Cell Communication & Cycle

What this unit covers

The topics below follow the published Biology course framework for Unit 4. This unit is worth 10–15% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Lessons in this unit

• Cell Signaling & Signal Transduction14 min · 3 objectivesOrder the three stages of cell signaling: reception, transduction, and response · Contrast G-protein-coupled receptors and receptor tyrosine kinases · Explain how second messengers and phosphorylation cascades amplify a signal
• Feedback Mechanisms & Homeostasis12 min · 3 objectivesDefine negative feedback and explain how it maintains homeostasis · Distinguish positive feedback from negative feedback using biological examples · Predict whether a given loop stabilizes or amplifies a change
• The Cell Cycle & Mitosis14 min · 3 objectivesDescribe the events of interphase (G1, S, G2) and identify when DNA is copied · Order the four phases of mitosis and the key event of each · Distinguish mitosis (nuclear division) from cytokinesis (cell division)
• Cell Cycle Checkpoints & Cancer13 min · 3 objectivesIdentify the G1, G2, and M checkpoints and what each verifies · Explain how cyclins and Cdks drive the cell cycle forward · Relate loss of checkpoint control to the development of cancer
• Signal Transduction Pathways in Depth15 min · 3 objectivesCompare G-protein-coupled receptors, receptor tyrosine kinases, and intracellular receptors by ligand type and mechanism · Quantify signal amplification across a multi-step phosphorylation cascade and explain the role of second messengers · Explain how pathways are terminated and how signaling triggers apoptosis
• Cell-Cycle Control & Cancer14 min · 3 objectivesState what the G1, G2, and M checkpoints each verify and how cyclin-Cdk complexes drive transitions · Explain the Rb/E2F switch and how p53 enforces the G1 checkpoint · Distinguish proto-oncogenes from tumor suppressor genes and relate their mutations, plus loss of density-dependent inhibition, to cancer

Formulas in Unit 4

Every term in Unit 4

All 37 terms we publish for Cell Communication & Cycle, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Sources of genetic variation in meiosis

Crossing over, independent assortment and random fertilization — together far more powerful than mutation in generating diversity each generation.

Signal transduction pathway

Reception by a receptor, transduction through a relay of molecules, then a cellular response. The relay amplifies the signal at every step.

Ligand

The signaling molecule that binds a receptor. Specificity comes from shape complementarity, which is why a hormone affects only cells with its receptor.

G protein-coupled receptor

Membrane receptor that activates a G protein on ligand binding, which then activates an enzyme — one of the most common receptor families.

Second messenger

Small intracellular molecule such as cAMP or Ca²⁺ that spreads a signal from the membrane through the cytoplasm rapidly.

Phosphorylation cascade

A chain of kinases each phosphorylating the next. Amplifies the signal enormously and gives multiple points of regulation.

Signal amplification

One ligand can produce millions of product molecules because each step activates many downstream molecules — how a few hormone molecules change a whole cell.

Paracrine, endocrine and synaptic signaling

Local diffusion to nearby cells; hormones traveling in blood to distant targets; neurotransmitters across a synapse. Distance defines the category.

Quorum sensing

Bacteria release and detect signal molecules to gauge population density, switching on group behaviors like biofilms only when numbers suffice.

Apoptosis

Programmed cell death by an orderly internal cascade. Removes damaged cells and sculpts tissue — the webbing between fingers goes this way.

Interphase

G1 (growth), S (DNA replication) and G2 (preparation). Most of the cell's life, and where chromosomes become sister chromatids.

Mitosis phases

Prophase (chromosomes condense), metaphase (align at the plate), anaphase (sister chromatids separate), telophase (nuclei re-form).

Cytokinesis

Division of the cytoplasm — a cleavage furrow in animal cells, a cell plate built from vesicles in plant cells.

G1 checkpoint

Assesses cell size, nutrients and DNA damage before committing to division. The most important control point; cells that fail it may enter G0.

G2 and M checkpoints

G2 verifies DNA has replicated correctly; the M (spindle) checkpoint verifies every chromosome is attached to the spindle before anaphase.

Cyclins and CDKs

Cyclin concentration rises and falls through the cycle; binding a cyclin activates its cyclin-dependent kinase, which phosphorylates targets that drive the next phase.

Cancer as checkpoint failure

Mutations in checkpoint genes let damaged cells divide. Proto-oncogene mutations push division; tumor-suppressor mutations remove the brakes.

p53

Tumor suppressor that halts the cycle for repair or triggers apoptosis. Mutated in roughly half of human cancers.

Meiosis vs mitosis

Meiosis has two divisions and produces four genetically distinct haploid cells; mitosis has one and produces two identical diploid cells.

Crossing over

Homologous chromosomes exchange segments at chiasmata in prophase I, producing recombinant chromatids and new allele combinations.

Independent assortment

Homologous pairs align randomly at metaphase I, so each gamete gets a random mix — 2ⁿ combinations, over 8 million in humans before crossing over.

Receptor specificity

A cell responds to a signal only if it has the matching receptor, which is how a hormone in general circulation affects only some tissues.

Intracellular receptors

Steroid and other nonpolar signals cross the membrane and bind receptors inside, often acting directly as transcription factors.

Ligand-gated ion channel

A receptor that opens a pore on ligand binding, letting ions flow and changing membrane potential — the mechanism at most synapses.

Tyrosine kinase receptor

Dimerises and phosphorylates itself on ligand binding, creating docking sites so one ligand can trigger several pathways at once.

cAMP pathway

Adenylyl cyclase converts ATP to cAMP, which activates protein kinase A. The classic second-messenger cascade downstream of a GPCR.

Calcium as a second messenger

Held at very low cytosolic concentration and released from the ER on signal, so a small release produces a large relative change.

Signal termination

Phosphatases remove phosphates and ligands dissociate, so the pathway shuts down. Without termination a cell could not respond to the next signal.

Negative and positive feedback

Negative feedback opposes a change and maintains homeostasis; positive feedback amplifies it, as in childbirth and the action potential.

Chromatin vs chromosome

Chromatin is the extended DNA-protein complex of interphase; a chromosome is that same material condensed for division.

Sister chromatids vs homologous chromosomes

Sister chromatids are identical copies joined at a centromere; homologues are the maternal and paternal versions of the same chromosome, carrying the same genes but possibly different alleles.

Centrosome and spindle

Microtubule-organizing centers that build the spindle fibers attaching to kinetochores and pulling chromosomes apart.

Meiosis I vs meiosis II

Meiosis I separates homologues and halves the chromosome number; meiosis II separates sister chromatids like a mitotic division.

Synapsis and tetrads

Homologues pair along their length in prophase I, forming a four-chromatid structure where crossing over occurs.

Density-dependent inhibition

Normal cells stop dividing when they contact neighbors. Cancer cells ignore this, which is why they pile up.

Anchorage dependence

Most normal cells divide only when attached to a surface. Loss of this requirement is a hallmark of malignancy.

Lab: Mitosis and Meiosis

Onion root tip squashes are scored for the number of cells in each phase. The proportion in a phase estimates the fraction of the cycle that phase occupies.

What examiners penalize here

• For AP free-response, name the stages in order — **reception → transduction → response** — and tie the cascade explicitly to **amplification**. Saying "the signal is passed along" earns little; explaining that each step activates many molecules at the next step earns the point.
• On the AP exam, always state the loop’s *direction* explicitly: does the response **counteract** the change (negative) or **amplify** it (positive)? Anchor the claim to a set point for negative feedback, or to a runaway endpoint (birth, clotting) for positive feedback.
• The AP framing for cancer is "**accelerators and brakes**." Tie any loss of control back to a specific failure: an **oncogene** stuck on (gain of function) or a **tumor suppressor** switched off (loss of function), and name the checkpoint (often **G1**, guarded by **p53**) that fails as a result.
• For AP free-response, treat **termination as a required half of the answer**: name a specific off-switch (G protein hydrolyzes GTP → GDP, phosphodiesterase degrades cAMP, phosphatases remove phosphates) and note that a *sustained* response needs *continued* ligand. Losing an off-switch causes constant, unregulated signaling — a common cancer mechanism.
• Cancer answers should name **which control failed and how**: an **oncogene** stuck on (gain of function, one copy) or a **tumor suppressor** lost (loss of function, two hits — e.g. **p53** at G1 or **Rb**), plus loss of **density-dependent inhibition**. Stress that cancer needs **several mutations accumulating together**, not one.

Practice Biology

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 8 units of AP Biology

1. Unit 1 · Chemistry of Life
2. Unit 2 · Cell Structure & Function
3. Unit 3 · Cellular Energetics
4. Unit 4 · Cell Communication & Cycle
5. Unit 5 · Heredity
6. Unit 6 · Gene Expression & Regulation
7. Unit 7 · Natural Selection
8. Unit 8 · Ecology