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Unit 6 · Gene Expression & Regulation Flashcards Cheat Sheet Essentials Visual Review MC Practice FRQ Practice

AP Biology Unit 6 Visual Review

A topic-by-topic visual walkthrough of Gene Expression and Regulation — DNA/RNA structure, replication, transcription, translation, gene regulation, mutations, and biotechnology.

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TOPIC 6.1 DNA & RNA Structure DNA → RNA → Protein (the central dogma) DNA — the master blueprint • Double-stranded, antiparallel, twisted double helix • Sugar = deoxyribose • Bases: A, T, G, C (A–T, G–C pairing) • Stored in the nucleus; stable, long-lasting Backbone of sugar + phosphate; bases point inward. RNA — the working copy • Single-stranded • Sugar = ribose • Bases: A, U, G, C (uracil replaces thymine) • Can leave the nucleus; short-lived Carries DNA's message out to the ribosomes. Three types of RNA you must know mRNA (messenger) — carries the code from DNA to the ribosome. tRNA (transfer) — brings the correct amino acid to match each codon. rRNA (ribosomal) — makes up the ribosome and catalyzes bond formation. Nucleotide = phosphate + sugar + nitrogenous base (the shared monomer of both DNA and RNA). DNA stores the code; RNA carries it out. Key differences: strands, sugar, and T vs. U. The Review Hub · AP Biology Unit 6 TOPIC 6.2 DNA Replication helicase leading (continuous) lagging (Okazaki) the replication fork — new strands in amber Semiconservative The two strands separate, and each old strand serves as a template for a new one. Every daughter DNA is half old, half new — that's why it's called "semiconservative." The enzymes (in order) Helicase unwinds the helix · primase lays an RNA primer DNA polymerase adds new nucleotides (5'→3' only) Ligase seals the Okazaki fragments together Leading vs. lagging strand — why the 5'→3' rule matters DNA polymerase can only build in the 5'→3' direction. The leading strand is made continuously toward the fork. The lagging strand runs the other way, so it's built in short pieces (Okazaki fragments) that ligase joins. Replication is highly accurate — polymerase "proofreads" and corrects most errors. Replication is semiconservative — each new DNA keeps one original strand as a template. The Review Hub · AP Biology Unit 6 TOPIC 6.3 Transcription & RNA Processing DNA TEMPLATE STRAND RNA pol new mRNA (built 5'→3') RNA polymerase reads the DNA template and builds a matching mRNA strand (A→U, T→A, G→C, C→G). Transcription (in the nucleus) RNA polymerase binds the promoter, unwinds the DNA, and synthesizes a pre-mRNA copy of one gene. No primer is needed, and it stops at a terminator sequence. RNA processing (eukaryotes) Before leaving the nucleus, pre-mRNA is edited: a 5' cap and poly-A tail are added (protection & export), and introns are spliced OUT while exons are joined. Introns out, exons expressed Exons are the coding regions kept in the final mRNA; introns are removed. Alternative splicing lets ONE gene produce several different proteins by keeping different combinations of exons — a key source of protein diversity. Prokaryotes lack a nucleus, so they transcribe and translate at the same time with no processing. Transcription copies a gene into mRNA; processing adds a cap, tail, and splices out introns. The Review Hub · AP Biology Unit 6 TOPIC 6.4 Translation mRNA — READ IN CODONS (3 bases each) AUG GCA UUC GGA UAA START (Met) amino acids added → STOP Each codon codes for one amino acid. tRNA anticodons pair with codons to match them. Where & how Ribosomes (in the cytoplasm) read mRNA codon by codon. tRNA carries the matching amino acid; the ribosome links them with peptide bonds into a growing polypeptide. The genetic code Start codon AUG (methionine) begins every protein. Three stop codons (UAA, UAG, UGA) end it. The code is redundant (many codons per amino acid) and near-universal. Three ribosome sites: A, P, E A site — incoming tRNA with the next amino acid arrives · P site — holds the growing chain · E site — the empty tRNA exits. The near-universal code is strong evidence that all life shares a common ancestor. Ribosomes read mRNA codons; tRNA anticodons deliver amino acids to build the protein. The Review Hub · AP Biology Unit 6 TOPIC 6.5 Regulation of Gene Expression Prokaryotes: operons Related genes are grouped and switched on/off together. lac operon (inducible) Normally OFF; turns ON when lactose is present so the cell can digest it. trp operon (repressible) Normally ON; turns OFF when tryptophan is abundant (no need to make more). A promoter, operator, and repressor control the switch. Eukaryotes: many control points • Transcription factors must bind the promoter for RNA polymerase to start • Enhancers & silencers tune the transcription rate • RNA processing & alternative splicing • mRNA stability & degradation • Protein modification after translation Control at ANY of these steps changes how much protein is made. Epigenetics — expression without changing the DNA sequence Chemical tags can turn genes on or off reversibly. DNA methylation generally silences genes; histone modification loosens or tightens how DNA is packaged. These marks can respond to the environment and sometimes be passed to offspring — without altering the underlying genetic code. Cells control which genes are expressed and when — via operons, transcription factors, and epigenetics. The Review Hub · AP Biology Unit 6 TOPIC 6.6 Gene Expression & Cell Specialization One genome nerve cell muscle cell skin cell Same DNA — different genes switched on Differential gene expression Every cell in your body has the SAME complete genome, but each cell type expresses a DIFFERENT subset of genes. That's what makes a neuron different from a muscle cell. Differentiation & development A fertilized egg divides into cells that gradually turn on specific genes and specialize. Signals and master regulatory genes (e.g., Hox genes) guide body layout. Stem cells vs. specialized cells Stem cells are unspecialized and can divide and become many cell types. As cells differentiate, they commit to a specific role by permanently silencing some genes and expressing others. Specialization is about REGULATION, not about losing DNA — the full genome stays intact in nearly every cell. Every cell shares the same DNA — which genes are ON makes a neuron different from a muscle cell. The Review Hub · AP Biology Unit 6 TOPIC 6.7 Mutations Point mutations (one base) Silent — codon still codes the same amino acid (no effect) Missense — changes one amino acid Nonsense — creates an early STOP codon (truncated protein) A single base substitution — e.g., sickle-cell anemia. Frameshift mutations (insertion / deletion) Adding or removing bases (not in multiples of 3) shifts the whole reading frame, so every codon downstream is misread. Usually far more damaging than a single substitution. Chromosomal mutations Large-scale changes: deletions, duplications, inversions, translocations, or nondisjunction (wrong chromosome number, e.g., trisomy 21). Causes & timing Spontaneous (replication errors) or caused by mutagens — UV light, radiation, certain chemicals. Only mutations in germ (gamete) cells are passed to offspring. Mutations aren't all bad — they fuel evolution Effects can be harmful, neutral, or occasionally beneficial. Mutations are the ORIGINAL source of all new genetic variation — the raw material that natural selection acts on (Unit 7). Mutations change the DNA sequence — harmful, neutral, or beneficial — and are the source of variation. The Review Hub · AP Biology Unit 6 TOPIC 6.8 Biotechnology PCR Polymerase chain reaction makes millions of copies of a DNA segment through repeated heating & cooling cycles. Amplifies tiny DNA samples. Gel electrophoresis Sorts DNA fragments by SIZE. An electric field pulls DNA through a gel; smaller pieces travel farther. Used for DNA fingerprinting. CRISPR A precise gene-editing tool that uses a guide RNA to target a specific DNA sequence and cut or change it. Edits genes at a chosen site. Transformation & plasmids A gene of interest is inserted into a bacterial plasmid, which the bacterium copies — making recombinant protein (e.g., insulin). DNA sequencing & GMOs Sequencing reads the exact base order of DNA. Genetically modified organisms carry genes engineered for useful traits. Why it works All these tools rely on the fact that the genetic code is universal — a gene from one organism can function in another. Applications include medicine, agriculture, forensics, and research. Each also raises ethical questions. Biotech tools — PCR, gel electrophoresis, CRISPR — copy, sort, and edit DNA for real-world uses. The Review Hub · AP Biology Unit 6
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How to use the visual review

Spend 30 seconds per slide before clicking next. Look at the diagram, then ask yourself: "Could I draw this from memory and explain it?"

Use the fullscreen button () on desktop for the best experience. Use arrow keys to navigate. Tap "Show all slides" to jump around.

This is great for review the night before the exam — fast, visual, and covers everything you need to remember about Unit 6's molecular biology content.