Gene Expression and Regulation

Quorum sensing

the way bacteria detect their population density by releasing and sensing signaling molecules; when enough cells are present, they coordinate group behavior (e.g., forming biofilms).

Taxis

a directed movement of an organism toward or away from a stimulus.

Chemotaxis

movement toward or away from a chemical signal (a type of taxis); e.g., immune cells moving toward chemicals at an infection site.

Ligands

signaling molecules that bind to a specific receptor, triggering a response. (The "key" that fits a receptor "lock.")

Receptors

proteins (on the cell surface or inside the cell) that bind a specific ligand and start a cellular response.

Signal transduction

the process of converting an extracellular signal into a cellular response through a series of molecular steps (reception → transduction → response).

Ligand-gated ion channel

a membrane receptor that opens or closes to let ions through when a ligand binds; converts a chemical signal into an electrical/ionic one.

Catalytic (or enzyme-linked) receptor

a membrane receptor that acts as an enzyme (or activates one) when a ligand binds, catalyzing a reaction inside the cell.

G-protein-linked receptor

a membrane receptor that activates an attached G protein when a ligand binds; the G protein then triggers downstream signaling.

Secondary messengers

small molecules (e.g., cyclic AMP, Ca²⁺) that relay and amplify a signal inside the cell after the receptor is activated.

Homeostasis

maintenance of a stable internal environment despite external changes.

Negative feedback pathway (feedback inhibition)

a loop where the output of a process shuts the process down, keeping conditions stable (e.g., a product inhibiting the enzyme that made it; temperature regulation). The most common form of biological regulation.

Positive feedback pathway

a loop where the output amplifies the process, pushing it further from the starting point (e.g., childbirth contractions, blood clotting).

Cell division

the process by which a parent cell splits into daughter cells.

Cell cycle

the ordered sequence of events a cell goes through from one division to the next (interphase → mitosis → cytokinesis).

Interphase

the longest part of the cycle, when the cell grows and copies its DNA; made of G₁, S, and G₂ phases.

G₁ phase

first growth phase; the cell grows and carries out normal functions before DNA is copied.

S phase

synthesis phase; DNA is replicated so each chromosome now has two sister chromatids.

G₂ phase

second growth phase; the cell prepares for division (makes proteins and organelles, checks copied DNA).

G₀ phase

a resting/non-dividing state; some cells (like mature neurons) exit the cycle and stay here.

Mitosis

division of the nucleus, producing two genetically identical nuclei. (Divided into prophase, metaphase, anaphase, telophase.)

Prophase

chromatin condenses into visible chromosomes; the nuclear envelope breaks down; spindle fibers begin to form.

Metaphase

chromosomes line up single-file along the metaphase plate (cell's midline).

Anaphase

sister chromatids are pulled apart to opposite poles of the cell.

Telophase

chromosomes reach the poles, decondense, and nuclear envelopes re-form around each set; two nuclei now exist.

Chromatin

the loose, uncondensed form of DNA + proteins found during interphase.

Sister chromatids

the two identical copies of a chromosome (made in S phase), joined at the centromere.

Centromere

the region where sister chromatids are joined and where kinetochores attach.

Spindle fibers

microtubules that attach to chromosomes and pull them apart during division.

Kinetochores

protein structures at the centromere where spindle fibers attach to the chromosome.

Metaphase plate

the imaginary midline of the cell where chromosomes align during metaphase.

Cytokinesis

division of the cytoplasm, splitting one cell into two; follows mitosis.

Cleavage furrow

the pinching-in of the membrane that divides an animal cell during cytokinesis.

Cell plate

the new cell wall that forms down the middle to divide a plant cell during cytokinesis.

Cyclins

regulatory proteins whose levels rise and fall through the cycle; they activate CDKs to drive the cell forward.

Cyclin-dependent kinases (CDKs)

enzymes that, when bound to a cyclin, add phosphate groups to target proteins to push the cell through the cycle. Active only when paired with a cyclin.

Cell cycle checkpoints

control points (G₁, G₂, and M/spindle checkpoints) where the cell verifies conditions are correct before continuing; stop errors from being passed on.

Apoptosis

programmed cell death; a controlled self-destruction that removes damaged or unneeded cells (also shapes tissues during development).

Cancer

uncontrolled cell division caused by failures in cell-cycle regulation, forming tumors.

Oncogene

a mutated/overactive version of a gene that normally promotes cell division; drives cancer by telling the cell to keep dividing (a "gas pedal stuck down").

Tumor suppressor gene

a gene that normally slows division or triggers repair/apoptosis; when it's mutated and lost, cancer can result (a "broken brake"). Example: p53.