Open by telling students that a single base change — one letter out of three billion — can bend a red blood cell into a sickle, block capillaries, and cause a stroke in a child. Then flip that idea: the same allele, in one copy, protects against Plasmodium falciparum malaria. Ask: 'If this mutation kills, why hasn't natural selection removed it?' Take 2-3 student ideas but don't resolve — tell students they will trace the exact base change from DNA → mRNA → amino acid → phenotype today, and by the end they will explain why 'all mutations are harmful' is wrong. Targets SP 6 Argumentation.

1. 7m

   Point mutations: silent, missense, nonsense

   Content

   A point mutation changes a single base in DNA, and the effect depends entirely on how the resulting mRNA codon reads on the codon table. Because the genetic code is redundant (most amino acids have 2-6 codons), a substitution in the third 'wobble' position often produces the same amino acid — a silent mutation, and the protein sequence is unchanged. If the new codon codes for a different amino acid, it's a missense mutation; the severity depends on whether the new residue has similar chemistry (conservative, often tolerated) or radically different chemistry (non-conservative, often disruptive — the sickle-cell Glu→Val swap replaces a charged residue with a hydrophobic one, creating a sticky patch that polymerizes hemoglobin). If the new codon is UAA, UAG, or UGA, translation terminates early — a nonsense mutation — producing a truncated, usually nonfunctional protein. Worked example on the codon strip: reference mRNA 5'-AUG-CCU-GAG-UGG-UAA-3' codes for Met-Pro-Glu-Trp-STOP. Change CCU → CCC: still Pro — silent. Change GAG → GUG: Glu → Val — missense (this is sickle cell). Change UGG → UGA: Trp → STOP — nonsense, truncates the protein.

   Delivery

   Walk through the three mutant strips shown on the slide side by side with the reference so students see that the SAME kind of change (one base) produces radically different outcomes. Emphasize that you cannot predict effect from 'it's just one letter' — you MUST consult the codon table. Pre-empt the misconception that any single-base change alters the protein by having students verbally identify the silent one first. Ask: 'Where in the codon is a substitution most likely to be silent, and why?' (third position, wobble). Quick check: hold up a random codon-to-codon change and cold-call for silent/missense/nonsense.

2. 7m

   Insertions, deletions, and frameshifts

   Content

   Insertions add bases; deletions remove them. The critical rule is the multiple-of-three rule. Because ribosomes read mRNA in non-overlapping triplets from a fixed start codon, adding or removing three bases changes the protein by only one amino acid — an in-frame indel — and downstream codons are untouched. But adding or removing one or two bases (or any number not divisible by three) shifts the reading frame, so every codon downstream is scrambled AND a premature stop codon almost always appears within ~20 codons by chance. That is a frameshift, and it is typically catastrophic — far worse than a missense change. Worked example on mRNA 5'-AUG-CCU-GAG-UGG-GCA-3' (Met-Pro-Glu-Trp-Ala): delete the first G of GAG → AUG-CCU-AGU-GGG-CA... = Met-Pro-Ser-Gly-... every codon after position 2 is different. Compare to deleting all three bases of GAG → AUG-CCU-UGG-GCA = Met-Pro-Trp-Ala; only Glu is gone, the rest of the protein is intact. Cystic fibrosis ΔF508 is a real-world in-frame deletion (three bases removed, one amino acid lost) — severe, but the protein still folds partially. Frameshifts in the same gene usually give no functional CFTR at all.

   Delivery

   The single most tested idea here is 'not all indels are equal — count the bases.' Walk the two parallel examples on the slide slowly, pointing at how the codon boundaries shift versus stay put. Ask students to predict: 'If I delete two bases instead of one, is the frame restored?' (No — need multiple of 3.) Pre-empt the misconception that inserting three bases is as damaging as inserting one. Connect back to sickle cell: that was a SUBSTITUTION, not an indel — different mechanism, different severity logic.

3. 6m

   Germ-line vs somatic; why mutations fuel evolution

   Content

   Mutations only enter a population's gene pool if they occur in germ-line cells (cells that make gametes). A somatic mutation — say, a UV-induced mutation in a skin cell — can cause disease in that individual (melanoma) but cannot be inherited and cannot change allele frequencies in the next generation. This is why the sickle-cell allele matters evolutionarily: it arose in germ cells and is transmitted. Second key idea: mutations are not all harmful. Most are neutral (silent substitutions, changes in non-coding regions, synonymous codon choices). Some are beneficial in a given environment (lactase persistence in dairying populations; the CCR5-Δ32 deletion conferring HIV resistance; the sickle-cell heterozygote advantage in malarial regions — a case of balancing selection where homozygotes for either allele have lower fitness than heterozygotes). Some are harmful. Which category a mutation lands in depends on the ENVIRONMENT, not the mutation itself. Mutation is the ONLY source of new alleles; without it, natural selection would have nothing new to act on.

   Delivery

   Anchor this with a two-column mental sort: 'heritable?' and 'helpful, neutral, or harmful in THIS environment?' Emphasize that these are two independent axes. Pre-empt two misconceptions at once: (1) 'all mutations are harmful' — no, most are neutral, some are beneficial; (2) 'a mutation in my lung cell can be inherited' — no, only germ-line. Close by returning to the hook: the sickle allele persists because in malarial regions heterozygotes survive BETTER than either homozygote — the 'harmful' label depends on environment. This sets up Unit 7.

1. 25m

   Mutation Analysis Lab: From Base Change to Phenotype

   Hand out the worksheet below. Students work in pairs for 18 minutes, then 7 minutes whole-class debrief where you cold-call pairs to defend classifications on the board. Walk around and check: are they reading the codon table correctly? Are they distinguishing frameshift from in-frame? Are they using the term 'non-conservative' when the chemistry changes drastically? Targets SP 1 Concept Explanation, SP 2 Visual Representations, and SP 6 Argumentation. Student handout — Mutation Analysis Lab Reference sequence (wild-type HBB fragment, coding strand DNA): 5'- A T G · G T G · C A C · C T G · A C T · C C T · G A G · G A G -3' Part 1 — Establish the wild-type protein. 1. Transcribe the DNA coding strand into mRNA (5' → 3'): 5'- ______ -3' 2. Using the codon table, translate the mRNA into amino acids 1-8: ______ ______ ______ ______ ______ ______ ______ ______ Part 2 — Apply four mutations to the wild-type DNA and analyze each. For each row, write the mutant mRNA, translate it, classify the mutation (silent / missense / nonsense / in-frame indel / frameshift), and predict the effect on the protein. - Mutant A: codon 7 GAG → GAA (third base G → A) - Mutant mRNA codon 7: ______ - New amino acid 7: ______ - Classification: ______ - Predicted effect on protein: ______ - Mutant B: codon 7 GAG → GTG (middle base A → T) — this is the real sickle-cell mutation - Mutant mRNA codon 7: ______ - New amino acid 7: ______ - Classification: ______ - Is this conservative or non-conservative? Why? ______ - Mutant C: codon 5 ACT → ATT is changed further so the mRNA codon becomes UAA - Mutant amino acid 5: ______ - Classification: ______ - How many amino acids does the protein now contain (from Met)? ______ - Mutant D: delete the very first G of codon 7 (GAG → AG…) so the sequence downstream reads AGG AG… - Rewrite mRNA from codon 7 onward: ______ - Translate codons 7-8 in the NEW frame: ______ ______ - Classification: ______ - Compare severity to Mutant B and justify: ______ Part 3 — Argue from evidence (SP 6). In 3-5 sentences, answer: Two patients each carry one mutation in HBB. Patient 1 has Mutant A. Patient 2 has Mutant D. Which patient is more likely to have severely disrupted hemoglobin, and why? Use the terms frameshift, premature stop codon, and reading frame in your answer. Part 4 — Heritability sort. For each scenario below, mark G (germ-line, heritable) or S (somatic, not heritable), and state whether it could change allele frequencies in the next generation. - A UV-induced point mutation in a melanocyte on a sunburned shoulder: ______ - A mutation in a sperm cell precursor caused by ionizing radiation: ______ - A frameshift in an intestinal epithelial cell after exposure to benzo[a]pyrene: ______ - A mutation arising during oogenesis in the ovary of a pregnant person: ______

   Materials

   • Printed student handout (below)
   • Standard mRNA codon table (printed on back of handout)
   • Pencil

   Example outputs

   • Part 1: mRNA 5'-AUG GUG CAC CUG ACU CCU GAG GAG-3'; protein Met-Val-His-Leu-Thr-Pro-Glu-Glu.
   • Mutant A: codon GAA still codes Glu — silent, no effect on protein. Mutant B: codon GUG codes Val — missense, non-conservative (charged acidic Glu → hydrophobic Val), causes hemoglobin polymerization / sickling. Mutant C: STOP at position 5 — nonsense, protein is only 4 amino acids long, nonfunctional. Mutant D: single-base deletion → frameshift; codon 7 reads AGG (Arg), codon 8 reads AG_ shifted, every downstream residue is scrambled and a premature stop will appear soon; more severe than Mutant B because ALL downstream residues are altered, not just one.
   • Part 4: melanocyte = S, cannot enter gene pool (but can cause melanoma); sperm precursor = G, heritable; intestinal epithelial = S; oogenesis = G, heritable.

1. A wild-type mRNA reads 5'-AUG-UCA-GGA-CUU-UAA-3'. A single-base substitution changes the third codon to GGG. Classify the mutation and justify your classification using the codon table. (Targets SP 1 Concept Explanation.)

   short answerGGA codes for Glycine; GGG also codes for Glycine. The amino acid is unchanged, so this is a silent mutation. It occurred at the third (wobble) position, where the code's redundancy commonly makes substitutions silent. Predicted effect on protein: none.

2. Which of the following mutations is MOST likely to produce a completely nonfunctional protein? A) A substitution at the third position of codon 40 in a 300-amino-acid protein B) An in-frame deletion of three bases in codon 100 C) A single-base insertion in codon 5 D) A missense mutation changing leucine to isoleucine at codon 250 (Targets SP 6 Argumentation.)

   multiple choiceC. A single-base insertion causes a frameshift beginning at codon 5, so nearly the entire protein (codons 5-300) is scrambled and a premature stop codon almost always appears within ~20 codons. A is likely silent (third-position wobble). B removes only one amino acid, protein largely intact (like ΔF508 CFTR — impaired but partially folded). D is a conservative missense (Leu→Ile, both small hydrophobic), usually tolerated.

3. The sickle-cell allele HbS causes disease in homozygotes yet remains at frequencies above 10% in parts of sub-Saharan Africa where malaria is endemic. Explain, using evidence, why natural selection has not eliminated this allele. (Targets SP 4 Representing and Describing Data, SP 6 Argumentation.)

   short answerHeterozygotes (HbA HbS) have higher fitness than either homozygote in malarial environments: HbA HbA homozygotes are vulnerable to Plasmodium falciparum, and HbS HbS homozygotes suffer sickle-cell disease, but heterozygotes have partial malaria resistance without severe sickling. This is balancing selection (heterozygote advantage), which maintains both alleles in the population. The allele is 'harmful' only in the environmental context of no malaria — fitness effects depend on environment, not on the mutation alone.

4. A person develops a somatic frameshift mutation in a tumor-suppressor gene in a lung cell after years of smoking. Will this mutation be passed to the person's children? Will it contribute to evolution of the human population? Explain. (Targets SP 1 Concept Explanation.)

   short answerNo to both. The mutation occurred in a somatic (body) cell, not in a germ-line cell that produces gametes, so it cannot be transmitted to offspring and cannot enter the population's gene pool. It may cause cancer in that individual (loss of tumor-suppressor function via frameshift → premature stop → truncated nonfunctional protein), but it has no evolutionary consequence for the species.

• ☐Printed Mutation Analysis Lab handout (one per student)
• ☐Standard mRNA codon table printed on the back of the handout
• ☐Pencils
• ☐Projector for slide deck (codon strips, frameshift diagram, sickle-cell pathway, heritability sort)