AP Biology Unit 1.6–1.7: Nucleic Acids & Proteins
This interactive lesson turns the attached College Board material into a full AP Biology learning sequence focused on structure → interaction → function → consequence of change.
Learning objectives
- Compare DNA and RNA structure, including sugars, bases, strand organization, and 5′/3′ directionality.
- Explain how phosphodiester bonds create nucleic-acid backbones and how complementary base pairing stabilizes DNA.
- Describe amino-acid structure and peptide-bond formation.
- Distinguish primary, secondary, tertiary, and quaternary protein structure.
- Predict how mutation, temperature, or pH can alter folding and biological function.
- Use AP-style evidence and reasoning in model, data, and FRQ questions.
Warm-up
Part I — Nucleic Acids
1. Nucleotide structure
DNA and RNA are polymers made from nucleotide monomers. Each nucleotide contains a five-carbon sugar, phosphate group, and nitrogenous base.
Adjacent nucleotides are linked by covalent phosphodiester bonds, forming the sugar-phosphate backbone. Because the sugar is asymmetric, each strand has distinct 5′ and 3′ ends.
2. DNA vs RNA
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, C, G | A, U, C, G |
| Typical organization | Double-stranded | Usually single-stranded |
| Directionality | Paired strands are antiparallel | Has 5′ and 3′ ends |
| Backbone | Sugar-phosphate | Sugar-phosphate |
Video 1 — DNA vs RNA
Focus questions
- List three similarities between DNA and RNA.
- Identify the sugar in each molecule.
- Which nitrogenous base differs?
- Why are 5′ and 3′ ends important?
- How can a single RNA strand form complex shape?
Part II — Proteins
3. Amino-acid structure
Every amino acid contains an α-carbon bonded to an amino group, carboxyl group, hydrogen, and a variable R group. The R group determines chemical properties such as charge, polarity, and hydrophobicity.
4. Peptide bonds
A peptide bond forms between the carboxyl group of one amino acid and the amino group of another in a dehydration reaction. Repetition produces a polypeptide.
5. Four levels of protein structure
| Level | Meaning | Major interactions/features | Consequence of disruption |
|---|---|---|---|
| Primary | Linear amino-acid sequence | Peptide bonds | Sequence change can alter later folding. |
| Secondary | α-helices and β-sheets | Hydrogen bonds between backbone atoms | Local folding may be destabilized. |
| Tertiary | 3-D shape of one polypeptide | Hydrophobic interactions, hydrogen bonds, ionic interactions, van der Waals forces, disulfide bonds | Active/binding sites can change. |
| Quaternary | Association of multiple polypeptides | Interactions among subunits | Complex function can be reduced or lost. |
Video 2 — Protein Structure and Folding
Focus questions
- How does primary structure differ from secondary structure?
- Which interactions stabilize tertiary structure?
- Why do not all proteins have quaternary structure?
- How can denaturation affect function?
- Why can one amino-acid substitution have a large effect?
AP Biology Practice — Multiple Choice
Data Analysis — Enzyme Stability
A researcher incubates an enzyme for 10 minutes at different temperatures and then measures activity.
| Temperature (°C) | Relative activity (%) |
|---|---|
| 20 | 68 |
| 30 | 92 |
| 40 | 100 |
| 50 | 61 |
| 60 | 18 |
| 70 | 3 |
- Describe the trend from 20–40°C.
- Describe the trend from 40–70°C.
- Explain the decline above 40°C using protein structure.
- Predict whether cooling the 70°C sample back to 40°C must restore activity. Justify.
AP-Style FRQ Practice
FRQ 1 — Hemoglobin structure
Hemoglobin contains multiple polypeptide subunits. A mutation prevents one subunit from associating with the others.
(a) Identify the level of structure most directly affected. [1]
(b) Predict the effect on oxygen-binding function. [1]
(c) Justify the prediction using structure–function reasoning. [2]
FRQ 2 — Mutation and folding
A mutation replaces a hydrophobic amino acid buried in a protein interior with a charged amino acid.
(a) Predict the effect on tertiary structure. [2]
(b) Explain how function could change. [2]
(c) Describe one measurement that could test whether the protein changed. [1]
FRQ 3 — DNA/RNA model
Polymer X is double-stranded and contains thymine. Polymer Y is single-stranded and contains uracil.
(a) Identify X and Y. [2]
(b) State one shared structural feature. [1]
(c) Explain how nucleotide chemistry produces 5′→3′ directionality. [2]
FRQ 4 — Experimental design
A student claims that changing pH alters enzyme activity because it changes interactions that stabilize tertiary structure. Design an experiment that includes the independent variable, dependent variable, one controlled variable, and the expected result pattern that would support the claim. [4]
FRQ 5 — Claim, Evidence, Reasoning
Evaluate the claim: “A mutation that changes one amino acid can change phenotype even when the rest of the protein sequence is unchanged.” Use molecular evidence and reasoning.
Exit Ticket
- Give two similarities and two differences between DNA and RNA.
- Explain why peptide bonds define primary structure but do not alone define tertiary structure.
- Explain how an R-group substitution can alter protein function.
- Predict what may happen if one subunit of a multimeric protein can no longer bind the others.
- Write one remaining question you have about nucleic acids or proteins.