Week 13 — Reading the Ladder: The Chargaff and Copying Report — Traditional alternative (no AI)

Instructors: publish this instead of the adaptive version if you prefer, and unpublish or delete the adaptive one.

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 week you learned that DNA is a double helix whose rungs pair adenine with thymine and guanine with cytosine, that its two strands are complementary rather than identical, and that a cell copies it by semiconservative replication, with helicase, DNA polymerase, and DNA ligase working in that order. In this assignment you work four short problems, interpret each one, and explain to someone who has never studied biology why a copy of DNA comes out right. This week's lecture tutorial, quiz, discussion, and Biology Lab & Inquiry practise the same ideas from other angles.

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.

Show your arithmetic. A reader should be able to reproduce every number you report from the values in the problems without guessing which values or steps you used.

Deliverables

Submit a single report containing:

• the four answers, each with the arithmetic shown and the brief interpretation the task names for that problem;
• your message to your friend, explaining in everyday words why a copy of DNA comes out right, with one limit; and
• a one-line restatement of the base percents you used, so your results are reproducible.

Keep it to roughly one page. Clear and correct beats long.

Choose how you show it. Present your report as written text, or as a recorded audio or video explanation of about three minutes 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. 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.

How you'll be graded

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

Academic integrity and AI use

This version is designed to be completed without AI: do the arithmetic, the interpretations, and the writing yourself, without a chatbot or other AI tool. You may use a calculator or a spreadsheet for the arithmetic. Put no quotation in any scientist's mouth. Submitting AI-generated work as your own, or copying another student's work, is an integrity violation under the course policy.

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