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#chromosome segregation

13 public questions tagged with this topic.

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

Ref: Fukagawa & Earnshaw, Dev Cell 2014, Centromere Function. Alberts 7th ed., Chapter 17, Centromere.

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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 mitot

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

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Cell Cycle Regulation

What is the role of the Spindle Assembly Checkpoint (SAC)?

Chromosome missegregation produces aneuploidy implicated in tumorigenesis, congenital disorders, and embryonic lethality, necessitating surveillance by spindle assembly checkpoint. Sensor is kinetochore assembled on CENP-A containing centromeric nucleosomes that binds microtubule plus ends via Ndc80 complex. Unattached kinetochores act as catalytic platforms where Mad1-Mad2 heterotetramer converts soluble open Mad2 into closed conformation that binds Cdc20. Together with BubR1, Bub3, and Bub1 phosphorylated by Mps1 kinase, they generate diffusible mitotic checkpoint complex that inhibits APC/C

Ref: Musacchio, Current Biology 2015, SAC Mechanism. Alberts 7th ed., Chapter 17, Mitosis Quality.

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Cell Cycle Regulation

What ensures the proper segregation of chromosomes at anaphase?

Correct chromatid partitioning during anaphase integrates mechanical forces and biochemical resetting pathways. Physical separation depends on earlier APC/C-Cdc20 mediated securin degradation freeing separase to cleave cohesin subunit Scc1 at centromeres, allowing kinetochore microtubules to pull sister chromatids poleward via depolymerization coupled movement. However exit from high CDK state also essential. In budding yeast model, mitotic exit network monitors spindle elongation, activating Tem1 GTPase when daughter pole enters bud overcoming checkpoint inhibition by Kin4-Bub2-Bfa1 GAP compl

Ref: Amon, Mitotic Exit Network and Cdc14 in Segregation, Current Biology; Bardin & Amon, Coordination of Anaphase and Exit.

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Cell Cycle Regulation

Which kinase family plays a role in spindle formation and chromosome segregation?

Spindle construction requires cooperation between CDK1 and two mitotic kinase families distinct from CDKs. Aurora kinases A, B, C share conserved catalytic domains but differ in localization. Aurora A at centrosomes phosphorylates TACC3, ch-TOG, LATS2, CEP192 increasing pericentriolar material recruitment and microtubule nucleation. Aurora B as catalytic subunit of chromosomal passenger complex with INCENP, Survivin, Borealin resides at inner centromere until metaphase and midbody during cytokinesis. It phosphorylates Ndc80/Hec1, MCAK, Dam1/Ska complexes to destabilize attachments lacking tens

Ref: Nigg, Mitotic Kinases Aurora and Polo Regulation of Cell Division, Science 2001; Carmena et al., Aurora B in Error Correction.

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Cell Cycle Regulation

What is the primary role of the Mps1 kinase?

Spindle assembly checkpoint activation hinges on kinase Mps1, also known as TTK, a dual specificity protein kinase recruited to unattached kinetochores. In prometaphase, Mps1 binds Ndc80 complex through competition with microtubules, positioning it near scaffold KNL1. Mps1 phosphorylates multiple MELT repeats on KNL1 at conserved methionine-glutamate-leucine-threonine motifs, converting them into phospho-docking sites for Bub3-Bub1 and Bub3-BubR1 complexes. Additionally, Mps1 phosphorylates Mad1 at C-terminus facilitating catalytic conversion of open O-Mad2 to closed C-Mad2, the conformer capa

Ref: London & Biggins, Mps1 Dependent KNL1 MELT Phosphorylation, Genes Dev 2014; Alberts et al., Chapter 17, SAC Kinase Cascade.

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Cell Cycle Regulation

Which factor ensures proper chromosome segregation at anaphase?

Sister chromatid cohesion, mediated by the ring-shaped cohesin complex composed of Smc1, Smc3, kleisin Scc1 or Rad21 and SA proteins, must be abruptly destroyed only after bioriented attachment. Separase provides the decisive enzymatic activity. This large cysteine endopeptidase recognizes a conserved E-x-x-R motif within the kleisin subunit and cleaves after arginine, effectively opening the ring. During early mitosis separase remains constrained by two inhibitory locks: stoichiometric binding of securin which occupies the catalytic groove as pseudosubstrate and chaperone, and CDK1-Cyclin B1

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 17: Cell Cycle, Sister-Chromatid Separation and Separase Regulation.

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Cell Cycle Regulation

Which of the following prevents premature mitotic exit?

In budding yeast, mitotic exit is controlled by mitotic exit network, abbreviated MEN, culminating in release of phosphatase Cdc14 from nucleolus to dephosphorylate CDK substrates and activate APC/C-Cdh1 and Sic1 to drive G1 entry. Premature exit would cause cytokinesis before chromosome segregation, generating aneuploid progeny. Two mechanisms prevent this: inhibition of upstream GTPase Tem1 keeps MEN inactive until anaphase when Tem1 is recruited to spindle pole body and activated by Lte1, ensuring that only elongated spindles trigger exit. Separately, Cdh1, alternative activator of APC/C, i

Ref: Visintin et al., Mol Cell 1998, Mitotic Exit Network; Stegmeier & Amon, Ann Rev Genet 2004, Closing Mitosis.

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Microbial Physiology

The bacterial partitioning system consists of:

Bacterial genome partitioning analogous mitosis uses conserved tripartite system. Components include ParA ATPase Walker box, ParB DNA binding protein, centromere like DNA site parS. parS consists of short palindrome repeats 16 base pairs each bound specifically by ParB via helix turn helix domain, ParB CTPase forms focus spreading several kilobases forming partition complex that also loads SMC condensin MukBEF. ParA ATP dimer binds nonspecifically to nucleoid periphery, forming oscillating pattern. Interaction ParB triggers ATP hydrolysis releasing ParA allowing movement. Tripartite ParA ParB

Ref: Livny et al., Mol Microbiol 2007, Bacterial partitioning system ParA ParB parS components.

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Microbial Physiology

In bacterial chromosome segregation, ParA is responsible for:

In tripartite ParABS chromosome segregation ParA role dynamic scaffolding providing motive force. ParA proteins family Walker A ATPase dimerize upon ATP binding forming DNA binding competent form coating bacterial chromosome as cargo independent cloud. Upon ATP hydrolysis to ADP affinity for DNA drops, detachment occurs. ParB parS partition complex stimulates ATPase converting ParA ATP to ADP, so region behind moving complex depleted ParA, while front retains high concentration. Newly synthesized ParA ATP rebinds nucleoid generating gradient that directs chemophoretic movement of ParB parS ori

Ref: Lutkenhaus, Annu Rev Biochem, 2020, ParA filament formation pulling chromosomes apart during segregation.

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Microbial Physiology

What is the primary function of ParA and ParB proteins in bacterial cells?

Stable inheritance of chromosome and low copy plasmids cannot rely on random diffusion given small cell volume, requires active segregation machinery. ParA and ParB constitute conserved partition system. ParB centromere binding protein binds parS sequence palindromic heptad repeats near origin or on plasmid forming large nucleoprotein complex via spreading oligomerization stimulated by CTP binding, also recruits SMC condensin compacting DNA. ParA Walker ATPase dimeric ATP bound associates non specifically with nucleoid DNA forming gradient or filamentous cloud. ParB triggers ParA ATPase releas

Ref: Baxter & Funnell, J Bacteriol, 2014, ParA ParB plasmid and chromosome segregation system.

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Nucleosome Packing

SMC proteins are involved in:

Structural Maintenance of Chromosomes proteins represent large ATPase machines forming elongated coiled-coil dimers with hinge domain closing ring-like structures embracing DNA duplexes. Condensin complexes SMC2/SMC4 compact mitotic chromosomes via loop extrusion, cohesin SMC1/SMC3 holds sister chromatids together from S phase until anaphase cleavage by separase, SMC5/SMC6 aids homologous recombination repair at collapsed forks. Mutations produce chromosome missegregation, aneuploidy and repair defects. Proteins localize to scaffold attachment regions and utilize ATP hydrolysis to extrude loop

Ref: Hirano T 2016 Nat Rev Mol Cell Biol SMC; Lodish et al., Chapter 19: SMC Proteins Chromosome Maintenance

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