Unit 6: Gene Expression and Regulation dives deep into the molecular biology of how DNA codes for proteins, how genes are turned on and off, and how genetic information flows in cells. It makes up 12–16% of the AP Biology Exam and is one of the most heavily tested units.
In this RevisionDojo Unit 6 Review, we’ll cover:
- DNA structure and replication
- Transcription and translation
- Regulation of gene expression in prokaryotes and eukaryotes
- Mutations and their effects
- Viruses and biotechnology connections
- Common AP exam question types
- Study tips to master this high-yield unit
Unit 6 Overview
The College Board organizes Unit 6 into six big concepts:
- DNA and RNA structure
- Replication of DNA
- Transcription and RNA processing
- Translation and protein synthesis
- Gene regulation in different cell types
- Mutations and viral biology
1. DNA and RNA Structure
DNA (Deoxyribonucleic Acid):
- Double helix, antiparallel strands
- Sugar-phosphate backbone with nitrogenous bases (A–T, C–G)
- Hydrogen bonds hold base pairs together
RNA (Ribonucleic Acid):
- Single-stranded
- Uracil (U) replaces thymine
- Types: mRNA, tRNA, rRNA, regulatory RNAs
2. DNA Replication
- Semi-conservative model: Each daughter DNA has one old strand and one new strand
- Key enzymes:
- Helicase (unwinds DNA)
- DNA polymerase (adds nucleotides)
- Ligase (joins Okazaki fragments)
- Leading vs. lagging strands: Continuous vs. discontinuous synthesis
AP Tip: Know how replication directionality (5′ → 3′) affects the process.
3. Transcription and RNA Processing
- Transcription: DNA → pre-mRNA (in eukaryotes)
- Enzyme: RNA polymerase
- RNA processing in eukaryotes:
- 5′ cap
- Poly-A tail
- Splicing (introns removed, exons joined)
4. Translation and Protein Synthesis
- Translation: mRNA → polypeptide chain
- Ribosome structure: rRNA + proteins
- tRNA role: Matches anticodon to mRNA codon, carries amino acids
- Start codon: AUG (methionine)
- Stop codons: UAA, UAG, UGA
5. Gene Regulation
In Prokaryotes:
- Operon model (lac operon, trp operon)
- Inducible vs. repressible systems
In Eukaryotes:
- Transcription factors
- Enhancers and silencers
- Epigenetic regulation (DNA methylation, histone modification)
- RNA interference (siRNA, miRNA)
6. Mutations
- Point mutations: Silent, missense, nonsense
- Frameshift mutations: Insertion/deletion changes reading frame
- Effects vary—some neutral, some harmful, some beneficial
7. Viruses and Biotechnology
- Viral replication cycles: lytic vs. lysogenic
- Retroviruses (HIV) use reverse transcriptase
- Biotechnology tools: PCR, gel electrophoresis, CRISPR, DNA sequencing
8. Connections to Other Units
- Unit 5: Heredity determines which alleles are expressed
- Unit 7: Mutations drive natural selection
- Unit 8: Viral spread affects ecosystems
Common AP Biology Exam Questions for Unit 6
Multiple Choice Examples:
- Predict effect of methylation on gene expression
- Identify the function of RNA polymerase
FRQ Examples:
- Explain how mutations in regulatory sequences affect protein production
- Describe how CRISPR can be used to modify genes in an organism
Study Tips for Unit 6
- Draw the Central Dogma: DNA → RNA → Protein, with key enzymes labeled.
- Practice Operon Diagrams: Be ready to explain gene regulation in prokaryotes.
- Know Codon Chart Usage: Be able to decode amino acid sequences quickly.
- Review Lab Applications: Especially PCR and gel electrophoresis questions.
Frequently Asked Questions
1. How much of the AP Bio exam is Unit 6?
About 12–16%—one of the most heavily weighted units.
2. Do I need to memorize codons?
No, you’ll be given a codon chart—but you must know how to use it.
3. Is biotechnology always tested?
Yes—at least one question involves biotech or viral genetics.
4. Are operons hard to learn?
Not if you study them visually—lac and trp operons are predictable systems.
Call to Action
Unit 6 is molecular biology at its core—mastering it is essential for both MCQs and FRQs.
👉 Use RevisionDojo’s AP Bio gene expression practice sets and biotech labs to strengthen your Unit 6 mastery and get closer to that 5.
