Cytoskeleton, Cell Division, and Apoptosis

Vocabulary practice flashcards covering the cytoskeleton (intermediate filaments, actin filaments, and microtubules), molecular motors, the mechanisms of mitotic cell division, and the apoptotic cell death pathways.

Cytoskeleton

An organized network of protein polymers within eukaryotic cells that provides mechanical strength, establishes cellular architecture, and acts as tracks for directed intracellular transport.

Intermediate Filaments

Cytoskeletal fibers made of fibrous proteins with high tensile strength that enable animal cells and tissues to withstand mechanical stress; they form rope-like polymers lacking structural polarity.

Intermediate Filament Subunit

A staggered antiparallel tetramer composed of two coiled-coil dimers; because the dimers point in opposite directions with $N$-termini exposed at both ends, the subunit is symmetric and non-polar.

Epidermolysis Bullosa Simplex

A rare genetic disorder caused by mutations in keratin intermediate filaments, leaving the skin highly vulnerable to mechanical injury so that even gentle pressure causes blistering and tissue rupture.

Nuclear Lamins

A class of intermediate filament proteins found in all animal cells that form a two-dimensional meshwork beneath the inner nuclear membrane to support and strengthen the nuclear envelope; mutations cause Progeria.

Filament Polarity

The structural distinction between the two ends of an actin filament or microtubule, resulting from the asymmetric assembly of its subunits into a fast-growing plus end and a slow-growing minus end.

Myosin II

An actin-based motor protein that forms dimers with coiled-coil tails and pairs of head domains; multiple dimers assemble into bipolar filaments with heads pointing in opposite directions that slide actin filaments past one another.

Actin Monomer

The asymmetric single-polypeptide subunit of actin filaments that binds ATP in a central cleft and polymerizes into a two-stranded helical filament.

Nucleation (Actin)

The initial, rate-limiting lag phase of actin polymerization in which free monomers collide to form a stable trimer nucleus; occurs only above the critical concentration ($C_c$).

Treadmilling

A steady-state dynamic behavior of actin filaments in which ATP-actin adds to the plus end at the exact rate that ADP-actin dissociates from the minus end, keeping filament length constant while subunits travel through.

ARP Complex

An actin-nucleating protein complex that binds to the side of an existing actin filament and nucleates a new daughter filament while capping its minus end, creating the branched actin meshwork seen in lamellipodia.

Formin

A ring-shaped actin-nucleating protein that stabilizes an actin trimer and remains tracked at the growing plus end to promote the assembly of unbranched, linear actin filaments.

Actin Severing Proteins

Regulatory proteins that bind along the length of actin filaments and sever them into shorter fragments, promoting disassembly and network remodeling.

Lamellipodium

A broad, sheet-like protrusion at the leading edge of a crawling cell, composed of a dense, branched actin network nucleated by the ARP complex that pushes the plasma membrane forward.

Tubulin Heterodimer

The asymmetric subunit of microtubules, formed by a tightly bound heterodimer of $\alpha$-tubulin (with non-hydrolyzable GTP) and $\beta$-tubulin (with an exchangeable, hydrolyzable GTP).

Centrosome

The primary microtubule organizing center (MTOC) in animal cells, consisting of a pair of centrioles surrounded by pericentriolar material (PCM) rich in $\gamma$-tubulin ring complexes.

Dynamic Instability

The intrinsic property of microtubules switching stochastically between phases of assembly (growth) and rapid disassembly (shrinkage), governed by the presence or loss of a GTP cap.

Catastrophe

The sudden transition of a microtubule from a growing state to a shrinking state when GTP hydrolysis outpaces subunit addition, causing the protective GTP cap to be lost.

Rescue

The switch of a depolymerizing microtubule back to a growing state upon the regain of a GTP cap.

$\gamma$-Tubulin Ring Complex ($\gamma$-TuRC)

A ring-shaped protein complex containing 13 $\gamma$-tubulin subunits embedded in the centrosomal matrix that templates the 13 protofilaments of a new microtubule at its minus end.

Augmin

A microtubule-associated branching protein that binds to the side of an existing microtubule and recruits a $\gamma$-tubulin ring complex to nucleate a branched microtubule array.

Kinesin-13

A specialized, catastrophe-inducing kinesin that enriches at microtubule plus ends and uses ATP to peel protofilaments apart, promoting microtubule depolymerization.

Katanin

An ATP-dependent severing protein (named after the Japanese Katana sword) that cuts microtubules along their length to promote rapid disassembly.

Melanosomes

Pigment granules in melanophore cells whose cellular distribution along radial microtubules is regulated by cAMP; high cAMP activates kinesin to disperse granules outward, whereas low cAMP activates dynein to aggregate them at the center.

Cohesin Complex

A ring-shaped protein complex loaded onto DNA during S phase that holds duplicated sister chromatids together like molecular glue until cleaved by separase at anaphase.

Condensin Complex

A ring-shaped protein complex that uses ATP to loop and compact chromatin during M phase, achieving a $10,000$-fold reduction in chromosome length to facilitate mitotic segregation.

Interpolar Microtubules

Mitotic spindle microtubules originating from opposite poles that overlap in an antiparallel orientation in the central overlap zone and are stabilized by cross-linking motor proteins.

Kinesin-5

A tetrameric, bipolar kinesin motor localized to the spindle overlap zone with two pairs of heads that walk toward the plus ends of antiparallel interpolar microtubules, generating outward sliding force to push the spindle poles apart.

Kinetochore

A multilayered protein complex assembled on the centromeric DNA of each sister chromatid; its outer fibril proteins bind along the sides of microtubule plus ends so the microtubules can remain dynamic without detaching.

Chromosome Biorientation

The state in which the two sister kinetochores of a mitotic chromosome are captured by microtubules originating from opposite spindle poles, creating mechanical tension that confirms correct attachment.

Anaphase A

The stage of chromosome segregation in which sister chromatids are pulled toward their respective spindle poles primarily by the depolymerization and shortening of kinetochore microtubules at their plus ends.

Anaphase B

The stage of chromosome segregation in which the spindle poles separate further apart, driven by Kinesin-5 pushing interpolar microtubules outward and cortical Dynein pulling astral microtubules toward the plasma membrane.

Contractile Ring

A transient ring of antiparallel actin filaments and bipolar Myosin II motors assembled at the cell equator during telophase that constricts the plasma membrane inward to complete cytokinesis.

Apoptosis

A tightly programmed and controlled process of cell death where a cell shrinks, collapses its cytoskeleton, disassembles its nuclear envelope, fragments its DNA, and produces apoptotic bodies cleared by phagocytes without causing inflammation.

Necrosis

Accidental or unprogrammed cell death occurring in response to acute physical or chemical trauma, characterized by cell swelling, membrane rupture, spill of intracellular contents, and a resulting inflammatory response.

Procaspases

Inactive proenzyme precursors of caspases; initiator procaspases exist as inactive monomers activated by adaptor-mediated dimerization and cross-cleavage, whereas executioner procaspases exist as inactive dimers activated by initiator caspase cleavage.

CAD Nuclease

Caspase-Activated DNase, an endonuclease kept inactive in healthy cells by the inhibitor protein iCAD; once an executioner caspase cleaves iCAD, CAD is liberated to degrade genomic DNA between nucleosomes.

Extrinsic Apoptotic Pathway

An apoptotic route initiated when an external signal, such as Fas ligand on a killer lymphocyte, binds death receptors (Fas receptor) on the target cell to assemble the DISC complex and activate initiator caspase-8.

Apoptosome

A large, wheel-shaped heptameric complex formed in the cytosol by cytochrome C-activated adaptor proteins (Apaf-1) during intrinsic apoptosis that recruits and activates initiator procaspase-9.

Bcl2 Family

A family of related proteins that govern the intrinsic apoptotic pathway by regulating cytochrome C release, comprising pro-apoptotic channel formers (Bax and Bak) and anti-apoptotic proteins (Bcl2) that inhibit them.

Survival Factors

Extracellular signaling proteins that bind cell-surface receptors to activate intracellular transcription factors, which stimulate the expression of anti-apoptotic Bcl2 to block cell death.