#cell division

Basics of Cell Biology

Which scientist contributed to the cell theory by stating that all cells arise from pre-existing cells?

Cell doctrine progressed through microscopic observations of plant tissues by Matthias Schleiden in 1838 stating plants consist of cells and Theodor Schwann 1839 extending to animals proposing cells as basic units with nuclei, but mechanism of new cell formation remained speculative with crystallization theory suggesting spontaneous formation. Rudolf Virchow, German pathologist analyzing diseased tissues and thrombosis, in 1855 aphorism Omnis cellula e cellula challenged spontaneous generation, arguing cells originate only by division of pre-existing cells via mitosis involving chromatin condensation, metaphase plate alignment, sister chromatid separation driven by microtubule kinetochores, and cytokinesis via actin-myosin contractile ring. This principle placed cell division at heart of growth, wound healing, embryonic development, and tumorigenesis where uncontrolled proliferation causes malignancy. Pasteur's swan-neck flask experiments disproved microbial spontaneous generation supporting biogenesis, while Schleiden and Schwann established structural unity. Virchow's contribution completed three pillars of cell theory taught today: all organisms composed of cells, cell as basic unit, and cells arise from pre-existing cells.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Virchow and Omnis Cellula e Cellula.

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Basics of Cell Biology

Which phase of the cell cycle is responsible for DNA replication?

Eukaryotic cell cycle comprises four organized stages orchestrated by cyclin-CDK oscillations, checkpoint kinases, and ubiquitin ligases ensuring faithful duplication. Gap1 G1 with cyclin D bound to CDK4 and CDK6 senses mitogen availability and growth factors via Ras-MAPK, phosphorylates retinoblastoma protein Rb releasing E2F transcription factors to produce cyclin E and replication factors. Entry into S phase, DNA synthesis phase, requires activation of pre-replication complexes assembled in late M and G1: origin recognition complex ORC binds origin DNA, Cdc6 and Cdt1 load MCM2-7 double hexamer licensing origins to ensure once per cycle. At G1/S transition, Dbf4-dependent kinase DDK and cyclin E/A-CDK2 phosphorylate MCM and Sld2/3 leading to recruitment of Cdc45, GINS forming active CMG helicase that unwinds parental duplex, RPA coats single stranded DNA, Pol alpha-primase synthesizes short RNA primers, Pol delta elongates lagging strand Okazaki fragments and Pol epsilon leading strand with PCNA sliding clamp increasing processivity and fidelity. Histone synthesis couples tightly. ATR-Chk1 monitors stalled forks preventing collapse. By end S phase genome duplicated from 2N to 4N preparing for G2 and mitosis.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Cell Cycle S Phase and DNA Replication.

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Cytoskeleton

Katanin severing microtubules during mitosis results in:

Severing of microtubules by AAA ATPases introduces breaks that alter polymer fate profoundly influencing network remodeling. Katanin hexamer composed of p60 catalytic and p80 regulatory subunits plus spastin fidgetin assembles into ring encircling filament extracting tubulin C terminal tails disrupting lattice non covalent bonds via threading mechanism using ATP hydrolysis energy. Cut generates new plus end exposing GDP tubulin not protected by GTP cap and new minus end intrinsically unstable prone to depolymerization. Both ends unstable plus lacking cap quickly depolymerizes unless rescued by polymerase XMAP215 and EB1 plus end tracking network, minus rapidly shrinks unless capped by patronin CAMSAP or gamma TuRC anchoring. Therefore severing increases overall depolymerization rate and turnover rate enhancing dynamic remodeling. During mitosis localized severing at poles amplifies microtubule number creates short fragments for transport to chromosomes controls spindle length and disassembles interphase array. Without rescue factors increased depolymerization dominates explaining result of increased depolymerization after cut.

Ref: Roll-Mecak & McNally, Trends Cell Biol 2010 – Katanin severing results in increased depolymerization of microtubules.

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Cytoskeleton

During mitosis, kinetochore microtubules:

Kinetochore microtubule fibers K fibers dynamic behavior changes across mitotic stages governed by tension sensing checkpoint signaling and motor forces generating chromosome movement. During prometaphase microtubules nucleated at centrosomal poles probe cytoplasm via dynamic instability growth at plus end at several microns per minute capture kinetochore through NDC80 complex forming bundles of 20 to 40 microtubules stabilized by CLASP and formins. Until anaphase kinetochore microtubules continue polymerization at plus end while minus ends depolymerize at poleward flux mediated by kinesin-13 and katanin maintaining tension across sister chromatids bi oriented at metaphase plate. This tension stabilizes attachments via Aurora B spatial separation and silences spindle assembly checkpoint via removal of Mad2 BubR1. At anaphase onset after separase cleaves cohesin switch to coordinated depolymerization at both ends: plus end depolymerizes at kinetochore Pacman mechanism driven by kinesin-13 and Dam1 ring, minus end at pole, pulling chromosomes poleward segregation of genetic material without continuous growth throughout mitosis. Continuous growth not observed switch essential.

Ref: Cheeseman & Desai, Annu Rev Cell Dev Biol – Kinetochore microtubules polymerize until anaphase then depolymerize.

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Cytoskeleton

What is the function of kinesin-5 in mitotic spindle dynamics?

Mitotic spindle bipolarity depends on antagonistic motor force balance between outward pushing and inward pulling motors establishing steady state length and facilitating chromosome bi orientation. Kinesin-5 family member Eg5 KSP is homotetrameric motor formed by antiparallel dimerization of dimers through centrally located tetramerization domain placing two motor heads at each end of dumbbell shape about 80 nm long. This geometry enables simultaneous crosslinking of antiparallel interpolar microtubules emerging from opposite poles and walking toward plus ends of each at about 20 nm per second sliding microtubules apart. Resulting force pushes poles outward during prometaphase and maintains spindle length at metaphase opposing minus directed kinesin-14 NCD HSET and dynein that pull poles together. Inhibitors like monastrol STLC block ATPase trapping Eg5 in ADP state causing monopolar spindles mitotic arrest apoptosis used therapeutically. Function not organelle transport but bipolar sliding essential for chromosome segregation fidelity checkpoint satisfaction and anaphase spindle elongation. Additional regulatory inputs including phosphorylation, small GTPases, and cargo adaptors fine tune filament assembly stability and motor activity matching cellular demands during division, migration, and mechanical stress responses efficiently.

Ref: Wittmann et al., Nat Cell Biol 2001 – Kinesin-5 function bipolar sliding of antiparallel microtubules.

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Cell Division

Individual chromosomes are clearly visible in which phase?

Visibility of individual chromosomes under light microscope depends on degree of condensation regulated by condensin complexes and histone modifications across cell cycle. Interphase chromatin exists as 10 nanometer beads-on-string fiber further folded into 30 nanometer fiber and looped domains anchored to nuclear matrix, occupying distinct chromosome territories where active transcription and replication occur, but overlapping extensively making individual chromosomes unresolvable. Entry into M phase, triggered by cyclin B-CDK1 rising activity, activates condensin I and II ATPases that extrude DNA loops and supercoil, producing 300 to 700 nanometer thick highly compact rod-shaped chromatids, while topoisomerase II decatenates sister intertwines. Phosphorylation of histone H3 serine 10 and histone H2A contributes. Resulting condensed chromosomes with distinct morphology including centromere constriction become individually discernible during prophase, prominently aligned at metaphase plate, ideal for karyotyping after hypotonic swelling. In G0, G1, S, G2, chromosomes remain decondensed diffuse, not recognizable as separate entities, so clear visualization exclusive to M phase.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Mitotic Chromosome Condensation.

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Cell Division

How many kinetochores are present in a human cell at mitosis?

Kinetochore count reflects chromosome and chromatid numbers fundamental to ploidy understanding. Human diploid somatic cell harbors 46 chromosomes, each defined by single centromere region where kinetochore assembles providing attachment site to spindle microtubules and checkpoint signaling platform. In G1, each chromosome contains one chromatid and one functional centromere kinetochore assembly site. During S phase, DNA replication converts each chromosome into two sister chromatids held together by cohesin, each sister chromatid retains its own centromere capable of assembling independent kinetochore structure composed of constitutive centromere-associated network and outer KMN network. Consequently by G2 and throughout mitosis until anaphase when sisters separate, total kinetochore number doubles to 92 distinct microtubule-binding interfaces. Electron tomography shows each mammalian kinetochore binds 20 to 30 microtubules forming K-fiber. Quantification via immunofluorescence for CENP-A or Ndc80 provides experimental readout for ploidy changes, polyploidization, and centromere inactivation events during tumorigenesis and development. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Cleveland et al., Cell 2003, Centromere and Kinetochore Number. Alberts 7th ed., Chapter 17.

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Cell Division

Which phase of mitosis is characterized by nuclear envelope reformation?

Telophase reverses many mitotic changes to re-establish interphase nuclear architecture in daughter cells following sister chromatid arrival at poles. Decline in cyclin B levels due to APC/C-Cdh1 mediated proteolysis extinguishes CDK1 activity, allowing protein phosphatases PP1 recruited to kinetochores by Repo-Man and PP2A-B55 to dephosphorylate key substrates. Dephosphorylation causes dissociation of condensin I and II complexes leading to chromatin decondensation and transcriptional restart, reassembly of nuclear lamina from lamin A/C and B1/B2 depolymerized during prophase, recruitment of nucleoporins through ELYS chromatin-binding to reform nuclear pore complexes restoring nucleocytoplasmic transport, and reformation of nucleolus around nucleolar organizer regions with RNA polymerase I activity resuming rRNA synthesis. Spindle microtubules depolymerize, while central spindle transiently persists forming midbody. Decondensation restores accessibility of genome for G1 gene expression programs, completing mitotic exit concurrent with cytokinesis abscission mediated by ESCRT-III filaments. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Telophase Restoration.

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Cell Division

What is the function of the spindle assembly checkpoint?

Reliability of chromosome segregation depends on surveillance mechanism preventing anaphase until every chromosome correctly attached to spindle, because single missegregation yields aneuploid daughter prone to tumor development or cell death. Spindle assembly checkpoint monitors occupancy and tension at kinetochores, large protein structures assembled on CENP-A containing centromeric chromatin. Unattached kinetochores recruit Mad1-Mad2 complex that catalyzes conversion of cytosolic open Mad2 to closed form bound to Cdc20. Together with BubR1, Bub3, Mps1-phosphorylated Bub1 they assemble mitotic checkpoint complex MCC that diffuses to inhibit APC/C-Cdc20 ubiquitin ligase activity, blocking degradation of securin and cyclin B. Aurora B kinase at inner centromere phosphorylates Ndc80 complex under low tension destabilizing erroneous syntelic attachments, recreating unattached kinetochore that re-engages checkpoint. Upon biorientation where sister kinetochores attach to opposite poles generating tension, Mad1-Mad2 recruitment ceases, MCC disassembles, APC/C activates separase, and synchronous segregation proceeds, preventing errors. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Musacchio & Salmon, Nature Reviews Mol Cell Biol 2007, SAC Mechanism. Alberts 7th ed., Chapter 17.

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Cell Division

Which enzyme is responsible for breaking down cohesin to allow sister chromatid separation?

Sister chromatid cohesion mediated by cohesin ring complex with SMC1 and SMC3 coiled-coil proteins forming V-shaped dimer bridged by kleisin Rad21 that closes tripartite ring embracing both sister DNAs from replication until anaphase. Opening requires proteolytic cleavage rather than dissociation. Separase, large 230 kDa cysteine endopeptidase belonging to CD clan with catalytic histidine-cysteine dyad analogous to caspases, serves as cleaving enzyme. Before anaphase, separase kept inactive through two layers: binding of securin pseudosubstrate occupying active site, and cyclin B-CDK1 mediated phosphorylation at serine 1126 causing cytoplasmic retention and auto-inhibition. At anaphase onset, APC/C-Cdc20 ubiquitinates securin with K11 chains for proteasomal destruction, cyclin B degradation reduces CDK1 activity allowing PP2A-dependent dephosphorylation, releasing separase to chromosomes where it cleaves Rad21 at conserved EXXR sequences separating N-terminal and C-terminal fragments unable to hold ring closed. Non-cleavable Rad21 mutant blocks sister separation causing metaphase arrest, proving cleavage essential. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Uhlmann et al., Nature 2000, Mechanism of Separase. Nasmyth, Science 2002, Cohesin Cleavage.

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Cell Division

In which phase of the cell cycle do cells prepare for mitosis?

G2 phase operates as final growth and quality control interval preparing cells materially and regulatory for mitosis. After completion of DNA replication in S phase, cells enter G2 where synthesis continues: transcription of genes encoding tubulin isotypes, gamma-tubulin ring complex proteins gamma-TuRC, pericentrin, Cep192, and microtubule-associated proteins rises to stockpile building blocks for spindle assembly. Centrosomes duplicated in S phase mature by acquisition of pericentriolar material becoming competent microtubule organizing centers. Organelles including Golgi apparatus and mitochondria expand through lipid synthesis and fusion events driven by mitofusins. Checkpoint kinase ATR activated by persistent RPA-coated single-stranded DNA and Chk1 maintain inhibitory Wee1-dependent Tyr15 phosphorylation on CDK1, holding cyclin B-CDK1 inactive while replication finishes. When replication and repair complete, Cdc25 phosphatases remove inhibitory marks, triggering feedback amplification that drives lamina breakdown, chromosome condensation, and Golgi unlinking. G2 thus ensures daughters inherit complete genome and sufficient organelle mass. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17, G2 Phase Preparation for Mitosis.

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Cell Division

The key determinant of the plane of cytokinesis in mammalian cells is the position of:

Placement of contractile ring determining cleavage plane is actively specified by mitotic spindle rather than cell geometry alone. During anaphase, after chromatids separate, antiparallel interpolar microtubules overlapping in middle zone are bundled by PRC1 protein and kinesins KIF4 and MKLP1, forming central spindle enriched with chromosome passenger complex including Aurora B kinase and centralspindlin heterotetramer of MKLP1-MgcRacGAP. This platform concentrates Rho guanine nucleotide exchange factor ECT2 at adjacent equatorial cortex, converting inactive RhoA-GDP to active RhoA-GTP in narrow band precisely midway between segregated genomes. Active RhoA stimulates formin mDia1 to nucleate linear actin filaments and ROCK-mediated phosphorylation of myosin regulatory light chain driving myosin II bipolar filament assembly and constriction. Astral microtubules radiating to polar cortex deliver inhibitory signals via MP-GAP suppressing Rac-mediated branched actin and Aurora A mediated inhibition, ensuring furrow ingresses only at equator perpendicular to central spindle, guaranteeing equal genome partitioning between daughters. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: D'Avino et al., J Cell Sci 2015, Cytokinesis Positioning. Glotzer, Science 2005, Central Spindle Signals.

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