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#G2 phase

15 public questions tagged with this topic.

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

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 G2 phase is characterized by:

G2 phase is second gap phase dedicated to growth, quality surveillance, and accumulation of components required for mitosis. After S phase completes DNA synthesis, cells continue protein synthesis, particularly tubulin alpha/beta heterodimers and gamma-tubulin ring complex proteins for spindle assembly, histone variants H2AZ for chromatin stability, and membrane lipid synthesis for organelle expansion. Centrosomes duplicated during S phase mature in G2 by recruitment of pericentriolar material including pericentrin, Cep192, and Cdk5Rap2, acquiring ability to nucleate large astral microtubule arrays. Mitochondria undergo fusion driven by mitofusins to enhance ATP production for mitosis. Simultaneously DNA damage checkpoint kinase ATR monitors incomplete replication intermediates and stalled forks, maintaining cyclin B-CDK1 inactive via Wee1 mediated Tyr15 phosphorylation until genome fully duplicated. Transcription of mitotic cyclin B and Polo-like kinase rises, preparing wave of mitotic entry. G2 thus bridges synthesis and segregation, ensuring daughters inherit intact genome and sufficient cytoplasm. 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 as Preparation for M Phase.

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Cell Division

What is the role of cyclins in cell cycle regulation?

Cyclins constitute regulatory subunits that impose temporal order on constitutively expressed but inactive cyclin-dependent kinases. In mammals, D-type cyclins respond to mitogens via Ras-MAPK induction, E-type governs G1/S boundary, A-type controls S and G2 progression, B-type drives mitosis. Binding of cyclin to CDK induces conformational change in T-loop exposing activation site for phosphorylation by CAK complex CDK7-cyclin H-Mat1, stabilizing fully active kinase competent for substrate phosphorylation. Substrate specificity derives from cyclin hydrophobic patch interacting with RXL motifs and subcellular localization. Cyclin D-CDK4/6 phosphorylates Rb pocket proteins, cyclin E-CDK2 phosphorylates p27 and Treslin, cyclin A-CDK2 phosphorylates ORC and Cdc6 to block re-licensing, cyclin B-CDK1 phosphorylates lamins, condensins, and nuclear pore proteins to disassemble interphase structures. Ubiquitin ligases SCF and APC/C confer switch-like inactivation by degrading cyclins at distinct cell cycle phases, producing oscillations. Without cyclin partner, CDKs exhibit negligible kinase activity, explaining functional dependency. 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: Morgan, The Cell Cycle: Principles of Control, Chapter 3: Cyclin-CDK Regulation. Alberts 7th ed., Chapter 17.

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Cell Division

Which phase of the cell cycle is the most metabolically active?

Cellular metabolism is tightly coupled to cell cycle progression, with G1 phase exhibiting maximal biosynthetic activity to support doubling of mass before DNA replication. Upon growth factor binding, PI3K-Akt-mTORC1 pathway stimulates glucose uptake through GLUT1, increases glycolytic flux and pentose phosphate pathway for nucleotide precursors, enhances mitochondrial biogenesis via PGC1-alpha and mitochondrial transcription factor TFAM, and activates ribosome biogenesis through RNA polymerase I mediated rDNA transcription and S6K phosphorylation of ribosomal protein S6. Translation initiation factor eIF4E cap-binding protein synthesizes cyclins, CDKs, replication factors Cdt1, Cdc6, and histone chaperones. Lipid synthesis provides membrane for organelle growth, while amino acid transport sustains protein synthesis. By contrast, S phase devotes resources to dNTP synthesis, M phase shuts transcription, and G0 quiescent cells greatly reduce metabolic rate. Therefore G1 concentrates growth, accumulating tubulin, actin, and centrosome components monitored by size-sensing pathways. 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: G1 Metabolism and Growth.

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Cell Cycle Regulation

Wee1 kinase inhibition in G2 phase leads to:

G2 progression is gated by inhibitory phosphorylation of master mitotic kinase CDK1 that prevents premature mitotic entry while last preparations complete. Nuclear kinase Wee1 and membrane-associated Myt1 phosphorylate CDK1 subunit within cyclin B-CDK1 complex at Tyr15 and Thr14 within ATP-binding loop, rendering kinase catalytically inactive despite abundant cyclin B synthesized during G2, allowing centrosome maturation, Golgi segregation, and DNA repair synthesis to proceed. At G2/M border, dual-specificity phosphatases Cdc25A/B/C remove these phosphates, causing explosive autoamplification where CDK1 phosphorylates Cdc25 to activate it and phosphorylates Wee1 to inhibit it. Pharmacologic inhibition or siRNA depletion of Wee1 eliminates inhibitory phosphate, leading to immediate activation of cyclin B-CDK1 before replication fully completed or damage repaired, driving cells into mitosis with under-replicated DNA, premature chromosome condensation, pulverization, and activation of mitotic catastrophe program. This vulnerability underlies therapy where Wee1 inhibitors enhance genotoxic chemotherapy efficacy in p53-deficient cancers. 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: Harper & Elledge, Mol Biol Cell 2007, Wee1 Control. Alberts 7th ed., Chapter 17, CDK1 Regulation.

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