Week 15 — Gels and Typos: The DNA Profiling and Mutation Report — Traditional alternative (no AI)

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

Assignment 14 of the term — read the virtual gel, follow two typos through Gene 1, choose the right tool for a job, and explain to a relative what a DNA match does and does not prove.

This week you learned how a gel sorts DNA by size, how a single typo or a missing letter changes a gene's message, and what PCR, recombinant DNA, and CRISPR-Cas9 each do. In this assignment you work four short problems, interpret each one, choose the right tool for a job, and explain to someone who has never studied biology what a DNA match does and does not show. This week's lecture tutorial, quiz, discussion, and Biology Lab & Inquiry practise the same ideas from other angles. It builds on Week 14's codon reading, which is how you tell what a typo does to a protein. This is the last assignment of the course, so it also asks you to choose among the tools you have learned and say why one fits a job better than another.

The task

The virtual gel is an illustrative dataset constructed for this course (not real data). Its size ladder has bands at 500, 1,000, 2,000, and 4,000 bp (base pairs), and smaller fragments travel farther from the wells. Three sample lanes sit beside the ladder:

• the crime-scene lane, with bands at 500, 2,000, and 4,000 bp;
• Suspect 1, with bands at 1,000, 2,000, and 4,000 bp;
• Suspect 2, with bands at 500, 2,000, and 4,000 bp.

Gene 1 from Week 14 returns too: its mRNA is 5′-AUG GCU UAU UGA-3′, which reads Met–Ala–Tyr and then stop.

Work these four problems, showing your arithmetic:

1. On the illustrative virtual gel, the size ladder's largest band is 4,000 bp and its smallest band is 500 bp. How many times larger is the largest band than the smallest?
2. Gene 1's mRNA, 5′-AUG GCU UAU UGA-3′, has 12 bases. If the first base after AUG is deleted, how many bases remain?
3. On the illustrative virtual gel, each ladder band is twice the size of the band below it, and the smallest band is 500 bp. What size, in bp, is the band two steps above the smallest?
4. On Gene 1's mRNA, the codon UAU at codon position 3 becomes the stop codon UAA. Counting AUG as codon position 1, how many amino acids does the shortened protein carry?

Beside each answer, add one brief interpretation:

• Problem 1: explain why the 4,000 bp band sits nearest the wells, and choose the tool: a lab has only a trace of DNA from a scene, so which tool would it use to get enough DNA before running this gel, and why do gel electrophoresis alone and CRISPR-Cas9 fit that job less well?
• Problem 2: name the kind of mutation, and regroup the remaining bases in threes from AUG to explain what happens to every codon after the deletion and to the UGA stop.
• Problem 3: read the crime-scene lane against both suspects, say which suspect matches and which is excluded, and explain why one band that differs is a firmer call than a match.
• Problem 4: name the kind of mutation, and contrast it with the change GCU to GAU on the same gene.

Explain it to a non-expert

A relative who has been called for jury duty asks, "If the DNA at a crime scene matches someone, does that prove they did it?" Write a 3–5 sentence message that explains, in everyday words, what a matching band pattern does and does not show, connects it to the virtual gel, and names one limit, including that the gel's numbers are illustrative rather than real case data. 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 rises. Model answer: "Bread dough rises because yeast, a tiny living fungus, breaks down sugar without oxygen and gives off carbon dioxide gas. The gas gets trapped in the stretchy dough and puffs it up, the same way air fills a balloon. Biologists call this alcoholic fermentation. It explains the rise, but it does not tell you exactly how big a loaf will get, which depends on the yeast, the sugar, the warmth, and the time."

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

Show your arithmetic. A reader should be able to reproduce every number you report from the band sizes, base counts, and codon positions 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, including your tool choice with Problem 1;
• your message to your relative, explaining in everyday words what a matching DNA band pattern does and does not show, with one limit; and
• a one-line restatement of the gel lanes and the Gene 1 mRNA 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 hand-drawn gel 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.

Looking ahead to the final

The cumulative final comes in Week 16, the finals week. Choosing a tool by its job, as you did in Problem 1, is the same move the final asks for across the whole course: name what a process or tool does, put its steps in order, and say where each one happens. To prepare, use the final study guide, the Final Practice Exam, and the exam-prep tutorial in the finals-week module.

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, a spreadsheet, or a codon table. 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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