it: Use this route if you cannot get strawberries, rubbing alcohol, or a safe place to run the protocol.

Use the illustrative fallback scores below, constructed for this lab and not real data, as your observation table, then complete every other scaffold table, analysis question, and the critique step from them and submit the same items for equal credit. Fill the empty column yourself with what each score tells you.

This route earns the same credit as the primary route.

Illustrative fallback scores for the three cups, constructed for this lab, that stand in for your own observations.

Fallback model data
Cup	What went into the liquid	Cloud score (0 to 3)
A	Soap and salt	3
B	Salt, no soap	1
C	Soap, no salt	2

Part 4 — The Scaffold

Name every part of the experiment before you mash any fruit, so your prediction is on record first.

Student scaffold table 1
Part of the experiment	Your answer
Hypothesis (if… then… because…)
Prediction: the cup with the thickest cloud
The one ingredient that changes between cups
What you hold the same in every cup

Record what each cup showed, so you can compare the full recipe with the two cups that leave out one ingredient.

Student scaffold table 2
Cup	What went into the liquid	Cloud score (0 to 3)	What you saw
A	Soap and salt
B	Salt, no soap
C	Soap, no salt

Work Chargaff's rules on the illustrative composition, which is 22% cytosine, with every step shown.

Student scaffold table 3
Quantity	Your arithmetic	Result
Guanine
Guanine plus cytosine
Adenine plus thymine
Adenine
Thymine

Explain the three results that matter most, from what you know about DNA and the cell.

Student scaffold table 4
Question	Your reasoning
What job does the soap do, and which cup shows it?
What job does the salt do, and which cup shows it?
Why can you not see the double helix in the white cloud?

Part 5 — Analysis Questions

Show every step of the composition: guanine from cytosine, guanine plus cytosine, the percent left for adenine and thymine, each of those two, and a check that all four add to 100.
Rank your three cups by cloud score and describe how the strands looked in each. Which cup differed most from the full recipe?
Does your evidence support or fail to support your hypothesis? Write one conclusion sentence that uses supports or fails to support rather than proves, and explain the result using the job of the ingredient you left out.
The strands in your cup came out of cells that each copied their DNA before dividing. Using the pairing rule, explain why the two strands of one helix are complementary rather than identical, and name the three enzymes that copy DNA in order with their jobs.
Choose one and answer it in 3–4 sentences. (a) A cloud score is a judgment made by eye: name one way two people could score the same cup differently, and say what you would do to make the scores more trustworthy. (b) Your cups could differ in more than one way, such as one strawberry being riper or mashed longer: name one such difference, say how it could change your result, and say how you would control it next time.

Part 6 — AI-Critique Moment

Choose one equal-credit critique route. Route A: ask an approved AI tool to critique your completed work, then check its response against your own evidence. Route B is the no-AI equivalent printed below.

Approved tools: Claude, Gemini, ChatGPT, Grok, Microsoft Copilot.

Prompt to paste:

I've pasted my completed Week 13 Biology Lab & Inquiry tables above. I pulled DNA out of mashed strawberries at home with three cups: one with dish soap and salt, one with salt but no soap, and one with soap but no salt, then layered cold rubbing alcohol on top of each and scored the white cloud from 0 to 3. I also worked an illustrative base composition that is 22% cytosine. Please do six things. First, explain what the soap, the salt, and the cold alcohol each do, and describe what the white cloud is. Second, check my base percents for adenine, thymine, guanine, and cytosine using Chargaff's rules, showing each step. Third, describe the structure of DNA as a labeled ladder, and say whether its two strands are identical. Fourth, list the three enzymes that copy DNA in the order they act, with each one's job. Fifth, say in which part of the cell cycle a strawberry cell copies its DNA and how many cells one round of mitosis makes. Sixth, tell me the exact amount of DNA in one strawberry and how many chromosomes a strawberry cell has, with a source.

Test the response for these likely failures:

Chatbots sometimes list the copying enzymes out of order or give DNA polymerase the job of unzipping the helix.
Chatbots sometimes say the DNA is copied during mitosis, or that one round of mitosis makes four cells, which describes meiosis.
Chatbots sometimes match cytosine to adenine and report 22% adenine for a sample that is 22% cytosine, or split the leftover percent unevenly.
Chatbots sometimes call the two strands identical copies, put the bases on the rails and the sugar on the rungs, or pair adenine with guanine.
Chatbots often say the white cloud is one DNA molecule or that you can see the double helix in it, or say salt is what breaks the cells open.
Asked for the exact DNA amount in a strawberry and its chromosome number with a source, chatbots often state precise figures and cite a source that was never given.

Write 3–5 sentences naming one thing the chatbot got right and at least two places it went wrong or overreached, quoting its words and correcting each with evidence from your own tables or this week's chapter.

Equivalent route (no AI required)

No AI account is required for this lab. Instead of the chatbot, critique this flawed analysis, printed here in full: "My strawberry lab showed that salt is what breaks the cells open, and the white cloud in my cup was a single DNA molecule; if you look closely you can see the double helix twisting. In DNA the rails are made of the bases and the rungs are made of sugar, adenine pairs with guanine, and the two strands are identical copies. My composition was 22% cytosine, so adenine is also 22%. A cell copies DNA with DNA polymerase unzipping first, then ligase adding bases, then helicase sealing the ends, and this copying happens during mitosis, which makes four cells. A strawberry holds exactly 3 milligrams of DNA, as a study shows." Mark it up: quote each flawed sentence, name what is wrong (a step out of order, mitosis confused with meiosis, a garbled ratio, a mislabeled structure, an over-simplified mechanism, or an invented figure), cite the evidence from your tables or this week's chapter, and write a corrected sentence beneath it. This route assesses the same objective (Objective 7 — explain the structure and semiconservative replication of DNA, apply Chargaff's rules and complementary base pairing, and trace gene expression through transcription and translation with the genetic code (the central dogma), decoding a sequence from template to protein) as the chatbot critique, takes comparable effort (about 30 minutes and 200–300 words), is scored by the same scoring criteria for the same credit, and its own submission artifact is the marked-up flawed analysis, posted in place of the chatbot share link and report.

Part 7 — What to Submit

All four completed scaffold tables, filled from your own cups or the illustrative fallback scores
Written answers to all five analysis questions
The chatbot critique report with its share link, or the equivalent route's marked-up flawed analysis

Deliverable:

A short Biology Lab & Inquiry report on the strawberry extraction: four completed tables (the named parts of the experiment, the three cups' scores and observations, the Chargaff composition, and the reasoning about each ingredient), written answers to five analysis questions, and a critique of a chatbot's account of your lab, or, on the equivalent route, a marked-up correction of the printed flawed analysis. Submit in any accessible file type or as a link; a recorded response may be submitted as audio or video with a transcript, or as text. Choose a written, recorded, or visual form for your answers; every form is graded by the same scoring criteria.

Rubric

The rubric below shows how each submitted part is evaluated; bands are full, partial, or none.

Workshop evaluation criteria
Criterion	Full	Partial	None
Testable hypothesis and controlled design	A falsifiable if… then… because… hypothesis and a prediction appear before any fruit is mashed, the one changed ingredient is named, and at least two things held constant are listed.	A hypothesis appears but is not falsifiable or lacks a prediction, or the changed ingredient or the constants are missing.	No hypothesis, or the parts of the experiment are not named.
Completed observation and composition tables	All three cups have a score and a description from the student's own cups or the fallback scores, and every composition step is shown with cytosine matched to guanine and the four percents adding to 100.	The tables are filled, but one or two composition steps are wrong or unshown, or a cup lacks its score or description.	No scores are recorded, or the scores or percents are invented rather than observed, taken from the fallback, or computed.
Analysis of the ingredients and the DNA they reveal	Questions 1 to 4 are answered accurately: the conclusion uses supports or fails to support, the left-out ingredient's job explains the result, the strands are called complementary rather than identical, and helicase, DNA polymerase, and DNA ligase are given in order with their jobs.	The questions are attempted, but one or two answers call the strands identical, give the enzymes out of order, misstate an ingredient's job, or claim the result proves the hypothesis.	The analysis questions are missing or do not address the results.
Critique of the chatbot or of the printed flawed analysis	On either route, at least two errors are quoted and corrected with evidence from the student's own tables or the chapter, and one correct point is acknowledged; both routes earn this band equally.	On either route, errors are named but corrected without evidence, or only one error is caught.	No critique appears, or pasted output is submitted without any checking.
Reflection on the limits of the scores	The chosen reflection option is answered in 3–4 sentences that tie a specific limit of the scoring or of the cups to a concrete change in the method or in what can be claimed.	The reflection is answered but stays general or names no specific change.	No reflection is submitted.

Quality Gate

Every cloud score comes from your own cups or the printed fallback scores, never from memory or a chatbot.
Each composition step is shown, cytosine is matched with guanine, and the four percents add to 100.
The hypothesis was written before any fruit was mashed and has the if… then… because… form.
The two strands are described as complementary, not identical, and the copying enzymes are given in order.
No measurement, amount, chromosome count, study, or source is invented anywhere in the submission.
The critique quotes the chatbot or the flawed analysis and corrects at least two errors with evidence from your tables or the chapter.

Built with The Course Maker · samples.thecoursemaker.com