Cell Division

Cell Division

Which factor regulates the exit of cells from mitosis?

Exit from mitosis, defined transition from high CDK1 activity driving chromosome condensation and spindle assembly to low CDK state permitting G1 growth, is regulated primarily by anaphase-promoting complex/cyclosome APC/C ubiquitin ligase. After anaphase onset mediated by APC/C-Cdc20 degradation of securin and cyclin B, second wave of APC/C activity associated with coactivator Cdh1 becomes dominant as CDK1 inactivation allows Cdc14 and PP2A phosphatases to dephosphorylate Cdh1 permitting its binding. APC/C-Cdh1 polyubiquitinates remaining mitotic cyclin B, Polo-like kinase, Aurora kinases A and B, Cdc20 itself, and other regulators targeting them to 26S proteasome, ensuring CDK activity precipitously falls and remains low throughout G1. Decline permits phosphatases to dephosphorylate lamins for nuclear envelope reassembly, condensin dissociation for chromatin decondensation, Golgi reassembly, and licensing of replication origins via pre-RC formation. Without functional APC/C, cells arrest in late anaphase with high cyclin B, unable to reform nuclei, demonstrating central regulatory role governing mitotic exit irreversibly.

Ref: Sullivan & Morgan, Nature Rev Mol Cell Biol 2007, Mitotic Exit Control. Alberts 7th ed., Chapter 17.

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Cell Division

What happens during anaphase II of meiosis?

Meiosis consists of two divisions following single S phase, with meiosis II equationally separating sisters after reductional meiosis I segregated homologs. Cells exit meiosis I entering interkinesis without replication, centrosomes duplicate, chromosomes recondense, and spindle reforms often orthogonal to first division axis. At metaphase II plate, sister chromatids align with kinetochores bioriented to opposite poles similar to mitosis, achieved despite haploid chromosome number. Protection of centromeric cohesion established in meiosis I by Shugoshin SGO2 bound to PP2A phosphatase that dephosphorylates Rec8 cohesin preventing separase cleavage is now removed as SGO2 degrades from centromeres. Upon APC/C-Cdc20 activation at anaphase II, securin degraded, separase active cleaves remaining centromeric Rec8, dissolving cohesion. Sister chromatids, now genetically non-identical due to crossing over in prophase I exchanging non-sister segments, separate toward opposite poles driven by kinetochore microtubule depolymerization and spindle elongation, generating four haploid daughter cells essential for sexual reproduction and complementation after fertilization. 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 21: Meiosis II Sister Chromatids.

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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

Which of the following proteins is responsible for regulating the G1/S transition?

G1/S transition also known as restriction point in mammals marks commitment to DNA replication independent of extracellular mitogens thereafter. Regulation centers on retinoblastoma protein phosphorylation cascade. Mitogens induce cyclin D-CDK4/6 via Ras-MAPK pathway which initiates partial phosphorylation of Rb family proteins pRb, p107, p130 at specific serine sites, displacing histone deacetylases and allowing transcription of cyclin E gene driven by E2F1-3. Cyclin E then associates with CDK2, activated by CAK phosphorylation, forming cyclin E-CDK2 holoenzyme that hyperphosphorylates Rb at additional sites including Thr373, Ser612, causing complete dissociation and full E2F-dependent transcription of S-phase genes encoding cyclin A, MCM2-7, PCNA, DNA polymerase alpha and delta, thymidine kinase, and dihydrofolate reductase. Cyclin E-CDK2 also phosphorylates p27Kip1 Thr187 creating phosphodegron for SCF-Skp2 ubiquitination establishing positive feedback loop making transition switch-like irreversible. Loss of cyclin E arrests cells in G1 despite mitogen presence, proving essential role in governing G1/S progression. 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: Dulic et al., Cell 1992, Cyclin E-CDK2 at G1/S. Alberts 7th ed., Chapter 17, Restriction Point Control.

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Cell Division

Which phase of the cell cycle is referred to as the quiescent phase?

Quiescent phase G0 represents reversible exit from cell cycle distinct from terminal differentiation and senescence, providing flexible pause aligning proliferation with physiological needs. Cells enter G0 from early G1 when mitogens withdraw, contact inhibition activates Hippo signaling with YAP nuclear exclusion, or nutrient deprivation suppresses mTORC1. Molecularly, cyclin D transcription declines, p27Kip1 and p130 Rb2 pocket protein accumulate, assembling DREAM complex with E2F4-DP1 repressing over 800 cell cycle genes including cyclin A, cyclin B, MCM helicases, and PCNA. Ribosomal RNA transcription by RNA polymerase I drops, reducing ribosome biogenesis and metabolic rate. Chromatin acquires repressive marks but retains plasticity. Upon re-addition of growth factors, Ras-ERK induces Myc and cyclin D, Skp2 mediated degradation reduces p27, DREAM disassembles, and cells re-enter G1. Lymphocytes, fibroblasts, hepatocytes, and stem cells exemplify G0. This reversible quiescence preserves proliferative capacity, prevents stem cell exhaustion, and matches tissue renewal to injury or demand, essential for longevity.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: G0 and Quiescence Regulation.

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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

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

The site of the division plane during cytokinesis in animal cells is determined by:

Cytokinesis in animal cells determines placement of furrow such that daughter cells inherit single genome copy and appropriate cytoplasmic volume. Experimental embryology demonstrated furrow forms where central spindle contacts cortex. Mechanistically, anaphase central spindle assembled from overlapping antiparallel interpolar microtubules bundled by PRC1 and centralspindlin component KIF4 recruits centralspindlin heterotetramer consisting of kinesin MKLP1 and RhoGAP MgcRacGAP. Centralspindlin clusters ECT2 RhoGEF, generating zone of active RhoA-GTP precisely at equatorial membrane. RhoA-GTP activates formin mDia1 to polymerize linear actin filaments and ROCK kinase phosphorylating myosin regulatory light chain serine 19 promoting bipolar myosin II assembly in antiparallel bundles contracting ring similarly to muscle. Astral microtubules reaching polar cortex deliver inhibitory cues through Aurora A phosphorylating ECT2 and RacGEF Trio suppressing ectopic contractility at poles. Net result furrow ingresses perpendicular to central spindle axis between chromosome masses. Manipulation shifting spindle relocates furrow accordingly, proving spindle dominance over chromosomes for division site. 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: Fededa & Gerlich, Current Biology 2012, Division Plane Specification. Alberts 7th ed., Chapter 17.

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Cell Division

What happens if a proteasome inhibitor is added to cells in G2 phase?

Cell cycle progression relies heavily on ubiquitin-proteasome pathway to confer irreversibility to transitions via degradation of cyclins and CDK inhibitors. Proteasome comprises 20S catalytic core with chymotrypsin-like, trypsin-like, caspase-like activities and 19S regulatory particle recognizing polyubiquitinated proteins. Inhibitors such as MG132 peptide aldehyde, lactacystin, and clinically used bortezomib block catalytic threonine residues, causing accumulation of polyubiquitinated substrates. When applied in G2, proteasome blockade prevents degradation of SCF substrates like p21 and Wee1, APC/C substrates like cyclin A needed for S/G2 transition, and also blocks NF-kB inhibitor IkB turnover altering transcriptional programs. Consequence includes sustained Chk1 signaling from replication stress, stabilization of CDK inhibitors, and failure to degrade cyclin B after mitotic entry attempt, leading to persistent inhibitory phosphorylation on CDK1 and activation of G2/M checkpoint kinase Wee1. Cells therefore arrest at G2/M boundary with 4N DNA content, unable to satisfy APC/C requirements, eventually undergoing apoptosis if arrest prolonged, explaining chemotherapeutic efficacy.

Ref: Ciechanover, Nature Reviews Mol Cell Biol 2005, Proteasome and Cycle. Alberts 7th ed., Chapter 3.

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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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