Signal molecule
A chemical messenger secreted by one cell to influence another; examples include hormones, neurotransmitters, and growth factors.
Receptor
A protein that binds a signaling molecule (ligand) and initiates a cellular response.
Ligand
A molecule that specifically binds to a receptor to activate or inhibit signaling.
Signal transduction
The process by which an extracellular signal is converted into an intracellular response through molecular cascades.
Effector protein
The final target of a signaling pathway that directly brings about a cellular change (e.g., enzymes, transcription factors).
Amplification
A single activated receptor triggers multiple downstream molecules, strengthening the signal.
Integration
When multiple signaling pathways interact and combine to determine a single cellular response.
Distribution
A single signaling pathway can influence multiple cellular processes simultaneously.
Feedback regulation
Signaling outputs feed back to regulate earlier steps; can be positive (enhancing) or negative (inhibiting).
Desensitization
Reduced receptor responsiveness after prolonged exposure to a signal.
Endocrine signaling
Signal molecules (hormones) travel long distances through the bloodstream to reach target cells.
Paracrine signaling
Signal molecules act locally on nearby target cells.
Autocrine signaling
A cell secretes a signal that acts on itself.
Synaptic signaling
Nerve cells transmit neurotransmitters across synapses to target cells.
Contact-dependent signaling
Requires physical contact between cells; signaling molecule remains membrane-bound.
Steroid hormone signaling
Hydrophobic hormones pass through the membrane, bind intracellular receptors, and directly alter gene expression.
Intracellular vs cell-surface receptors
Intracellular receptors bind small, nonpolar ligands like steroid hormones; cell-surface receptors bind polar molecules.
Loss-of-function mutation
Removes or reduces protein activity; reveals necessity of a gene in a pathway.
Gain-of-function mutation
Creates a constitutively active protein or new function; shows sufficiency of a gene in signaling.
Constitutively active mutant
Remains active without a signal, mimicking constant pathway activation.
Kinase-dead mutant
Carries an inactivating mutation in the kinase domain, blocking phosphorylation activity.
Epistasis
One gene’s mutation masks or alters the phenotype of another; helps determine order of signaling components.
Double mutant analysis
Compares phenotypes of combined mutations to map pathway order (which gene acts upstream or downstream).
In vivo experiment
Performed in a living organism. to test and study how a disease, drug, or treatment affects a whole, living organism
