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#G1 phase

15 public questions tagged with this topic.

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

The role of p27 in G1 phase is to:

p27Kip1 product of CDKN1B gene exemplifies Cip/Kip family that imposes G1 arrest and integrates nutrient availability with size control. In quiescent and early G1 cells, p27 accumulates due to low proteasomal turnover and binds cyclin E-CDK2 heterodimer, inserting 3_10 helix into ATP-binding pocket, preventing catalysis and blocking Rb phosphorylation, maintaining E2F repression. This prevents premature S-phase entry when growth factors limited. Mitogen signaling induces cyclin D-CDK4/6 complexes that sequester p27 in trimeric assembly without fully inhibiting them, lowering free p27 pool. Subsequent phosphorylation of p27 at threonine 187 by progressively activated cyclin E-CDK2 creates high-affinity phosphodegron recognized by F-box protein Skp2 assembled in SCF complex with Skp1, Cul1, Rbx1, and accessory Cks1. SCF-Skp2 polyubiquitinates p27 targeting it to 26S proteasome. Declining p27 levels unleash full CDK2 activity, hyperphosphorylating Rb and committing to DNA replication when nutrient conditions favorable. 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: Sherr & Roberts, Genes & Dev 2004, p27 Biology. Morgan, Cell Cycle, Chapter 4, G1 Control.

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Cell Cycle Regulation

The APC/C-Cdh1 complex is required for:

Specificity and timing of APC/C activity arise from sequential association with two adaptors, Cdc20 and Cdh1, both containing seven WD40 repeats forming beta-propeller that binds D-box and KEN motifs. During prometaphase and metaphase, APC/C-Cdc20 polyubiquitinates securin and cyclin B to trigger anaphase and mitotic exit. At telophase, cyclin B degradation reduces CDK1 activity, allowing Cdc14 phosphatase to dephosphorylate Cdh1, enabling its binding to APC/C core. APC/C-Cdh1 then operates from late mitosis through entire G1, continuously ubiquitinating mitotic cyclins A and B, mitotic kinases Polo and Aurora, Geminin inhibitor of licensing, and even Cdc20 itself, ensuring low CDK environment. Low CDK permits pre-replication complex formation consisting of ORC, Cdc6, Cdt1, and double hexamer of MCM2-7 helicase loaded at origins. As cyclin E-CDK2 accumulates at G1/S, it phosphorylates Cdh1 creating 14-3-3 sites, displacing it from APC/C and inactivating ligase, allowing S-phase cyclin accumulation and transition to next cycle. 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: Peters, Nature Reviews Mol Cell Biol 2006, APC/C Mechanisms. Alberts 7th ed., Chapter 17.

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Cell Cycle Regulation

What is the role of Cyclins in the cell cycle?

Cyclins earned name due to fluctuating concentrations through cell cycle while CDKs remain relatively constant. Function is to activate CDKs by allosteric remodeling and to direct substrate choice through additional binding motifs. In G1, Cyclin D with CDK4/6 senses mitogens, Cyclin E with CDK2 triggers S-phase entry, Cyclin A with CDK2 drives S-phase progression and later with CDK1 primes mitosis, Cyclin B with CDK1 executes mitosis. Each cyclin contains conserved cyclin box domain and hydrophobic patch recognizing RXL motifs on substrates, plus localization signals determining nuclear or centrosomal enrichment. Binding causes CDK PSTAIRE helix rotation aligning catalytic lysine glutamate salt bridge and opening activation segment for substrate accommodation. Resulting holoenzyme phosphorylates diverse targets including Rb family proteins for transcription derepression, lamins for envelope breakdown, condensins for chromosome compaction, replication factors for origin firing control and microtubule associated proteins for spindle assembly. Ubiquitination by SCF in interphase and APC/C in mitosis ensures periodic disappearance of cyclins, creating irreversible forward direction. Thus cyclins convert basal low affinity kinase into temporal specific effector.

Ref: Murray, Cyclins: Roles in Cell Cycle and Evolution, Cell; Alberts et al., Molecular Biology of the Cell, Chapter 17, Cyclin-CDK Functions.

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Cell Cycle Regulation

Which event marks the commitment to cell division?

Late G1 restriction point defines irreversible commitment to divide independent of external growth factors. Its molecular basis revolves around retinoblastoma protein Rb and E2F transcription factors. In quiescence, hypophosphorylated Rb binds E2F-DP1 heterodimers, recruiting histone deacetylases, SWI/SNF remodelers and H3K9 methyltransferases to repress genes needed for S phase. Mitogen signaling induces Cyclin D synthesis via Ras-MAPK and PI3K pathways. Cyclin D-CDK4/6 complexes accumulate, phosphorylate Rb at multiple C-terminal residues, partially reducing repression and derepressing limited E2F target transcription including Cyclin E gene. Cyclin E then binds CDK2, forming potent kinase that hyperphosphorylates Rb at additional pocket domain residues, creating complete disruption of Rb-E2F interaction. This generates positive feedback loop producing large burst of E2F activity transcribing DNA polymerase subunits, dihydrofolate reductase, Cdc6, Cyclin A and MCM proteins. After this switch, growth factor removal no longer halts progression; cells complete division autonomously, highlighting Rb phosphorylation as commitment event essential for proliferative control and cancer relevance. Additional feedback loops involving polo-like kinases, phosphatases and SCF-mediated degradation reinforce irreversibility and protect against premature progression that would compromise genome integrity and viability.

Ref: Weinberg, Retinoblastoma Protein and Cell Cycle Control, Cell 1995; Alberts et al., Molecular Biology of the Cell, Chapter 17, Rb-E2F Switch.

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