Week 13 — Reading the Ladder: The Chargaff and Copying Report

Assignment 12 of the term — use the pairing rule to fill in a DNA sample, count the bonds that hold a short stretch together, follow two rounds of copying, and explain to a friend why a copy of DNA comes out right.

This adaptive assignment uses a coaching dialogue to help you turn a first attempt into a clear, evidence-based report on this week's DNA structure and replication. You remain the author. The coach asks questions, checks the reasoning you show, and waits for you to revise before it produces the required report. This week's lecture tutorial, quiz, discussion, and Biology Lab & Inquiry practise the same ideas from other angles; this assignment is where you put Chargaff's rules, the pairing rule, and semiconservative replication together.

The task

The Chargaff example is an illustrative dataset constructed for this course (not real data). One model DNA sample has these base percents:

• adenine (A): 30%;
• thymine (T): 30%;
• guanine (G): 20%;
• cytosine (C): 20%.

Chargaff's rules say that in DNA the percent of adenine equals the percent of thymine, and the percent of guanine equals the percent of cytosine. An A–T base pair is held by 2 hydrogen bonds and a G–C base pair by 3.

Work these four problems, showing your arithmetic:

1. In the Chargaff example (illustrative), a model DNA sample is 30% adenine and 30% thymine. What percent of its bases are guanine and cytosine together?
2. In the Chargaff example (illustrative), guanine and cytosine together make up 40% of the bases, and guanine pairs only with cytosine. What percent of the bases are guanine?
3. The DNA strand 5′-AATGC-3′ pairs with its complement through 3 A–T base pairs and 2 G–C base pairs. An A–T pair is held by 2 hydrogen bonds and a G–C pair by 3. How many hydrogen bonds hold this stretch together?
4. One DNA double helix is copied twice by semiconservative replication, giving 4 double helices, and 2 of those 4 still carry one original strand. What fraction of the helices still carry an original strand?

Beside each answer, add one brief interpretation:

• Problem 1: explain why the thymine percent equals the adenine percent, and why the rest of the 100% belongs to guanine and cytosine together.
• Problem 2: explain why that remainder splits evenly between guanine and cytosine, and why setting cytosine equal to adenine is a slip; then say how real measured samples compare with these clean numbers.
• Problem 3: write the complementary strand with both its 5′ and 3′ ends labeled, and explain why the two strands are complementary rather than identical and why every rung keeps the helix the same width.
• Problem 4: explain what "semiconservative" means, what Meselson and Stahl's single in-between band after one round ruled out, and the jobs of helicase, DNA polymerase, and DNA ligase, in order.

Explain it to a non-expert

Your friend, who has started baking bread for a farmers market stall, asks, "Every cell copies all its DNA before it splits. With that much copying, how does the copy come out right?" Write a 3–5 sentence message that explains, in everyday words, how the pairing rule and the cell's own checking make the copy reliable, and names one limit, such as that the checking catches most slips but not every one, or that the percents in your report are illustrative rather than measured. Explain any technical term in everyday words. More than one good answer is possible.

For a different, ungraded example, imagine a friend asking why bread dough puffs up as it rises. Model answer: "Bread yeast are tiny living cells. When they feed on sugar without much air, they break it down and give off carbon dioxide gas as a by-product. That gas gets trapped in the stretchy dough and puffs it up. This explains the puffing, but it does not tell you how long to let the dough rise, which depends on the recipe."

Use that pattern for your friend's question, with your own wording and evidence.

Part 1 — Student instructions

1. Draft all four answers, the four brief interpretations, and your message to your friend before opening a chatbot.
2. Open any course-approved chatbot. Copy the entire Coach Prompt box, including its first and last marker lines, and paste it as one message. The coach will ask for your draft and then work one question at a time.
3. Answer in your own words. Show calculations when asked. Use the coach's feedback to revise at least one calculation or explanation, even if your first attempt was correct.
4. Continue until the prompt's exit condition is met.
5. Save the chat's share link and copy the final report exactly as the coach gives it.

The chatbot is a coach, not a ghostwriter. It must draw out your reasoning rather than write the report for you.

Coach Prompt

COPY EVERYTHING BELOW THIS LINE
ROLE: You are a careful learning coach for an introductory biology course. Draw out and challenge MY thinking about this week's DNA structure and replication. Do not write my assignment for me, do not invent work I did not show, and never invent a measurement, a study, a date, a percentage, or a quotation.

ASSIGNMENT: Work through these four problems in order, one at a time. Do not invent, replace, or reorder a problem. The data: the Chargaff example, an illustrative dataset constructed for this course (not real data). One model DNA sample is adenine 30 percent, thymine 30 percent, guanine 20 percent, cytosine 20 percent. Chargaff's rules: the percent of adenine equals the percent of thymine, and the percent of guanine equals the percent of cytosine. An A-T base pair is held by 2 hydrogen bonds and a G-C base pair by 3.
PROBLEM P1: In the Chargaff example (illustrative), a model DNA sample is 30% adenine and 30% thymine. What percent of its bases are guanine and cytosine together?
PROBLEM P2: In the Chargaff example (illustrative), guanine and cytosine together make up 40% of the bases, and guanine pairs only with cytosine. What percent of the bases are guanine?
PROBLEM P3: The DNA strand 5′-AATGC-3′ pairs with its complement through 3 A–T base pairs and 2 G–C base pairs. An A–T pair is held by 2 hydrogen bonds and a G–C pair by 3. How many hydrogen bonds hold this stretch together?
PROBLEM P4: One DNA double helix is copied twice by semiconservative replication, giving 4 double helices, and 2 of those 4 still carry one original strand. What fraction of the helices still carry an original strand?
After the four answers, I must briefly interpret each one: for P1, why the thymine percent equals the adenine percent, plus why the rest of the 100 percent belongs to guanine and cytosine together; for P2, why that remainder splits evenly between guanine and cytosine and why setting cytosine equal to adenine is a slip, plus how real measured samples compare with these clean numbers; for P3, the complementary strand with both its 5' and 3' ends labeled, plus why the two strands are complementary rather than identical and why every rung keeps the helix the same width; for P4, what semiconservative means, what Meselson and Stahl's single in-between band after one round ruled out, and the jobs of helicase, DNA polymerase, and DNA ligase in order. I must then write a 3-5 sentence message to my friend who bakes bread for a farmers market stall, explaining in everyday words how the pairing rule and the cell's own checking make a copy of DNA come out right, and naming one limit. I must explain technical terms in everyday words.

SCORING CRITERIA: Score only evidence I supply: correct computations (40 points), biological reasoning (32 points), clarity for a non-expert reader (16 points), and showing work so it can be checked (12 points). For each of P1-P4, give 2, 1, or 0 evidence units per criterion; one unit is worth 5 computation points, 4 reasoning points, 2 clarity points, or 1.5 showing-work points. Sum all four problems within each criterion. Count my message to my friend only with P4, not again with P1-P3.
COMPUTATION UNITS: A correct G-plus-C percent, guanine percent, hydrogen-bond count, or old-strand fraction = 2; the right values and operation with only an arithmetic slip = 1; a missing result or the wrong values or operation = 0. Do not penalize an alternative valid method, such as filling in each base one at a time, or equivalent correct notation, such as a fraction or a decimal.
REASONING UNITS: For P1-P3 separately, an interpretation that meets both parts named for that problem = 2; exactly one part = 1; neither = 0. For P4, BOTH an accurate explanation of semiconservative replication, the Meselson and Stahl first-round band, and the three enzymes in order AND a friend message that explains in everyday words why a copy of DNA comes out right with a named limit = 2. If full P4 evidence is absent but at least one accurate substantive part appears, give 1. No accurate reasoning = 0; pairing A with G, calling the two strands identical, setting cytosine equal to adenine, calling replication conservative, saying polymerase unzips the helix, or putting a quotation in a scientist's mouth is not accurate. An arithmetic slip alone is not a reasoning error.
CLARITY UNITS: For each problem, labeled evidence understandable to someone new to biology, with technical terms explained = 2; intelligible evidence that needs rereading because of ambiguity or jargon = 1; absent or unintelligible evidence = 0. Include the friend message with P4 if submitted. Missing reasoning alone does not lower clarity for otherwise clear submitted work.
SHOWING-WORK UNITS: For each problem, checkable values and steps = 2; a relevant but incomplete value or step = 1; a result alone or no method = 0. A visible method stays checkable despite an arithmetic slip.

Do NOT invent grading rules.

DIALOGUE RULES: Ask exactly one question per message and wait for my answer. Start by asking for my complete first attempt. Build each next question from my exact words. Probe an unsupported claim and ask me to examine its evidence. Keep the conversation focused on questions rather than completed work.

ENGAGEMENT GUARDS: A greeting, copied prompt, yes/no reply, or one-word answer is not a substantive exchange. Do not flatter or accept agreement as evidence. Praise only a concrete move I made. If evidence is missing, say what is missing and ask for it. Require a real revision plus a short reflection on why the revision is better. Do not issue the report early.

EXIT CONDITION: Continue until at least five substantive exchanges have occurred AND I have supplied all four answers and their brief interpretations AND written my message to my friend AND revised at least one calculation or explanation AND reflected on the revision. Stop only after whichever requirement happens last.

REPORT: After the exit condition is satisfied, output exactly these labelled lines in this order. Replace X with the exact earned score; it may end in .5 and must not be rounded to a whole number. Base every line only on evidence in this chat; write "Not supplied" for missing evidence and never raise the score for polished chatbot prose.
STUDENT'S SCORE: X/100
EVIDENCE FOR THE SCORE: [specific evidence from my calculations and reasoning]
REVISION MADE: [what I changed and why]
STRENGTH: [one earned strength]
NEXT STEP: [one concrete improvement]
Then tell me to submit this report, my revised DNA report, and the share link.
COPY EVERYTHING ABOVE THIS LINE

Deliverables

Submit these three items in the assignment entry:

• your revised DNA report with all four answers, their brief interpretations, and your message to your friend — written, or recorded as audio or video with a transcript, or as a labelled sketch of the ladder and its complementary strand with a short note; every form is graded by the same scoring criteria;
• the coach's five-line report; and
• the conversation share link.

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.

Equivalent route (no AI required)

This route assesses the same Week 13 skills — applying Chargaff's rules to find base percents, writing a complementary strand and counting the hydrogen bonds that hold it, and explaining semiconservative replication, the Meselson and Stahl test, and the jobs of the copying enzymes — for comparable time and effort, about 60 to 90 minutes, the same as the coached route. Complete the same four problems, the four brief interpretations, and the message to your friend. Then audit your own first draft against the attached scoring criteria, criterion by criterion, and write a second draft. Add a 150–200 word revision memo that names one calculation or explanation you changed, the evidence that prompted the change, and one question you still have. Submit the revised DNA report, your completed criteria checklist, and the revision memo in the assignment entry; together they replace the share link and the coach's report. This route uses the same scoring criteria and the same points, with no penalty for choosing it.

How you'll be graded

The full analytic rubric is attached to this assignment in Canvas.

Academic integrity and AI use

The calculations, reasoning, revision, and final report must be your own. Do not ask the chatbot to create the assignment or add evidence you did not produce, and put no quotation in any scientist's mouth. Submit an honest share link from your own dialogue. The equivalent route keeps the same workload and evidence and requires no AI account.

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