Cell Communication
& Cell Cycle
How do cells talk to each other — and how do they know when to divide? Unit 4 covers the molecular machinery of cellular signaling (reception, transduction, response) and the tightly regulated cell cycle that governs growth and division. Disruptions to either process are at the heart of cancer biology.
Cell Communication
Cells constantly communicate — to coordinate development, maintain homeostasis, mount immune responses, and regulate tissue growth. Communication can occur via direct cell contact or at a distance via chemical signals. The key principle: a signal molecule (ligand) is only effective if the target cell has the matching receptor protein.
Modes of Cell-to-Cell Communication
| Mode | Distance | Mechanism | Examples |
|---|---|---|---|
| Direct Contact | Zero — cell touching cell | Surface proteins on one cell bind receptors on adjacent cell; gap junctions allow direct cytoplasmic exchange | Immune cell recognition (T cells + APCs); embryonic development (induction); gap junctions in cardiac muscle |
| Local Signaling (Paracrine) | Short — nearby cells | Signal molecule diffuses through extracellular fluid to neighboring cells only | Growth factors, cytokines, histamine from mast cells, morphogens in development |
| Synaptic Signaling | Synapse gap (~20 nm) | Neurotransmitter released into synaptic cleft; binds postsynaptic receptors | Acetylcholine at neuromuscular junction; dopamine, serotonin in brain |
| Endocrine Signaling | Long — via bloodstream | Hormones secreted into blood; travel to distant target cells bearing specific receptors | Insulin (pancreas → liver/muscle), testosterone, estrogen, thyroid hormone, human growth hormone |
Signal Molecules and Receptor Location
Where a receptor is located depends on the chemical nature of the signal molecule:
Large protein/peptide hormones (insulin, glucagon, growth hormone) and charged molecules bind to cell surface (plasma membrane) receptors. The signal itself never enters the cell — it causes a conformational change in the receptor that initiates an intracellular cascade.
Steroid hormones (testosterone, estrogen, cortisol) are lipid-soluble — they diffuse directly through the plasma membrane and bind to intracellular receptors in the cytoplasm or nucleus, forming a hormone–receptor complex that acts as a transcription factor to alter gene expression. Thyroid hormones are also lipid-soluble and use a similar intracellular receptor mechanism, though their receptor details differ from steroids. For AP Biology, the key distinction is: lipid-soluble signals → intracellular receptors → gene expression changes; water-soluble signals → cell-surface receptors → intracellular signaling cascades.
Signal specificity is determined by the receptor, not the signal. The same signal molecule (e.g., epinephrine) can cause different responses in different cell types because those cells have different receptor types linked to different transduction pathways. "Specificity" is a very common AP exam concept.
Steroid hormones vs. protein hormones: Steroids cross the membrane → intracellular receptor → direct gene regulation. Protein hormones cannot cross → surface receptor → signal transduction cascade → indirect gene regulation. This distinction connects Topic 4.1 to all of Topics 4.2 and 4.3.
Insulin is a protein hormone secreted by the pancreas to regulate blood glucose. Which of the following correctly describes how insulin affects liver cells?
- (A) Insulin diffuses across the liver cell membrane and binds to a nuclear receptor, directly activating glucose uptake genes.
- (B) Insulin binds to a receptor on the liver cell surface, initiating a signal transduction cascade that leads to increased glucose uptake.
- (C) Insulin travels through gap junctions into liver cells, where it activates glycogen synthase directly.
- (D) Insulin binds to an intracellular receptor in the cytoplasm of liver cells, forming a complex that enters the nucleus.
Introduction to Signal Transduction
Signal transduction is the process by which a cell converts an extracellular signal into an intracellular response. Every signaling pathway follows a universal three-stage framework: Reception → Transduction → Response.
Stage 1 — Reception
A ligand (signal molecule — can be a protein, peptide, steroid, or small molecule) binds to a specific receptor protein. The binding is highly specific — the ligand's shape must be complementary to the receptor's binding domain. Binding causes a conformational change in the receptor, initiating the cascade. Key receptor types:
The most common type of receptor in eukaryotes. A 7-transmembrane protein linked to a G protein on the cytoplasmic side. When a ligand binds: G protein activates → activates effector enzyme (e.g., adenylyl cyclase) → produces second messenger (e.g., cAMP). Examples: epinephrine receptor, odorant receptors, many hormone receptors.
Enzyme-linked receptor. When ligand binds (typically as a dimer): the receptor autophosphorylates tyrosine residues on its cytoplasmic domain → creates docking sites for relay proteins → activates multiple downstream pathways simultaneously. Examples: insulin receptor, growth factor receptors. Mutations frequently cause cancer.
Channel protein that opens or closes when a specific ligand binds. Direct ion flow (Na⁺, K⁺, Ca²⁺, Cl⁻) changes the membrane potential of the cell. Very fast response — milliseconds. Critical in nervous system (nicotinic acetylcholine receptors, GABA receptors). The binding of ligand = channel opens = ions flow.
For lipid-soluble signals (steroids, thyroid hormones). The hormone-receptor complex acts as a transcription factor — binds to specific DNA sequences (hormone response elements) in the nucleus → directly activates or represses gene transcription. Response is slow (hours) but long-lasting.
Stage 2 — Transduction: Phosphorylation Cascades
Most signal transduction pathways involve a phosphorylation cascade: a series of protein kinases (enzymes that add phosphate groups) that sequentially activate each other. Each step amplifies the signal — one activated protein kinase can phosphorylate and activate hundreds of substrate proteins. This is signal amplification.
Second messengers are small intracellular molecules that rapidly relay and amplify the signal from the receptor deeper into the cell. The most important second messenger in AP Biology is cyclic AMP (cAMP):
Stage 3 — Response
The final response can include:
- Change in enzyme activity (e.g., glycogen phosphorylase activated → glycogen → glucose)
- Changes in gene expression (transcription factors activated/repressed → new proteins made)
- Changes in membrane transport (ion channels open/close)
- Cell growth or division (growth factor → RTK → Ras → kinase cascade → cell cycle progression)
- Apoptosis (programmed cell death triggered by specific signals)
- Secretion (exocytosis of hormones, neurotransmitters, digestive enzymes)
Signal amplification: One ligand binding to one receptor can ultimately activate millions of effector molecules. This amplification occurs at each step of the phosphorylation cascade — each activated kinase phosphorylates many substrates. The AP exam frequently asks "why can a tiny amount of signal produce a large response?" → amplification through cascade.
cAMP as second messenger: cAMP is produced by adenylyl cyclase (activated by a G protein) from ATP. It is broken down by phosphodiesterase. Caffeine and theophylline work by inhibiting phosphodiesterase → cAMP stays elevated → prolonged signaling. Cholera toxin locks G protein in the ON state → uncontrolled cAMP → massive Na⁺/water secretion into intestines → diarrhea.
Epinephrine binds to a receptor on liver cells, leading to the activation of a G protein, which then activates adenylyl cyclase. Adenylyl cyclase converts ATP to cAMP, which activates Protein Kinase A (PKA). PKA then phosphorylates and activates glycogen phosphorylase, breaking down glycogen to glucose. If a drug blocks adenylyl cyclase, which of the following would most likely occur?
- (A) Epinephrine would be unable to bind to its receptor.
- (B) The G protein would remain permanently activated.
- (C) cAMP levels would decrease, PKA would not be activated, and glycogen breakdown would be reduced.
- (D) Glycogen phosphorylase would be activated by a different pathway.
Signal Transduction Pathways — Outcomes & Disruptions
Signal transduction pathways don't just trigger short-term responses — they can fundamentally alter cell behavior including gene expression, cell fate, and cell death. Understanding what happens when these pathways are mutated or chemically disrupted is essential for AP Biology and connects directly to cancer biology.
Cellular Responses to Signal Transduction
Many signaling pathways ultimately activate or repress transcription factors that alter gene expression. Example: Growth factor → RTK → Ras GTPase → MAP kinase cascade → transcription factors enter nucleus → growth genes expressed → cell divides. Cytokines trigger gene expression changes that allow immune cell replication.
A tightly regulated process in which a cell self-destructs in an orderly way. Triggered by specific signals (e.g., DNA damage beyond repair, developmental cues, immune elimination of infected cells). Caspase enzymes are activated → cell is dismantled from within → fragments are engulfed by phagocytes. No inflammation (unlike necrosis). Essential for embryonic development (finger separation) and immune function.
Growth factors bind RTKs → activate downstream kinase cascades → promote G1 entry and S phase DNA replication → cell division. In embryonic development, morphogens (gradient-forming signals) and HOX genes (transcription factors) regulate body plan formation. Ethylene (a plant hormone) triggers fruit ripening by altering enzyme expression.
Bacteria release signaling molecules that accumulate in proportion to population density. When the concentration reaches a threshold (quorum), bacteria collectively switch gene expression — forming biofilms, producing virulence factors, or bioluminescing. This is a prokaryotic form of cell communication via chemical signaling.
Disruptions to Signaling Pathways
Mutations or chemicals that alter any component of a signaling pathway can cause disease:
| Disruption | Effect | Disease Example |
|---|---|---|
| Constitutively active receptor (stuck ON) | Continuous signaling without ligand → uncontrolled cell proliferation | Certain breast cancers (HER2 overexpression); chronic myelogenous leukemia (BCR-ABL fusion kinase) |
| Impaired signal reception or transduction | Reduced or absent response to signal even when ligand is present | Type 2 diabetes — impaired insulin signaling / insulin resistance (cells fail to respond normally to insulin; involves receptor and post-receptor signaling defects); androgen insensitivity syndrome |
| Mutant Ras GTPase (stuck ON) | G protein cannot hydrolyze GTP → permanently active → continuous proliferation signal | ~30% of all human cancers have Ras mutations |
| Loss of tumor suppressors (e.g., p53) | Checkpoint proteins that stop uncontrolled division are lost → cells divide without restriction | p53 mutation in ~50% of cancers; BRCA1/2 mutations in breast/ovarian cancer |
| Drugs targeting signaling components | Block aberrant signals → stop cancer cell growth | Imatinib (Gleevec) blocks BCR-ABL kinase; trastuzumab (Herceptin) blocks HER2 receptor |
Cancer arises when normal cell division signals become permanently switched on (proto-oncogene → oncogene) OR when braking mechanisms are lost (tumor suppressor gene mutation). The AP exam frequently presents a scenario with a mutated receptor or kinase and asks you to predict the cellular outcome.
Proto-oncogene: A normal gene that promotes cell division when activated by appropriate signals. Oncogene: A mutated proto-oncogene that causes constitutive (always-on) signaling → uncontrolled division. Examples: Ras, Her2/neu.
Tumor suppressor gene: Normally puts the brakes on the cell cycle (e.g., p53 triggers apoptosis after DNA damage; Rb blocks G1→S transition). When both copies are lost (two-hit hypothesis) → loss of growth control.
A researcher discovers a mutation in the gene encoding a receptor tyrosine kinase (RTK) that causes the receptor to be continuously active even in the absence of its growth factor ligand. Predict and explain the likely effect of this mutation on the cell and organism.
Explanation: Normally, RTKs are only active when their specific growth factor ligand binds, causing dimerization and autophosphorylation. The activated RTK then initiates a kinase cascade (e.g., Ras → MAP kinase) that ultimately activates transcription factors promoting cell cycle entry and division.
With a constitutively active RTK, the cascade is permanently switched on without ligand. The cell receives a continuous "divide now" signal. The downstream kinases and transcription factors remain active, driving the cell through the cell cycle repeatedly and bypassing normal growth controls.
At the organismal level, this could result in tumor formation (cancer) — a mass of cells dividing uncontrollably because the growth signal is never "turned off." This is equivalent to a proto-oncogene becoming an oncogene, which is observed in several human cancers (e.g., HER2 amplification in breast cancer).
Feedback Mechanisms
Organisms use feedback loops to maintain homeostasis — stable internal conditions necessary for life. A feedback loop detects a deviation from a set point and triggers a response to correct it. Two types exist: negative feedback (far more common) and positive feedback.
Negative Feedback — Maintaining Homeostasis
In negative feedback, the system's output opposes (negates) the initial stimulus, returning conditions toward the set point. This is the dominant homeostatic mechanism in biology. The response reduces the deviation from normal.
↑ Blood glucose → pancreatic β cells secrete insulin → liver/muscle take up glucose + convert to glycogen → blood glucose falls back to set point (~90 mg/dL).
↓ Blood glucose → pancreatic α cells secrete glucagon → liver breaks down glycogen → releases glucose → blood glucose rises back to set point.
↑ Temperature → hypothalamus signals: vasodilation of skin blood vessels + sweating → heat lost → temperature falls back to 37°C.
↓ Temperature → vasoconstriction + shivering (muscle contractions generate heat) → temperature rises back to set point.
Hypothalamus → TRH → Pituitary → TSH → Thyroid → T3/T4.
High T3/T4 in blood → inhibits hypothalamus AND pituitary → less TRH and TSH released → less T3/T4 produced. This three-tier negative feedback is a classic AP exam example.
End product of a metabolic pathway allosterically inhibits the first enzyme of that pathway. Connects to Topic 3.2. Example: excess isoleucine (amino acid) inhibits threonine deaminase (first enzyme in isoleucine synthesis pathway) → pathway slows when product is plentiful.
Positive Feedback — Amplifying a Response
In positive feedback, the system's output amplifies the initial stimulus — driving the variable further away from the set point. This is not for maintaining homeostasis; it drives processes toward a definitive endpoint quickly.
| Example | Trigger | Amplification | Endpoint |
|---|---|---|---|
| Childbirth (Labor) | Uterine contractions stretch the cervix | Oxytocin released → more contractions → more cervical stretching → more oxytocin → stronger contractions | Baby born — cervix no longer stretched → feedback stops |
| Blood Clotting | Vessel injury exposes collagen | Platelets aggregate → release chemicals attracting more platelets → clotting factors activate more clotting factors | Clot seals wound → cascades stop |
| Fruit Ripening (Ethylene) | Fruit begins to ripen → releases ethylene | Ethylene triggers more ethylene production in nearby fruit → accelerating ripening throughout the fruit/cluster | Fully ripe fruit — ethylene production slows |
| Action Potential (Nervous System) | Membrane depolarization reaches threshold | Na⁺ channels open → Na⁺ rushes in → more depolarization → more channels open | Rapid full depolarization → then K⁺ channels restore resting potential |
Negative feedback = homeostasis; positive feedback = amplification to completion. The AP exam tests this distinction constantly. Never say positive feedback "maintains homeostasis" — it deliberately moves a variable away from its resting set point toward a definitive outcome.
Insulin and glucagon are antagonistic hormones. They have opposite effects on blood glucose and are both negative feedback regulators. Diabetes Type 1 = insufficient insulin production; Type 2 = insulin resistance (receptor/postreceptor defect). Either destroys blood glucose homeostasis.
During childbirth, uterine contractions cause oxytocin release, which leads to stronger contractions, which causes more oxytocin release. This process continues until birth. This is an example of
- (A) negative feedback, because the contractions eventually stop after birth.
- (B) negative feedback, because oxytocin is released in response to contractions.
- (C) positive feedback, because the response (oxytocin) amplifies the initial stimulus (contractions) rather than reducing it.
- (D) homeostasis, because contractions maintain a stable uterine environment.
The Cell Cycle
The cell cycle is the ordered sequence of events by which a cell grows and divides into two daughter cells. It is divided into two major phases: Interphase (growth and DNA replication) and the Mitotic Phase (nuclear and cytoplasmic division). Most of a cell's life is spent in interphase.
Organelle duplication
Protein synthesis
(G₁ checkpoint here)
Each chromosome → 2 sister chromatids
Centrosome replication begins
ATP production ↑
Centrosome replication
(G₂ checkpoint here)
→ Anaphase → Telophase
+ Cytokinesis
Quiescent state
Some cells can re-enter cycle; others (e.g., mature neurons, cardiac muscle) remain in G0 permanently
(liver cells can re-enter; most neurons cannot)
Interphase — Details
| Phase | Key Events | DNA State |
|---|---|---|
| G₁ (Gap 1) | Cell grows; organelles duplicated; proteins for DNA replication synthesized; G₁ checkpoint ("restriction point") — determines whether conditions are right to commit to division | 2n (diploid), unduplicated chromosomes (chromatin form) |
| S (Synthesis) | DNA replication — each chromosome is duplicated, creating two identical sister chromatids held together at the centromere by cohesins. Histone proteins synthesized simultaneously | Chromosome number remains 2n; DNA amount doubles by end of S phase (cell now has twice the DNA of G1, though it is still diploid) |
| G₂ (Gap 2) | Cell continues growing; centrosomes finish replicating (each centriole pair duplicated); proteins for mitosis (e.g., tubulin for spindle) synthesized; ATP reserves maximized; G₂ checkpoint — checks for complete DNA replication and DNA damage | Chromosome number still 2n; DNA amount = doubled (each of the 2n chromosomes consists of 2 sister chromatids) |
Mitosis — The Four Stages + Cytokinesis
| Stage | Key Events | Chromosome State |
|---|---|---|
| Prophase | Chromatin condenses into visible chromosomes; mitotic spindle begins forming (from centrosomes at poles); nuclear envelope breaks down; nucleolus disappears | Chromosomes visible as sister chromatid pairs; spindle attaches to kinetochores |
| Metaphase | Chromosomes align along the metaphase plate (cell equator); spindle fibers from opposite poles attach to kinetochores of each sister chromatid; M checkpoint verifies all chromosomes are attached | Chromosomes maximally condensed at equator; 2 sister chromatids per chromosome |
| Anaphase | Sister chromatids separate — cohesins cleaved by separase; sister chromatids pulled to opposite poles by shortening spindle fibers; cell elongates | Single chromatids (now called chromosomes) moving to each pole; each pole has full diploid set |
| Telophase | Chromosomes arrive at poles; nuclear envelope re-forms around each set; chromosomes begin to decondense; spindle dismantles; two nuclei present | Two sets of diploid chromosomes at opposite ends |
| Cytokinesis | Animal cells: actin/myosin form a cleavage furrow that pinches the cell in two. Plant cells: vesicles from Golgi fuse at cell equator → cell plate forms → new cell wall laid down between cells | Two genetically identical daughter cells (2n each) |
Mitosis: 1 division → 2 daughter cells; genetically identical to parent (2n → 2n); used for growth, tissue repair, asexual reproduction; no crossing over.
Meiosis: 2 divisions → 4 daughter cells; genetically unique (2n → 4 haploid, n); used for sexual reproduction (gametes); crossing over in Prophase I increases genetic diversity. (Full coverage in Unit 5.)
Both processes use a spindle apparatus (microtubules/tubulin) to move chromosomes. Both have prophase, metaphase, anaphase, telophase stages. Key meiosis-specific feature: synapsis and crossing over in Prophase I.
A researcher treats a population of cells with a drug that prevents DNA replication. In which phase of the cell cycle would these cells be arrested?
- (A) G₁ phase
- (B) S phase
- (C) G₂ phase
- (D) Metaphase
Regulation of the Cell Cycle
The cell cycle is tightly regulated to ensure cells divide only when appropriate and that each daughter cell receives an exact copy of the genome. Two molecular control systems govern this: cyclins/CDKs (molecular accelerators) and checkpoints (molecular brakes). Disruption of either system can lead to cancer or apoptosis.
Cyclins and Cyclin-Dependent Kinases (CDKs)
Cyclin-dependent kinases (CDKs) are protein kinases that are only active when bound to a cyclin protein. Cyclin concentrations fluctuate dramatically through the cell cycle — they rise and fall in waves, activating specific CDKs at specific times. The cyclin–CDK complex phosphorylates target proteins that drive cell cycle progression.
Cell Cycle Checkpoints
Checkpoints are surveillance mechanisms that pause the cell cycle if conditions are not met. They monitor for: (1) adequate cell size, (2) sufficient growth signals, (3) DNA integrity (no damage), (4) complete DNA replication, and (5) correct chromosome attachment to spindle fibers.
| Checkpoint | Location | What Is Checked | Effect if Failed |
|---|---|---|---|
| G₁ Checkpoint (Restriction Point) | Late G₁, before S phase | Cell size sufficient? DNA undamaged? Growth factors present? Sufficient nutrients? | Cell exits cell cycle → G₀ (quiescence) or apoptosis if DNA damaged |
| G₂ Checkpoint | End of G₂, before mitosis | Was DNA fully replicated? Is there any unreplicated DNA or DNA damage? | Cell cycle paused; DNA repair initiated; p53 can trigger apoptosis if damage irreparable |
| Spindle Assembly Checkpoint (M Checkpoint) | Metaphase, before anaphase | Are all chromosomes properly attached to spindle fibers from both poles (amphitelic attachment)? | Anaphase-promoting complex (APC) held inactive → sister chromatids cannot separate → cycle paused |
p53 — The Guardian of the Genome
p53 is a critical tumor suppressor protein (transcription factor) that is activated by DNA damage signals. Activated p53 can: (1) halt the cell cycle at G₁ or G₂ to allow DNA repair, or (2) trigger apoptosis if damage is irreparable. p53 is mutated or non-functional in approximately 50% of all human cancers — making it the most commonly mutated gene in cancer.
Apoptosis — Programmed Cell Death
Apoptosis is a controlled, energy-requiring process of cellular self-destruction. It differs fundamentally from necrosis (uncontrolled cell death from injury, which causes inflammation). Apoptosis:
- Is triggered by internal signals (DNA damage, developmental cues) or external signals (immune cytokines)
- Activates caspase enzymes → protein digestion, DNA fragmentation, membrane blebbing
- Cell fragments are packaged into apoptotic bodies → engulfed by phagocytes → no inflammation
- Is essential for normal development (separating fingers and toes in embryo, eliminating excess neurons), immune selection (eliminating autoreactive T cells), and cancer suppression
Cancer — Loss of Cell Cycle Control
Cancer results from accumulation of mutations in two classes of genes:
Mutated proto-oncogenes that cause constitutive cell cycle activation. Dominant — only ONE mutant copy needed. Leads to uncontrolled proliferation. Examples: Ras mutation (30% of cancers), HER2 amplification (breast cancer), Bcr-Abl fusion (leukemia).
Normally restrain cell division or trigger apoptosis. Recessive — BOTH copies must be lost (two-hit hypothesis, Knudson). Examples: p53 (DNA damage response); Rb (blocks G₁→S, retinoblastoma); BRCA1/2 (DNA repair, breast/ovarian cancer).
G₁ checkpoint is the "commitment point" — once a cell passes it, it is committed to dividing. Loss of the G₁ checkpoint (e.g., Rb mutation) means cells enter S phase without proper signals → uncontrolled replication. This is one of the most tested cancer mechanisms.
Contact inhibition: Normal cells stop dividing when they contact neighboring cells — a form of negative feedback via surface receptors. Cancer cells lose contact inhibition → keep dividing even in a confluent monolayer → pile up (tumor formation).
Apoptosis vs. necrosis: Apoptosis = ordered, no inflammation, serves a purpose. Necrosis = chaotic, causes inflammation. AP FRQs ask students to distinguish these. Apoptosis requires energy (active process); necrosis is passive.
A mutation eliminates the G₂ checkpoint in a population of liver cells. Predict and explain TWO specific consequences of this mutation on cell behavior and organism health.
Consequence 2 — Cells with DNA damage are not repaired before division: The G₂ checkpoint also detects DNA damage (double-strand breaks, mismatched bases). Normally, damaged cells are held in G₂ while repair occurs; if damage is irreparable, p53 triggers apoptosis. Without the checkpoint, cells with DNA damage proceed directly into mitosis, passing mutations on to all daughter cells. Accumulation of mutations over multiple rounds of division can activate oncogenes or inactivate tumor suppressors → tumor formation in the liver.
Mixed Practice Questions
A scientist discovers that a certain protein kinase in a cancer cell is permanently phosphorylated (active) and cannot be inactivated. Which of the following best describes the role of this protein in cancer development?
- (A) It is acting as a tumor suppressor, because it activates cell cycle inhibitors.
- (B) It is acting as an oncogene product, because it continuously promotes cell cycle progression even without an external growth signal.
- (C) It is initiating apoptosis, because permanently active kinases trigger programmed cell death.
- (D) It is functioning as a checkpoint protein, preventing cells from dividing prematurely.
A patient has a mutation in both copies of the gene encoding p53. Which of the following outcomes is most likely in the patient's cells?
- (A) Cells will arrest at the G₁ checkpoint when DNA damage occurs.
- (B) Cells will undergo apoptosis more frequently than normal cells.
- (C) Cells with damaged DNA will fail to arrest or undergo apoptosis, increasing the risk of uncontrolled division.
- (D) The cell cycle will be permanently arrested at the S phase checkpoint.
High-Frequency Errors to Avoid
- 📡Saying the signal molecule enters the cell to cause a response (for protein hormones)Protein/peptide hormones bind surface receptors and NEVER enter the cell. The signal stays outside; the conformational change in the receptor initiates an intracellular cascade. Only steroid/thyroid hormones cross the membrane to bind intracellular receptors.
- 🔁Saying positive feedback maintains homeostasisPositive feedback amplifies a signal toward a definitive endpoint (birth, clot formation, action potential). It deliberately moves away from the set point. Only negative feedback maintains homeostasis by opposing deviations.
- 🧬Saying sister chromatids separate in Meiosis I (or in Mitosis Metaphase)In Mitosis Anaphase and Meiosis II Anaphase, SISTER CHROMATIDS separate. In Meiosis I Anaphase, HOMOLOGOUS CHROMOSOMES separate (sister chromatids remain together). This is one of the most tested distinctions in genetics questions.
- 🔬Thinking DNA replication occurs during mitosisDNA replication occurs during S phase of INTERPHASE — before mitosis begins. By the time prophase starts, all chromosomes have already been replicated (each consisting of 2 sister chromatids). Mitosis only separates the already-replicated chromosomes.
- 🎯Confusing proto-oncogenes and tumor suppressorsProto-oncogenes promote division when mutated to oncogenes (dominant — one mutant copy is enough). Tumor suppressors inhibit division; both copies must be lost for cancer (recessive — two-hit hypothesis). Oncogene = stuck accelerator; tumor suppressor = cut brake lines.
- 💊Saying cAMP is produced by the receptor directlycAMP is produced by ADENYLYL CYCLASE, an enzyme that is activated by the G protein, which is activated by the receptor. The receptor → G protein → adenylyl cyclase → cAMP is a three-step relay. The receptor itself does not produce cAMP.
- 💀Confusing apoptosis with necrosisApoptosis = ordered, energy-requiring, no inflammation, cell fragments phagocytosed cleanly. Necrosis = chaotic injury-caused cell death, rupture, inflammation. Apoptosis is healthy and necessary (embryo development, immune selection). Never say "apoptosis causes inflammation."
Unit 4 — Key Takeaways
Direct contact vs. distance signaling. Hydrophilic signals → surface receptors. Steroids → intracellular receptors → direct gene regulation. Signal specificity determined by receptor type.
Reception → Transduction → Response. Ligand binds receptor → conformational change. GPCRs, RTKs, ligand-gated channels. Phosphorylation cascades amplify signal. cAMP = key second messenger.
Constitutively active receptors/kinases → uncontrolled division. Responses: gene expression, apoptosis, secretion. Oncogenes (dominant), tumor suppressors (recessive). p53 = guardian of genome.
Negative: opposes stimulus → homeostasis (blood glucose, temperature, hormone axes). Positive: amplifies stimulus → definitive endpoint (labor, clotting, action potential, fruit ripening).
G₁ (grow) → S (replicate DNA) → G₂ (prepare) → Mitosis (PMAT) → Cytokinesis. G₀ = non-dividing. Mitosis: prophase (condense), metaphase (align), anaphase (separate), telophase (reform). Cleavage furrow (animals) vs. cell plate (plants).
Cyclins activate CDKs → drive transitions. Checkpoints: G₁ (commit?), G₂ (DNA replicated?), M (spindle attached?). p53 halts cycle or triggers apoptosis on DNA damage. Cancer = loss of both accelerators and brakes.
Unit 4 = 10–15% of the AP Biology Exam. The highest-yield topics are: the three stages of signal transduction with a worked example (Reception→Transduction→Response), signal amplification through phosphorylation cascades, negative vs. positive feedback with real examples, the cell cycle phases and what occurs in each, the three checkpoints and what each monitors, and the molecular basis of cancer (oncogenes vs. tumor suppressors, p53). Always trace signal disruptions through the entire pathway — "what happens if step X is blocked/mutated?" is the most common question format.