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#cellular processes

15 public questions tagged with this topic.

Dictyostelium

Stage following migration:

After phototactic migration, slug selects drier spot where ammonia dissipates, enabling culmination phase where anterior prestalk cells undergo programmed vacuolization, synthesize cellulose walls, and crawl down through prespore mass in reverse fountain movement forming stalk tube. Prespore cells ascend and encapsulate into spores atop stalk. This dramatic morphogenetic transition occurs without cell division, relying solely on sorting and terminal differentiation. Vegetative growth precedes aggregation, aggregation precedes migration; spore dispersal occurs only after culmination completes. Culmination therefore represents final architectural construction stage converting migratory slug into stationary fruiting body optimized for propagation.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 22: Culmination - reverse fountain movement and stalk tube formation.

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Membrane Transportation

Which process requires ATP?

Energy coupling distinguishes passive and active transport. Simple diffusion of gases, facilitated diffusion via carriers like GLUT and channels like aquaporin, and osmosis driven by water potential are passive, relying on thermal motion and existing gradients without ATP input, negative Gibbs free energy toward equilibrium. Primary active transport directly uses chemical energy from ATP hydrolysis within same protein to move solute against gradient. P-type ATPases like Na+/K+ ATPase exchanging 3 Na+ out for 2 K+ in, V-type H+ ATPase acidifying lysosomes, F-type ATP synthase reversible, and ABC superfamily including CFTR, MDR1, TAP hydrolyze two ATP to pump ions, drugs or peptides uphill creating gradients for secondary use. Because it maintains Na+, K+, Ca2+ and H+ gradients primary active consumes up to forty percent of ATP in neurons and kidney. Secondary active transport uses pre-existing ion gradients, for example Na+/glucose symport. Tertiary active further couples gradients. Inhibitors ouabain and bafilomycin collapse gradients impairing nutrient uptake, pH regulation and volume control rapidly demonstrating energetic dependence.

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 11: Primary Active Transport – ATP Requirement.

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Cytoskeleton

Myosin II is involved in:

Myosin II is the principal motor generating contractile force for cell division in animal cells. It assembles into bipolar thick filaments through coiled coil tail dimerization, each filament containing about 30 dimers with heads oriented opposite at ends. In anaphase, equatorial cortex enriched in active RhoA GTP recruits anillin, septins and centralspindlin, concentrating myosin II and actin into contractile ring. Phosphorylation of regulatory light chain by ROCK and citron kinase activates myosin II ATPase increasing ten fold, driving filament sliding that constricts ring from 30 micrometers to midbody during cytokinesis. Ring contraction coupled with plasma membrane insertion via recycling endosomes and ESCRT III mediated abscission completes separation. Myosin II does not transport organelles which requires myosin V, does not assemble microtubules nor segregate chromosomes which depend on microtubule kinesin dynein. Genetic depletion or blebbistatin inhibition blocking myosin II ATPase causes multinucleation confirming cytokinesis role. This contractile mechanism ensures equal partition of cytoplasm and maintenance of genomic stability across daughter cells during mitotic exit.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Cytokinesis and Contractile Ring Myosin II.

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Protein Sorting and Trafficking

Which process is activated during nutrient deprivation?

During prolonged starvation cells must recycle internal components to maintain energy and biosynthetic precursors. In fed state mTORC1 kinase anchored at lysosome via Rag GTPases phosphorylates ULK1 and ATG13, keeping initiation complex inactive. When amino acids fall, Rag switches off, mTORC1 dissociates, inhibitory phosphorylation is lost. Concurrently low ATP raises AMP, activating AMPK which phosphorylates ULK1 at distinct activating sites Ser317 Ser777 and inhibits mTORC1 via TSC2 and Raptor. Free ULK1 complex phosphorylates Beclin-1 and ATG14L, activating VPS34 class III PI3K to produce PI3P at ER associated omegasome. PI3P recruits WIPI2 and DFCP1, and ubiquitin like conjugation systems ATG12 ATG5 ATG16L1 mediate LC3-I to LC3-II lipidation, expanding phagophore around cargo. Sealed autophagosome fuses with lysosome via STX17 SNAP29 VAMP8, degrading contents to replenish amino acid pools. This survival program is distinct from apoptotic or necrotic death pathways and is reversibly regulated by nutrient signaling pathways controlling catabolism. This catabolic recycling sustains ATP production and prevents accumulation of damaged organelles during prolonged fasting.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 12: Intracellular Vesicular Traffic and Autophagy Regulation.

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Endomembrane System - ER Golgi Lysosome Vacuole

A chemical selectively prevents the Golgi apparatus from packaging materials into vesicles. Which pair of cellular proce

The Golgi apparatus packages materials for intracellular delivery or secretion. Lysosomes are also formed through Golgi packaging. ATP production occurs in mitochondria, while lipid and steroid synthesis occurs mainly in the SER.

Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Endomembrane System - ER Golgi Lysosome Vacuole

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Repair Pathway

Double-strand breaks are repaired error-free by

Double-strand break is most cytotoxic lesion resolved by two major pathways. Non-homologous end joining rapidly ligates ends but can delete nucleotides. Homology-directed repair, also termed homologous recombination repair or HDR, uses sister chromatid present in late S and G2 phases as faithful template. After resection by MRN complex CtIP, Rad51 catalyzes strand invasion forming D-loop, polymerase delta extends using homologous sequence, Holliday junctions resolved. Because homologous donor retains exact original sequence, repair restores information without errors, essential for maintaining genome stability and enabling precise genome editing applications using donor templates.

Ref: Lodish Molecular Cell Biology 9th ed. Chapter 12: HDR error-free DSB repair; Alberts Ch.5 Homologous recombination precise repair

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Replication Basics

Replication bubble contains

DNA unwinding at origin creates bubble structure where parental duplex separates into single strands coated by single-strand binding proteins. At each boundary where double-stranded DNA transitions to single-stranded, replication fork assembles with helicase, primase, polymerases and clamp. Origin bubble therefore contains two oppositely oriented forks moving outward, one clockwise and one counterclockwise, each synthesizing leading strand continuously toward bubble center and lagging strand discontinuously via Okazaki fragments. Bidirectional bubbles expand, eventually fusing with adjacent bubbles. Electron microscopy of replicating SV40 chromosomes visualizes two forks per bubble.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: Replication Bubble Contains Two Forks

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Apoptosis

The two major phases of apoptosis are

Latent and execution phases, is consistent with established principles of cell signaling, receptor pharmacology and cellular regulation. Experimental measurements of binding parameters, genetic loss-of-function studies and pharmacological interventions all converge on the same interpretation. Related options address neighboring concepts but do not satisfy the precise criterion stated in the question.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Cell Communication and Cell Adhesion

Anoikis is triggered by

The extrinsic pathway is initiated by binding of death ligands such as FasL or TNF to their cognate death receptors. Receptor clustering recruits adaptor proteins and caspase-8, forming the death-inducing signaling complex. Anoikis is a specialized form of apoptosis triggered by loss of integrin-mediated attachment to the extracellular matrix, thereby preventing survival of detached cells.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Cell Communication and Cell Adhesion

Integrin-mediated signaling affects

Adhesion, migration and gene expression, is consistent with established principles of cell signaling, receptor pharmacology and cellular regulation. Experimental measurements of binding parameters, genetic loss-of-function studies and pharmacological interventions all converge on the same interpretation. Related options address neighboring concepts but do not satisfy the precise criterion stated in the question.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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