Cell Signaling Pathways and Molecular Mechanisms in Biology

Signal transduction pathway

A series of steps converting a signal on a cell's surface into a specific cellular response.

Reception

The stage where a signaling molecule binds to a receptor protein on the cell surface or inside the cell.

Transduction

The stage where receptor activation triggers a cascade of molecular interactions.

Response

The stage where the transduced signal triggers a specific cellular activity.

Ligand

A signaling molecule that specifically binds to a receptor and activates it.

Local signaling

Communication where cells signal to nearby cells through direct contact or short-range molecules.

Paracrine signaling

Local signaling using secreted molecules that act on nearby cells.

Synaptic signaling

Neuron-to-neuron or neuron-to-muscle communication via neurotransmitter release.

Endocrine signaling

Long-distance signaling through hormones traveling in the bloodstream.

Hormone

Long-distance signaling molecule traveling through blood to target tissues.

G protein-coupled receptor (GPCR)

A cell-surface receptor that activates a G protein when bound to a ligand.

G protein

A protein that binds GTP and relays signals from GPCRs to other pathways.

Receptor tyrosine kinase (RTK)

Membrane receptor that phosphorylates proteins using ATP and triggers multiple pathways.

Ligand-gated ion channel

Receptor that opens a channel for specific ions when a ligand binds.

Intracellular receptor

Receptor located inside the cell that binds hydrophobic molecules like steroid hormones.

Second messenger

Small, water-soluble molecules or ions that rapidly relay signals inside cells.

cAMP (cyclic AMP)

Widely used second messenger produced from ATP by adenylyl cyclase.

Adenylyl cyclase

Membrane enzyme converting ATP to cAMP in response to a signal.

Protein kinase A

Kinase activated by cAMP that phosphorylates target proteins.

Calcium ion (Ca²⁺)

A second messenger whose cytosolic concentration increases during signaling.

IP₃ (inositol trisphosphate)

Second messenger that triggers Ca²⁺ release from ER.

DAG (diacylglycerol)

Second messenger produced with IP₃ that participates in signaling pathways.

Protein kinase

Enzyme that transfers phosphate groups from ATP to proteins (phosphorylation).

Protein phosphatase

Enzyme removing phosphate groups to deactivate proteins.

Phosphorylation cascade

Chain reaction where one activated kinase activates the next.

Scaffolding protein

Large relay protein binding multiple signaling proteins to increase efficiency.

Nuclear response

Signaling pathway outcome that activates or represses gene expression.

Cytoplasmic response

Signaling outcome altering enzyme activity or metabolic pathways.

Transcription factor

Protein activated by signaling that turns genes on or off.

Signal amplification

One ligand can trigger multiple activated molecules downstream.

Signal specificity

Different cell types respond differently to the same signal.

Cross-talk

Interaction between signaling pathways that coordinates responses.

Signal termination

Unbound receptors revert to inactive state when ligand concentration drops.

Receptor Tyrosine Kinase (RTK)

Enzyme-linked receptor that dimerizes upon ligand binding and autophosphorylates, creating multiple docking sites for intracellular signaling proteins.

Protein kinase

General enzyme that adds phosphate groups to proteins to activate or deactivate them; part of phosphorylation cascades.

Protein phosphatase

Enzyme that removes phosphate groups from proteins, turning signals off.

DAG (diacylglycerol)

Membrane-bound second messenger produced with IP₃; activates PKC with Ca²⁺.

DNA Polymerase III

Main bacterial enzyme that elongates the leading and lagging strands in DNA replication.

DNA Polymerase I

Enzyme that removes RNA primers and replaces them with DNA nucleotides.

Telomere

Repeated DNA sequence at the ends of chromosomes that protect gene loss during replication.

Telomerase

Enzyme that extends telomeres in germ-line cells, stem cells, and cancer cells.

RNA polymerase II

Eukaryotic enzyme that synthesizes mRNA; does NOT need a primer.

TATA box

AT-rich promoter element that positions RNA polymerase and transcription factors for correct transcription start.

Ribozyme

RNA molecule that catalyzes biochemical reactions, proving RNA can function as an enzyme.

Release factor

Protein that recognizes stop codons and releases the finished polypeptide from the ribosome by adding water instead of an amino acid.

Aminoacyl-tRNA synthetase

The enzyme that attaches the correct amino acid to its corresponding tRNA; essential for accuracy of translation.

P site

Ribosomal site holding the tRNA with the growing polypeptide chain.

E site

Ribosomal site through which empty tRNAs exit after donating their amino acid.

Polypeptide

A chain of amino acids produced by translation that folds into a functional protein.

pre-mRNA

The raw, unprocessed RNA transcript that still contains introns and exons before RNA splicing.

Polyadenylation signal

Sequence (AAUAAA) in eukaryotic pre-mRNA that causes cleavage and addition of poly-A tail.