What you'll learn
This revision guide covers the essential aspects of cell communication and the cell cycle for AP Biology. You'll master how cells receive and respond to external signals through signal transduction pathways, and understand the precisely regulated stages of cell division. These concepts are fundamental to understanding how multicellular organisms coordinate activities and maintain controlled growth.
Key terms and definitions
Signal transduction pathway — A series of molecular steps by which a signal on a cell's surface is converted into a specific cellular response, involving relay molecules and second messengers.
Ligand — A molecule that binds specifically to a receptor protein, triggering a cellular response; may be a hormone, neurotransmitter, or growth factor.
Cyclin-dependent kinase (CDK) — A regulatory protein kinase that becomes active only when bound to a cyclin protein; controls progression through cell cycle checkpoints.
Apoptosis — Programmed cell death characterized by a series of controlled steps leading to cell self-destruction without damaging neighboring cells.
G1 checkpoint — The restriction point in late G1 phase where the cell commits to division or enters G0; checks for adequate size, nutrients, growth signals, and DNA integrity.
Phosphorylation cascade — A series of protein modifications in which each kinase activates the next kinase by adding phosphate groups, amplifying the signal.
Quorum sensing — A cell-to-cell communication process in bacteria where population density is monitored through secreted signaling molecules called autoinducers.
MPF (Maturation Promoting Factor) — A cyclin-CDK complex (cyclin B-CDK1) that triggers the cell's passage from G2 into M phase by phosphorylating proteins involved in mitosis.
Core concepts
Cell communication mechanisms
Cells communicate through three main methods: direct contact, local signaling, and long-distance signaling.
Direct contact involves:
- Gap junctions in animal cells allowing passage of small molecules and ions between adjacent cells
- Plasmodesmata in plant cells serving similar intercellular communication functions
- Cell-cell recognition through membrane-bound signaling molecules binding to receptors on adjacent cells
Local signaling includes:
- Paracrine signaling where secreted molecules affect nearby target cells (e.g., growth factors, neurotransmitters)
- Synaptic signaling in the nervous system where neurotransmitters cross synaptic clefts
- Autocrine signaling where cells respond to substances they themselves secrete
Long-distance signaling occurs primarily through:
- Endocrine signaling where hormones travel through the bloodstream to distant target cells
- Examples include insulin from pancreatic β-cells affecting liver, muscle, and fat cells
Signal transduction pathway stages
Cell signaling follows three sequential stages: reception, transduction, and response.
Reception occurs when a signaling molecule binds to a receptor protein. Receptors fall into two categories:
Intracellular receptors for hydrophobic ligands (steroids, thyroid hormones, nitric oxide) that can cross the plasma membrane. These typically function as transcription factors that directly regulate gene expression.
Cell-surface receptors for hydrophilic ligands that cannot cross membranes:
- G protein-coupled receptors (GPCRs) — largest receptor family; work with G proteins to activate enzymes or ion channels
- Receptor tyrosine kinases (RTKs) — dimerize upon ligand binding and phosphorylate tyrosines on cytoplasmic tails, activating multiple signal transduction pathways
- Ion channel receptors — ligand-gated channels that open or close in response to binding, allowing specific ions to flow
Transduction amplifies and converts the signal through:
- Second messengers (small, non-protein molecules that relay signals): cyclic AMP (cAMP), cyclic GMP, calcium ions (Ca²⁺), inositol trisphosphate (IP₃), diacylglycerol (DAG)
- Phosphorylation cascades where protein kinases sequentially activate each other, providing signal amplification
- Each activated enzyme can activate multiple downstream molecules, creating exponential amplification
Response may involve:
- Activation of cellular enzymes
- Rearrangement of the cytoskeleton
- Changes in gene expression
- Altered cell metabolism
Feedback and signal termination
Signal transduction pathways incorporate regulatory mechanisms:
- Negative feedback — the response reduces the initial signal (e.g., protein kinase A phosphorylates and inactivates enzymes that produce cAMP)
- Scaffolding proteins — organize groups of signaling proteins into signaling complexes for enhanced specificity and efficiency
- Signal termination — receptor desensitization, degradation of second messengers, removal of phosphate groups by protein phosphatases
The cell cycle phases
The cell cycle consists of interphase (G1, S, G2) and the mitotic phase (M).
Interphase comprises approximately 90% of the cell cycle:
G1 phase (Gap 1):
- Intensive cellular growth and metabolic activity
- Accumulation of enzymes and nutrients needed for DNA replication
- Production of organelles and proteins
- Duration varies greatly between cell types
S phase (Synthesis):
- DNA replication occurs
- Centrosome duplication
- Histones synthesized
- Cell continues to grow
G2 phase (Gap 2):
- Continued cell growth
- Protein synthesis, particularly tubulin for spindle fibers
- Organelle replication completed
- Preparation for mitosis
G0 phase — quiescent state where cells have exited the cell cycle and are not preparing to divide. Some cells (neurons, cardiac muscle cells) remain permanently in G0, while others (liver cells) can re-enter upon appropriate signals.
M phase (Mitotic phase) includes:
- Mitosis (nuclear division): prophase, metaphase, anaphase, telophase
- Cytokinesis (cytoplasmic division)
Cell cycle regulation and checkpoints
The cell cycle is regulated by cyclins and cyclin-dependent kinases (CDKs).
Cyclins are regulatory proteins whose concentrations fluctuate cyclically during the cell cycle. Different cyclins are active at different stages.
CDKs are protein kinases that are present throughout the cell cycle but active only when bound to appropriate cyclins. The cyclin-CDK complex phosphorylates specific target proteins to drive cell cycle events.
Major checkpoints ensure proper cell cycle progression:
G1 checkpoint (restriction point):
- Most critical checkpoint
- Assesses: cell size, nutrient availability, growth signals, DNA integrity
- If conditions are unfavorable, cells enter G0
- p53 protein (tumor suppressor) can halt the cycle here if DNA damage is detected
G2 checkpoint:
- Confirms: DNA replication is complete, DNA is undamaged, sufficient cell size
- Ensures cell is ready for mitosis
- MPF accumulation triggers passage into M phase
M checkpoint (spindle/metaphase checkpoint):
- Occurs during metaphase
- Verifies: all chromosomes are attached to spindle fibers, chromosomes are properly aligned at metaphase plate
- Prevents premature separation of sister chromatids
- Anaphase-promoting complex (APC) triggers progression to anaphase
Cell cycle regulation failure and cancer
Cancer results from loss of cell cycle control mechanisms.
Proto-oncogenes are normal genes that promote cell division. When mutated or overexpressed, they become oncogenes that can cause excessive cell proliferation. Examples include:
- Growth factor genes
- Receptor genes (e.g., HER2)
- Signal transduction proteins (e.g., Ras)
Tumor suppressor genes normally restrain cell division. Loss-of-function mutations allow uncontrolled growth. Key examples:
- p53 — "guardian of the genome"; arrests cell cycle or triggers apoptosis when DNA damage is detected
- Rb (retinoblastoma protein) — inhibits progression past G1 checkpoint
Cancer typically requires multiple mutations accumulating over time (multi-hit hypothesis). Characteristics of cancer cells include:
- Evade growth suppressors
- Resist apoptosis
- Enable unlimited replicative potential (activate telomerase)
- Induce angiogenesis (blood vessel formation)
- Activate invasion and metastasis
- Alter cellular metabolism
Apoptosis and its regulation
Apoptosis is essential for normal development and tissue homeostasis.
Cellular events during apoptosis:
- Cell shrinkage and chromatin condensation
- Membrane blebbing
- DNA fragmentation
- Formation of apoptotic bodies
- Phagocytosis by neighboring cells or macrophages (no inflammatory response)
Apoptotic pathways:
- Extrinsic pathway — triggered by death signals binding to cell-surface receptors
- Intrinsic pathway — triggered by internal signals (DNA damage, oxidative stress)
- Both pathways activate caspases (proteolytic enzymes) that execute cell death
Regulation:
- p53 can induce apoptosis in cells with irreparable DNA damage
- Bcl-2 family proteins regulate the intrinsic pathway (some promote, others inhibit apoptosis)
- Failure of apoptosis contributes to cancer development
Worked examples
Example 1: Signal transduction pathway analysis
Question: Epinephrine binds to a GPCR on liver cells, ultimately leading to glucose release. Describe the signal transduction pathway from receptor activation to cellular response, explaining how signal amplification occurs. [6 marks]
Mark scheme answer:
Epinephrine (ligand) binds to GPCR on cell surface [1 mark]
Receptor changes shape, activating G protein which exchanges GDP for GTP [1 mark]
Activated G protein activates adenylyl cyclase, which converts ATP to cAMP (second messenger) [1 mark]
cAMP activates protein kinase A (PKA) by binding to regulatory subunits [1 mark]
PKA triggers phosphorylation cascade, activating enzymes that promote glycogen breakdown and glucose release [1 mark]
Signal amplification occurs because: one activated enzyme can activate many molecules of the next component in the pathway, so one epinephrine molecule ultimately produces many glucose molecules [1 mark]
Example 2: Cell cycle checkpoint analysis
Question: A researcher treats cells with a drug that prevents cyclin B degradation. Predict and explain the effect on cell cycle progression. [4 marks]
Mark scheme answer:
Cyclin B combines with CDK1 to form MPF, which drives cells from G2 into M phase [1 mark]
Normally, cyclin B is degraded during mitosis, allowing cells to exit M phase and return to interphase [1 mark]
If cyclin B cannot be degraded, MPF remains active [1 mark]
Cells would be unable to exit mitosis properly and could not re-enter interphase, blocking cell cycle progression [1 mark]
Example 3: Cancer genetics
Question: Explain why mutations in both copies of the p53 gene are more likely to lead to cancer than mutations affecting only one copy. [3 marks]
Mark scheme answer:
p53 is a tumor suppressor gene that normally halts the cell cycle when DNA damage is detected or triggers apoptosis if damage is irreparable [1 mark]
Tumor suppressor genes follow a recessive inheritance pattern at the cellular level; one functional copy is usually sufficient for normal regulation [1 mark]
Both copies must be mutated/lost for complete loss of function, removing the checkpoint that prevents damaged cells from dividing, thus allowing cancer development [1 mark]
Common mistakes and how to avoid them
Confusing reception with transduction: Reception is only the binding of ligand to receptor; transduction is the subsequent relay and amplification of the signal inside the cell. Be precise about which stage you're describing.
Incorrectly stating that all cyclins are present throughout the cell cycle: While CDKs are constitutively present, cyclin concentrations fluctuate cyclically. Cyclin synthesis and degradation regulate CDK activity at specific cell cycle stages.
Mixing up checkpoints and their criteria: Each checkpoint assesses different conditions. G1 checks for growth signals, nutrients, size, and DNA integrity; G2 verifies completed DNA replication; M checkpoint ensures proper chromosome attachment to spindle fibers.
Claiming cancer results from a single mutation: Most cancers require multiple mutations accumulating over time (typically 4-7 critical mutations). A single mutation in one proto-oncogene or tumor suppressor gene is generally insufficient.
Describing apoptosis as cell "death" without distinguishing it from necrosis: Apoptosis is programmed, controlled, energy-requiring, and doesn't trigger inflammation. Necrosis is uncontrolled cell death due to injury that causes inflammation. Use the correct term.
Forgetting signal amplification in transduction pathways: A key feature of phosphorylation cascades and second messenger systems is that one activated molecule can activate many downstream molecules, exponentially amplifying the signal. Always explain the mechanism of amplification when relevant.
Exam technique for "Cell Communication and Cell Cycle"
Sequence and timing questions: When describing cell cycle stages or signal transduction pathways, present events in correct chronological order. Use linking words ("subsequently," "this triggers," "followed by") to show the sequence clearly. Each step usually earns one mark.
"Explain" vs "Describe" command words: "Describe" requires stating what happens; "explain" requires both what happens and why/how it happens (mechanism or reason). For example, describing checkpoint function states what it checks; explaining includes the molecular mechanism (cyclin-CDK activity, p53 activation, etc.).
Naming specific molecules earns marks: Generic answers like "proteins are involved" score poorly. Name specific components: GPCR, RTK, cAMP, PKA, cyclin B, CDK1, p53, Rb. AP Biology mark schemes reward precision.
Link structure to function: When discussing receptors or signaling proteins, connect structural features (tyrosine kinase domain, ligand-binding site, DNA-binding domain) to their functional roles. This demonstrates higher-level understanding beyond memorization.
Quick revision summary
Cell communication involves reception (ligand-receptor binding), transduction (signal relay and amplification via second messengers and phosphorylation cascades), and response (altered cellular activity). The cell cycle progresses through G1-S-G2-M phases, regulated by cyclin-CDK complexes at three major checkpoints (G1, G2, M). Checkpoint failure can result from mutations in proto-oncogenes (becoming oncogenes) or tumor suppressor genes (p53, Rb), leading to cancer. Apoptosis is controlled programmed cell death essential for development and homeostasis, regulated by caspases and Bcl-2 family proteins.