AP Biology Unit 6 Review – Gene Expression & Regulation (2025 Guide)

• 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

1. Draw the Central Dogma: DNA → RNA → Protein, with key enzymes labeled.
2. Practice Operon Diagrams: Be ready to explain gene regulation in prokaryotes.
3. Know Codon Chart Usage: Be able to decode amino acid sequences quickly.
4. 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.