AcornPrep

AP Biology Unit 6: Gene Expression and Regulation

Unit 6: Gene Expression and Regulation — Quick Review

Replication

  • Semiconservative. 5' → 3' synthesis only.
  • Leading strand continuous; lagging strand = Okazaki fragments joined by ligase.
  • Key enzymes: helicase (unwind), primase (RNA primer), polymerase III (main synthesis), polymerase I (primer removal), ligase (seal).

Transcription

  • RNA polymerase reads template 3' → 5', writes mRNA 5' → 3'.
  • Eukaryotic processing: 5' cap, poly-A tail, splicing (introns out, exons in).
  • Alternative splicing = one gene → many proteins.

Translation

  • AUG = start (Met). UAA, UAG, UGA = stops.
  • Code is redundant but not ambiguous.
  • Ribosome sites: A (arrive) → P (peptide) → E (exit).

Operons

  • lac: inducible, default OFF; lactose→allolactose pulls repressor off; glucose LOW + lactose HIGH = maximum transcription.
  • trp: repressible, default ON; tryptophan is a corepressor.

Eukaryotic regulation (four levels)

  1. Chromatin — acetylation opens; DNA/histone methylation silences.
  2. Transcription factors + enhancers — activators loop enhancers to promoter.
  3. Post-transcriptional — alternative splicing, miRNA/siRNA.
  4. Post-translational — folding, cleavage, phosphorylation, ubiquitin.

Epigenetics

  • Heritable expression changes WITHOUT DNA sequence change.
  • Mechanisms: DNA methylation (mostly silencing), histone modification.

Mutation types

TypeEffect
SilentNone (same AA)
MissenseOne AA changed
NonsensePremature stop → truncated protein
FrameshiftShifts reading frame → usually severe

Viruses

  • Lytic = fast, kills cell. Lysogenic = integrates as prophage.
  • Retroviruses (HIV): RNA → DNA via reverse transcriptase → integrates as provirus.

Biotechnology (5 must-knows)

  1. PCR — amplifies DNA with primers + Taq polymerase.
  2. Gel electrophoresis — separates DNA by size.
  3. Restriction enzymes — cut at specific sequences.
  4. Plasmid cloning — foreign gene + vector → bacteria.
  5. CRISPR-Cas9 — gRNA targets, Cas9 cuts, edits the genome.

💡 Exam Tip: When the FRQ hands you a DNA sequence, your pipeline is DNA → mRNA → protein → phenotype. Always identify template vs. coding strand first.

Key Terms

  • Promoter / operator / enhancer — DNA-binding regulatory sites.
  • Transcription factor — protein that regulates RNA polymerase access.
  • Spliceosome — snRNP complex that removes introns.
  • Reverse transcriptase — RNA-to-DNA enzyme (retroviruses).
  • Prophage / provirus — integrated viral DNA.

Must-Know for the Exam

  • Explain why DNA synthesis requires a lagging strand.
  • Compare prokaryotic and eukaryotic gene expression (at least 3 differences).
  • Diagram or describe the lac operon in four states (± lactose, ± glucose).
  • Classify a mutation from a sequence and predict phenotype.
  • Describe how a retrovirus alters the host genome.
  • Map a biotech workflow (e.g., clone a gene from human to bacteria).

💡 Exam Tip: If you can't remember whether histone acetylation opens or closes chromatin, mnemonic: "Acetyl = Active; Methyl = Mute" (with the caveat that histone methylation context matters).