What the exam asks
- Multiple Choice (Section I): Expect stimulus sets containing DNA/mRNA sequences alongside a genetic code chart, asking you to classify a mutation or predict its impact on protein folding. Other sets present bacterial plating experiments testing antibiotic resistance via horizontal gene transfer.
- Free Response (Section II): Commonly tested in Question 1 (Experimental Results) or Question 4 (Conceptual Analysis), where a genetic disruption occurs and you must describe the mutation, predict the effect on a polypeptide or signaling pathway, and justify how an environmental change alters the selective advantage of that trait.
Core ideas
Gene mutations and protein structure
A mutation is a permanent change in the nucleotide sequence of DNA. When a mutation occurs within a coding region, its effect depends on how it alters the mRNA codons read during translation:
| Mutation Type | DNA / mRNA Change | Effect on Polypeptide & Phenotype |
|---|---|---|
| Silent (Point substitution) | Changes one nucleotide, but due to the redundancy of the genetic code, the new codon specifies the same amino acid. | Primary structure is unchanged; normally no effect on protein function or phenotype. |
| Missense (Point substitution) | Changes one nucleotide so that a different amino acid is incorporated. | Effect varies: if an R-group’s chemical properties (e.g., hydrophobic vs. hydrophilic) change near an active site, tertiary structure and function may be altered or lost. |
| Nonsense (Point substitution) | Changes an amino-acid-coding codon into a premature stop codon (, , or ). | Produces a truncated (shortened) polypeptide that is almost always nonfunctional. |
| Frameshift (Insertion or deletion) | Adds or removes a number of nucleotides not divisible by 3, shifting the reading frame. | Alters every codon downstream of the mutation, usually causing a completely different amino acid sequence and an early stop codon (nonfunctional protein). |