Unit 4: Cell Communication and Cell Cycle

Cell communication

The mechanisms cells use to detect information (signals) and convert it into a specific change in cell behavior (a response).

Signal

Any cue that can be detected and triggers a biological response (often a chemical signal such as a hormone, neurotransmitter, or growth factor).

Ligand

A signal molecule that binds to a receptor to start cell signaling.

Receptor

Usually a protein that binds a ligand and changes shape/activity to initiate signal transduction.

Reception

The stage of cell signaling in which a ligand binds its receptor (often causing a receptor shape change).

Transduction

The stage of cell signaling in which the receptor’s activation triggers a chain of intracellular molecular events (often with amplification).

Response

The final effect of signaling, such as changes in gene expression, enzyme activity, membrane transport, cytoskeleton, movement, or cell cycle decisions.

Signal specificity

The principle that the same ligand can produce different responses in different cell types due to differences in receptors, transduction proteins, or accessible genes.

Target cell

A cell that responds to a signal because it has the appropriate receptor (and downstream machinery).

Amplification

A property of many signaling pathways where one activated molecule activates many downstream molecules, creating a large response from a small signal.

Desensitization/resetting (signal termination)

Shutting down a signaling pathway via mechanisms such as ligand removal, receptor inactivation/internalization, dephosphorylation by phosphatases, or breakdown of second messengers.

Paracrine signaling

Local signaling in which cells secrete local regulators that diffuse short distances to affect nearby cells (often forming concentration gradients).

Synaptic signaling

Local signaling in which a neuron releases neurotransmitters into a synapse to signal a specific nearby target cell; fast and highly specific.

Endocrine signaling

Long-distance signaling in which hormones are released into a transport fluid (e.g., bloodstream) to reach distant target cells.

Hormone

A long-distance signaling molecule delivered through the bloodstream (animals) or transport tissues (plants) that affects only cells with the correct receptor.

Autocrine signaling

Signaling in which a cell releases a signal that binds to receptors on its own surface (or the same cell type), potentially stimulating itself.

Cell-surface receptor

A plasma membrane receptor that binds extracellular ligands (typically hydrophilic/charged) and transmits the signal into the cytoplasm.

Intracellular receptor

A receptor in the cytoplasm or nucleus that binds ligands that cross the membrane (often hydrophobic) and commonly alters gene expression.

Ligand-gated ion channel

A membrane receptor/channel that opens or closes in response to ligand binding, changing ion flow and membrane potential or ion concentration.

Receptor tyrosine kinase (RTK)

An enzyme-linked membrane receptor that typically dimerizes upon ligand binding and phosphorylates tyrosine residues (often on itself), creating docking sites that activate multiple pathways; often involved in growth/division.

G protein-coupled receptor (GPCR)

A membrane receptor that, upon ligand binding, activates an intracellular G protein to trigger downstream signaling (often via second messengers).

G protein

An intracellular relay protein activated by GPCRs that commonly switches between GDP-bound (inactive) and GTP-bound (active) states.

Second messenger

A small non-protein molecule or ion that relays and amplifies a signal inside the cell (after receptor activation).

Cyclic AMP (cAMP)

A common second messenger that can activate protein kinases and produce amplified cellular responses.

Protein kinase

An enzyme that transfers a phosphate group from ATP to a target protein, often changing that protein’s activity in signaling pathways.

Protein phosphatase

An enzyme that removes phosphate groups from proteins, helping turn pathways off and reset signaling.

Phosphorylation cascade

A series of protein kinases activating one another by phosphorylation; useful for amplification and can branch to multiple responses.

Apoptosis

Programmed cell death: a regulated process (often involving caspase proteases) that dismantles the cell without causing inflammation; important in development and cancer prevention.

Negative feedback

A regulatory loop in which the response reduces the initial stimulus, stabilizing a system around a set point (common in homeostasis).

Positive feedback

A regulatory loop in which the response increases the initial stimulus, amplifying change until a defined endpoint is reached.

Cell cycle

The ordered sequence of events from one cell division to the next, coordinating DNA replication, chromosome separation, and cytoplasmic division.

Interphase

The part of the cell cycle when the cell grows and replicates DNA (G1, S, and G2 phases); most of a cell’s life is spent here.

G1 phase

Interphase stage of cell growth and normal metabolism; the cell assesses nutrients, growth signals, and DNA damage before committing to DNA replication.

S phase

Interphase stage when DNA replication occurs, producing sister chromatids; DNA amount doubles but chromosome number (by centromeres) is considered unchanged.

G2 phase

Interphase stage of further growth and preparation for division; the cell checks replication completion and DNA damage before mitosis.

Mitosis

Nuclear division in which chromosomes separate so each daughter nucleus receives an identical set of chromosomes.

Cytokinesis

Division of the cytoplasm to form two separate daughter cells (cleavage furrow in animals; cell plate in plants).

Sister chromatids

Two identical DNA copies of a duplicated chromosome formed in S phase, joined together until they separate during mitosis.

Centromere

The chromosome region that holds sister chromatids together; chromosome number is typically counted by the number of centromeres.

Cell cycle checkpoint

A regulatory control point where the cell evaluates conditions and whether key events were completed correctly (notably at G1, G2, and the spindle checkpoint).

Cyclin

A regulatory protein whose concentration rises and falls during the cell cycle; binds and activates CDKs to time cell cycle transitions.

Cyclin-dependent kinase (CDK)

A kinase that drives cell cycle events by phosphorylating targets when activated by binding a cyclin; CDK levels are relatively stable while cyclins fluctuate.

Growth factor

An external protein signal that stimulates cell division; many cells require growth factors to pass the G1 checkpoint.

Density-dependent inhibition

A property of many normal animal cells in which they stop dividing when they become crowded.

Anchorage dependence

A requirement of many normal animal cells to be attached to a surface/extracellular matrix in order to divide.

Cancer

A breakdown of normal signaling, cell cycle checkpoints, and apoptosis control that leads to inappropriate cell division, tumor formation, and possibly spread.

Proto-oncogene

A normal gene that promotes cell division or survival in appropriate contexts; can contribute to cancer if altered.

Oncogene

A mutated/overactive form of a proto-oncogene that drives excessive cell division or survival (like a “gas pedal stuck down”).

Tumor suppressor gene

A gene whose products inhibit cell cycle progression, help repair damage, or trigger apoptosis; loss of function removes “brakes” on cell division.

p53

A tumor suppressor protein that can cause cell cycle arrest for DNA repair and can promote apoptosis if DNA damage is severe.