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#mitosis

147 public questions tagged with this topic.

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

Which microtubule-binding protein suppresses catastrophe and promotes stability?

Stable microtubule subsets resist cold and nocodazole depolymerization due to decoration by classical structural MAPs reducing dynamics. Members include MAP2 and tau families as well as MAP1B and CLASP. MAP2 specifically expressed in neurons predominantly dendritic compartment forms projection domain protruding from filament serving as spacing crosslinker and microtubule binding repeats with positive charge neutralizing acidic tubulin C terminal tails bridging adjacent protofilaments reinforcing lateral contacts mechanically stiffening filament suppressing catastrophe by maintaining straight lattice conformation resisting GDP induced curling. Activity promotes rescue and long lived polymer essential for dendrite morphogenesis. Modulated by phosphorylation through MARK CDK5 reducing affinity to allow remodeling during branching and synaptic plasticity. CLASP also suppresses catastrophe at plus ends but MAP2 binds along lattice providing continuous stability. Katanin severs rather than stabilizes tau similar to MAP2 in axons. Therefore MAP2 exemplifies microtubule binding protein that suppresses catastrophe promotes stability crucial for dendrite outgrowth and plasticity maintenance and transport fidelity.

Ref: Dehmelt & Halpain, Genome Biology 2005 – MAP2 suppresses catastrophe promoting stability via lattice binding.

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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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Cytoskeleton

The microtubule-severing enzyme activated during mitosis is:

During mitosis interphase long stable microtubules must be extensively remodeled into highly dynamic spindle capable of search and capture chromosome alignment and segregation. Two coordinated regulatory shifts occur: dramatic phosphorylation of stabilizing MAPs such as MAP2 MAP4 and tau by CDK1 Aurora B and other mitotic kinases reduces their affinity several fold detaching them from lattice exposing filament to remodeling enzymes and increasing turnover; concurrently microtubule severing AAA ATPases katanin spastin and fidgetin activated via phosphorylation recruitment and release from sequestration. Katanin forms hexameric ring that uses ATP hydrolysis to pull tubulin C terminal tail through pore breaking lateral contacts generating fragments. MAP2 itself not severing enzyme but mitotic phosphorylation of MAP2 permits severing enzymes access to lattice. Result increased creation of short fragments dynamic ends increased catastrophe frequency. Transition ensures spindle size scaling chromosome capture and segregation illustrating regulated switch from stabilization to severing dominance crucial for mitotic progression. 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: McNally & Roll-Mecak, J Cell Biol 2018 – Mitosis severing enzymes including katanin regulation and MAP2 phosphorylation.

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Cell Division

What is the primary function of the centromere during mitosis?

Centromere is specialized chromosomal domain that appears as primary constriction in metaphase chromosomes and functions as essential platform for kinetochore assembly linking chromosomes to spindle microtubules and providing tension sensor. Human centromeres built on megabases of alpha-satellite repeat DNA enriched with nucleosomes containing histone variant CENP-A replacing H3, deposited by chaperone HJURP in G1, epigenetically marking locus independent of sequence alone. CENP-A recruits constitutive centromere-associated network including CENP-C, CENP-I, CENP-H, CENP-T-W-S-X complex forming inner kinetochore bridging DNA to outer kinetochore KMN network composed of KNL1 scaffold, Mis12 complex, and Ndc80 complex that directly binds microtubule plus ends via calponin homology domains. This trilaminar structure translates microtubule dynamics into chromosome movement and recruits spindle assembly checkpoint proteins Mad1, Mad2, Bub1 until biorientation satisfied. Artificial chromosome assays show alpha-satellite plus CENP-B boxes required for stable inheritance, proving centromere function indispensable for segregation during mitosis and meiosis. 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: Fukagawa & Earnshaw, Dev Cell 2014, Centromere Function. Alberts 7th ed., Chapter 17, Centromere.

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Cell Division

What ensures that DNA damage is repaired before mitosis?

Faithful genome transmission requires that DNA lesions incurred during replication or from external mutagens be repaired before chromosomes condense and segregate to avoid transmitting broken chromosomes and rearrangements. G2/M checkpoint serves this function by integrating damage sensing with CDK1 control. Lesions such as double-strand breaks recruit MRN complex MRE11-RAD50-NBS1 that activates ATM kinase, while RPA-coated single-stranded DNA from resection or stalled forks recruits ATRIP-ATR and TopBP1 activator. Effector kinases Chk2 and Chk1 phosphorylate dual-specificity phosphatases Cdc25B and Cdc25C at serine 216 generating 14-3-3 binding site sequestering them in cytoplasm away from nuclear cyclin B-CDK1, preserving inhibitory Tyr15 phosphorylation added by Wee1. p53 pathway parallel reinforces arrest via p21, Gadd45, 14-3-3 sigma transcriptional induction. When repair completed through homologous recombination or non-homologous end joining restores duplex, checkpoint kinases inactivated, PP2A dephosphorylates Cdc25, allowing nuclear entry and dephosphorylation of CDK1 driving mitotic entry after genome integrity restored. 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: Sancar et al., Annu Rev Biochem 2004, DNA Repair Checkpoints. Alberts 7th ed., Chapter 17, G2/M Damage Control.

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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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