Unit 4 Ligand Tap / Space to flip
Unit 4 A signal molecule (often hormone or neurotransmitter) that binds to a specific receptor.
Unit 4 · 10–15% of exam
Cell Communication & Cell Cycle
Signal transduction, feedback loops, cell cycle regulation, checkpoints, and cancer.
Must-know content
- Cell signaling stages: Reception → Transduction → Response.
- Receptors: G-protein-coupled (GPCRs), receptor tyrosine kinases (RTKs), ligand-gated ion channels, intracellular (steroid hormones).
- Signal amplification via phosphorylation cascades and second messengers (cAMP, Ca²⁺, IP₃).
- Local vs. long-distance signaling: paracrine, synaptic, endocrine.
- Quorum sensing in bacteria; apoptosis as programmed cell death.
- Feedback:
- Negative — returns system to set point (thermoregulation, blood glucose).
- Positive — amplifies response (childbirth oxytocin, blood clotting).
- Cell cycle: Interphase (G1 → S → G2) → M (mitosis + cytokinesis).
- Checkpoints — G1 (most important; restriction point), G2, M (spindle assembly).
- Cyclins and Cdks regulate progression.
- Cancer: Loss of cell-cycle control via mutations in proto-oncogenes (gain-of-function) or tumor suppressors (loss-of-function, e.g., p53).
- Mitosis (PMAT): Prophase, Metaphase, Anaphase, Telophase. Two genetically identical diploid daughter cells.
Example questions
MCQ A signal molecule binds to a receptor tyrosine kinase. What occurs immediately after? (A) Receptor enters nucleus (B) Two receptor monomers dimerize and autophosphorylate tyrosines (C) cAMP is produced (D) Receptor degrades
Answer: B. RTKs form active dimers upon ligand binding and phosphorylate each other on tyrosine residues, creating docking sites for downstream signaling proteins.
FRQ Explain how a mutation in p53 can contribute to cancer.
Answer: p53 is a tumor suppressor that halts the cell cycle at G1 in response to DNA damage and can trigger apoptosis if damage is irreparable. A loss-of-function mutation in p53 lets cells with damaged DNA bypass this checkpoint, replicate, and divide. The damaged cells accumulate further mutations, and uncontrolled division produces tumors.
MCQ Which is an example of negative feedback? (A) Action potential firing (B) Insulin lowering blood glucose after a meal (C) Blood clotting cascade (D) Lactation initiated by suckling
Answer: B. Insulin reduces blood glucose, which in turn reduces insulin secretion — a classic homeostatic negative feedback loop. The other three are positive feedback: amplifying processes that drive a system away from baseline.
Drill flashcards
Unit 4 Second messenger Tap / Space to flip
Unit 4 Small intracellular molecule (e.g., cAMP, IP₃, Ca²⁺) that propagates and amplifies a signal.
Unit 4 Signal transduction Tap / Space to flip
Unit 4 Series of molecular changes (often a phosphorylation cascade) that converts a signal into a cellular response.
Unit 4 Apoptosis Tap / Space to flip
Unit 4 Programmed cell death — cell shrinks, fragments, and is engulfed without inflammation.
Unit 4 Cell cycle checkpoint Tap / Space to flip
Unit 4 Control point (G1, G2, M) where the cell verifies conditions before proceeding. G1 is the most important.
Unit 4 Cyclin Tap / Space to flip
Unit 4 Regulatory protein whose levels oscillate during the cell cycle; activates Cdks to drive progression.
Unit 4 Proto-oncogene Tap / Space to flip
Unit 4 Normal gene that promotes growth. A gain-of-function mutation converts it to an oncogene → uncontrolled division.
Unit 4 p53 (tumor suppressor) Tap / Space to flip
Unit 4 Halts the cell cycle at G1 in response to DNA damage and triggers apoptosis. Loss-of-function mutations are seen in many cancers.