AP Biology Unit 6 Cheat Sheet: Gene Expression & Regulation
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TLDR (Quick Summary)
The Central Dogma: DNA → RNA → Protein → Trait.
Know replication, transcription, and translation steps and enzymes.
Understand how operons and epigenetic factors regulate genes.
Review virus replication (lytic, lysogenic, retrovirus).
⚡ Download our AP Biology Unit 6 Cheat Sheet above for all key diagrams and mnemonics.
Why Unit 6 Matters
Unit 6 is where the molecular world of biology comes alive.
You move from “genes control traits” to exactly how that happens. Every FRQ on gene regulation or enzyme synthesis traces back to this unit — so mastering it means you can explain how cells express, control, and sometimes mutate their genetic code.
The Central Dogma: DNA → RNA → Protein
This is the flow of genetic information in every living cell.
Replication: DNA copies itself before cell division.
Transcription: DNA → mRNA in the nucleus.
Translation: mRNA → protein at the ribosome.
Tutor Tip: Always specify direction (5′→3′) and location (nucleus or ribosome) when explaining these processes — the AP exam loves detail!
DNA Structure & Replication
DNA = double helix, antiparallel strands (A-T, C-G).
Helicase unzips, DNA polymerase builds, ligase links.
Replication is semiconservative — each new DNA molecule keeps one old strand.
Mnemonic: “Helicase unzips, Polymerase builds, Ligase links.”
Transcription: Making mRNA
Steps:
RNA polymerase binds the promoter (start point).
Builds complementary mRNA strand (A→U, T→A).
Stops at a terminator sequence.
In eukaryotes, the pre-mRNA is modified:
Add 5′ cap and poly-A tail for stability.
Splice out introns, keep exons.
Common Pitfall: Forgetting that prokaryotes skip mRNA processing because they lack a nucleus.
Translation: Building Proteins
Happens at the ribosome.
tRNA reads mRNA codons with anticodons, bringing amino acids.
Start codon: AUG (methionine).
Stop codons: UAA, UAG, UGA.
Mnemonic: “AUG = Always the Start.”
Gene Regulation
Cells don’t express all their genes at once — they regulate which ones are active.
In Prokaryotes: The Operon Model
Lac operon (inducible): off until lactose is present.
Trp operon (repressible): on until tryptophan is present.
Mnemonic: “Lac = Lazy until lactose, Trp = Tired when tryptophan.”
In Eukaryotes
Controlled by transcription factors, enhancers, and epigenetic markers (DNA methylation and histone acetylation).
These determine which genes are “on” or “off.”
Viruses & Gene Expression
Viruses hijack gene expression systems.
Lytic cycle: virus replicates immediately, host bursts.
Lysogenic cycle: viral DNA hides in host genome.
Retroviruses (like HIV): use reverse transcriptase to turn RNA → DNA.
Mnemonic: “Lytic = Loud, Lysogenic = Latent.”
Common Pitfalls
Confusing transcription with replication enzymes.
Forgetting 5′ → 3′ directionality.
Skipping mRNA processing steps.
Mixing up lac vs trp operons.
Not connecting structure (like DNA methylation) to function (gene silencing).
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