GO:0044843 cell cycle G1/S phase transition: Commitment Point, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0044843 cell cycle G1/S phase transition is the biological process by which a cell in G1 phase commits to S phase, the point of no return for DNA replication.
• The transition is driven by cyclin-dependent kinases (CDKs), especially CDK4/6-cyclin D and CDK2-cyclin E, which phosphorylate RB1 and release E2F transcription factors.
• A unified quantitative model shows that the G1/S transition is a bistable switch controlled by the balance between CDK activity and its inhibitors, ensuring an all-or-none commitment.
• CDC7-dependent firing of replication origins is a key execution step, but CDC7-independent G1/S transition can occur when CDK activity is high, revealing redundancy.
• Higher-order chromatin organization and nuclear architecture compartmentalize G1/S regulatory events, influencing when and where replication starts.
• Dysregulation of the G1/S transition is a hallmark of cancer, and genes such as CCND1, CDK4, CDK6, RB1, and CDKN2A are frequent therapeutic targets.
Description
The cell cycle is the ordered series of events by which a cell duplicates its contents and divides. The transition from G1 phase to S phase, annotated as GO:0044843 cell cycle G1/S phase transition, is the critical commitment step at which a cell decides to replicate its DNA. This process is not a simple linear cascade but a highly regulated switch that integrates growth signals, nutrient status, and stress cues to ensure that DNA replication begins only when conditions are favorable. Understanding the G1/S transition is fundamental to cell biology because it controls proliferation, genome stability, and the response to DNA damage. At the molecular level, the G1/S transition is orchestrated by cyclin-dependent kinases (CDKs) and their regulatory cyclins. In mammalian cells, CDK4 and CDK6 partner with D-type cyclins to phosphorylate the retinoblastoma protein RB1, while CDK2-cyclin E further phosphorylates RB1 and other substrates to drive cells past the restriction point. This releases E2F transcription factors, which activate genes required for DNA replication and nucleotide biosynthesis. The process is also influenced by higher-order chromatin architecture and nuclear compartmentalization, which help coordinate the timing and location of replication origin firing. Researchers study the G1/S transition to understand normal development, tissue homeostasis, and diseases such as cancer, where unchecked proliferation is common. Recent work has revealed unexpected plasticity, including CDC7-independent G1/S transition when CDK activity is high, and the involvement of non-coding RNAs and micropeptides in regulating the transition. These findings underscore the importance of precise experimental models to dissect the genetic and biochemical control of this process.
cell cycle G1/S phase transition At A Glance
| GO ID | GO:0044843 |
|---|---|
| GO term | cell cycle G1/S phase transition |
| Ontology | biological_process |
| Synonym | None |
| Definition | The cell cycle process by which a cell in G1 phase commits to S phase. |
| Major function | Commitment to DNA replication and entry into S phase |
| Key regulators | CDK4/6-cyclin D, CDK2-cyclin E, RB1, E2F transcription factors, CDK inhibitors |
| Associated diseases | Cancer, developmental disorders, and diseases of uncontrolled proliferation |
| Research methods | Flow cytometry, live-cell imaging, CRISPR screens, transcriptomics, proteomics |
What Is GO:0044843?
GO:0044843 cell cycle G1/S phase transition is defined as the cell cycle process by which a cell in G1 phase commits to S phase [QuickGO]. In other words, it is the point at which a cell irreversibly decides to enter S phase and begin DNA replication, integrating internal and external signals to ensure faithful genome duplication.
Why Is cell cycle G1/S phase transition Important in Cell Biology?
The G1/S transition is a central control point in the cell cycle because it determines whether a cell will replicate its DNA and divide. Dysregulation of this transition can lead to uncontrolled proliferation, genomic instability, and cancer, making it a major focus for therapeutic intervention. Moreover, understanding the G1/S transition is essential for interpreting how cells respond to growth factors, DNA damage, and developmental cues, and for designing experiments that manipulate cell fate.
• Controls the commitment to DNA replication, ensuring genome integrity.
• Integrates growth signals, nutrient availability, and stress responses.
• Dysregulation is a hallmark of cancer, with frequent alterations in CCND1, CDK4, CDK6, RB1, and CDKN2A.
• Target of approved CDK4/6 inhibitors used in breast cancer therapy.
• Influenced by chromatin architecture and nuclear organization, affecting replication timing.
• Exhibits redundancy, with CDC7-independent G1/S transition under high CDK activity.
• Regulated by non-coding RNAs and micropeptides, adding layers of control.
• Critical for stem cell self-renewal and differentiation decisions.
• Provides a model for bistable switches in cell fate decisions.
• Key for understanding how cells exit the cell cycle into quiescence or senescence.
What Happens During cell cycle G1/S phase transition?
Integration of Growth Signals and Formation of Cyclin-CDK Complexes
In simple terms: The cell checks if conditions are right and builds the molecular engines that will push it into S phase.
During early G1, mitogenic signals induce the expression of D-type cyclins (CCND1, CCND2, CCND3), which assemble with CDK4 and CDK6 to form active kinases. These complexes phosphorylate RB1 and related pocket proteins, partially inactivating them. This initial phosphorylation is reversible and serves as a sensor for growth factor availability. The activity of CDK4/6-cyclin D is further modulated by CDK inhibitors such as p16INK4A (CDKN2A) and p21CIP1 (CDKN1A), which can block complex formation or activity. The balance between cyclins, CDKs, and inhibitors determines whether the cell will progress to the next step.
RB1 Hyperphosphorylation and E2F Release
In simple terms: The brake on the cell cycle is fully released, allowing the genes needed for DNA replication to be turned on.
As G1 progresses, CDK2-cyclin E complexes form and, together with CDK4/6-cyclin D, hyperphosphorylate RB1. Hyperphosphorylated RB1 dissociates from E2F transcription factors (E2F1-3), which then activate a large program of genes required for S phase, including DNA polymerases, origin recognition complex components, and nucleotide biosynthetic enzymes. This transcriptional program is essential for DNA replication and is a point of no return. The switch from partial to hyperphosphorylation of RB1 is a key event that commits the cell to S phase.
Origin Licensing and Firing
In simple terms: The cell marks all the starting points on its DNA where replication will begin and then activates them.
Before S phase, replication origins are licensed by the loading of MCM2-7 helicase complexes onto chromatin, a process that occurs in G1 and requires CDK activity to be kept low. As cells approach the G1/S transition, CDK2-cyclin E and CDC7-DBF4 kinase (DDK) promote the firing of licensed origins. CDC7 phosphorylates MCM subunits to activate helicase, while CDK2 phosphorylates other replication factors. Recent studies show that in the absence of CDC7, high CDK activity can still drive origin firing, revealing redundancy in the system. This step ensures that DNA replication begins at the right time and place.
Chromatin Architecture and Nuclear Compartmentalization
In simple terms: The 3D organization of DNA inside the nucleus helps control when and where replication starts.
Higher-order chromatin organization and nuclear architecture play important roles in regulating the G1/S transition. Specific genomic regions are positioned within the nucleus to facilitate or repress the transcription of cell cycle genes. For example, the nuclear matrix and chromatin loops bring together regulatory elements and gene promoters to coordinate expression. This architectural perspective shows that the G1/S transition is not just a biochemical cascade but also a spatially organized process.
Checkpoint Control and Stress Responses
In simple terms: If something is wrong, like DNA damage, the cell can pause the transition to repair or die.
The G1/S transition is monitored by checkpoints that can halt the cell cycle in response to DNA damage or other stresses. The ATM/ATR pathways activate p53 (TP53), which induces p21CIP1 (CDKN1A), a potent inhibitor of CDK2 and CDK4/6 complexes. This prevents cells with damaged DNA from entering S phase. Additionally, oncogenic stress can trigger senescence via p16INK4A and p14ARF, further illustrating the tight integration of the G1/S transition with cellular stress responses.
Key Genes Involved in GO:0044843 cell cycle G1/S phase transition
The following genes and proteins are central to the regulation and execution of the G1/S transition, based on published literature.
| Gene | Major Role | Research Relevance |
|---|---|---|
| CCND1 | Cyclin D1, partners with CDK4/6 to phosphorylate RB1 | Overexpressed in many cancers; target for CDK4/6 inhibitors |
| CDK4 | Catalytic subunit of CDK4-cyclin D complexes | Amplified or mutated in cancers; drug target |
| CDK6 | Catalytic subunit of CDK6-cyclin D complexes | Similar to CDK4; target for inhibitors |
| CCNE1 | Cyclin E1, activates CDK2 to drive G1/S transition | Overexpressed in cancers; marker of poor prognosis |
| CDK2 | Catalytic subunit of CDK2-cyclin E complexes | Essential for G1/S transition; target for therapy |
| RB1 | Retinoblastoma protein, gatekeeper of G1/S transition | Inactivated in many cancers; loss leads to unchecked proliferation |
| E2F1 | Transcription factor released by RB1 phosphorylation | Activates S-phase genes; oncogenic when deregulated |
| CDKN2A | p16INK4A, inhibits CDK4/6 | Frequently deleted or silenced in cancers |
| CDKN1A | p21CIP1, inhibits CDK2 and CDK4/6 | Mediates p53-dependent cell cycle arrest |
| TP53 | p53, induces p21 and other checkpoints | Mutated in many cancers; loss impairs G1/S checkpoint |
| CDC7 | Kinase that activates MCM helicase at origins | Target for cancer therapy; redundancy with CDK |
| DBF4 | Regulatory subunit of CDC7 | Required for CDC7 activity |
| MCM2 | Component of replicative helicase, licensed in G1 | Marker of origin licensing |
| MCM7 | Component of replicative helicase | Essential for DNA replication |
| SMIM30 | Micropeptide from LINC00998, regulates cytosolic calcium | Promotes G1/S transition; potential target |
| LINC00998 | Long non-coding RNA encoding SMIM30 | Regulates G1/S transition via calcium |
| PIP-FUCCI | Fluorescent reporter for cell cycle phases | Used to delineate G1/S transition in live cells |
How Is cell cycle G1/S phase transition Regulated?
The G1/S transition is regulated by a complex network of signaling pathways, including mitogenic signaling through RAS-MAPK and PI3K-AKT, which induce cyclin D expression. CDK inhibitors (CKIs) such as p21CIP1, p27KIP1, and p16INK4A provide negative regulation. The transition is also controlled by ubiquitin-mediated proteolysis of cyclins and CKIs, and by transcriptional feedback loops involving E2F and RB1. Recent studies have highlighted additional layers, including non-coding RNAs and micropeptides that modulate calcium signaling to influence the transition. A unified model proposes that the G1/S transition behaves as a bistable switch, with CDK activity and inhibitor thresholds determining the commitment point.
cell cycle G1/S phase transition and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| CCND1 | Breast cancer, lymphoma | Knockout or overexpression in cancer cell lines |
| CDK4 | Melanoma, breast cancer | Point mutation (e.g., R24C) knock-in mice |
| CDK6 | Lymphoma, leukemia | Knockout mice or CRISPR KO in cell lines |
| RB1 | Retinoblastoma, osteosarcoma | Knockout in retinal organoids or mice |
| CDKN2A | Melanoma, pancreatic cancer | Knockout in melanoma cell lines |
Cancer
Dysregulation of the G1/S transition is a hallmark of cancer. Overexpression of CCND1, CDK4, or CDK6, or loss of CDKN2A or RB1, leads to constitutive activation of the transition and uncontrolled proliferation. CDK4/6 inhibitors have shown clinical benefit in breast cancer, validating the G1/S transition as a therapeutic target. Mutations in TP53 impair the G1/S checkpoint, allowing damaged cells to replicate DNA and accumulate mutations.
Developmental Disorders
Proper control of the G1/S transition is essential for normal development. Mutations in genes such as CDKN1C (p57KIP2) cause Beckwith-Wiedemann syndrome, characterized by overgrowth, due to loss of cell cycle inhibition. Similarly, defects in RB1 cause retinoblastoma, a pediatric eye cancer, highlighting the importance of the G1/S transition in tissue-specific proliferation.
Neurodegeneration
Aberrant re-entry of postmitotic neurons into the cell cycle, including the G1/S transition, has been implicated in neurodegeneration such as Alzheimer's disease. However, direct evidence linking G1/S transition genes to neurodegeneration is limited, and further research is needed.
From cell cycle G1/S phase transition-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does gene X regulate G1/S transition? | CRISPR knockout in cell lines followed by flow cytometry |
| Does a specific point mutation in CDK4 alter G1/S transition? | Point mutation knock-in (e.g., R24C) in cells |
| Does overexpression of cyclin D1 drive G1/S transition? | Overexpression cell model with inducible promoter |
| Where is protein X localized during G1/S transition? | Tagged knock-in with fluorescent protein |
| What genes are essential for G1/S transition? | Genome-wide CRISPR library screening |
| How does gene X affect cell cycle transcriptome? | RNA-seq after knockout or overexpression |
How to Study the cell cycle G1/S phase transition Process
| Method | What It Measures | Typical Application |
|---|---|---|
| Flow cytometry | DNA content and cell cycle phase distribution | Quantify G1/S transition after gene knockout |
| PIP-FUCCI live imaging | Real-time cell cycle phase transitions | Track G1/S transition in single cells |
| RNA-seq | Transcriptional changes | Identify E2F target genes and pathways |
| Proteomics | Protein abundance and modifications | Measure RB1 phosphorylation and CDK levels |
| CRISPR screen | Genes affecting cell cycle progression | Discover novel regulators of G1/S transition |
| Kinase assay | CDK activity | Test inhibitors or mutations |
| EdU incorporation | DNA synthesis | Confirm S phase entry |
Flow Cytometry and Live-Cell Imaging
Flow cytometry using DNA dyes (e.g., propidium iodide) or EdU incorporation is a standard method to measure cell cycle distribution and identify G1/S transition defects. Live-cell imaging with reporters such as PIP-FUCCI allows real-time tracking of G1/S transition in individual cells. These methods are essential for validating genetic perturbations.
Transcriptomics and Proteomics
RNA sequencing (RNA-seq) can reveal changes in gene expression programs during the G1/S transition, including E2F target genes. Proteomics approaches, such as mass spectrometry, can quantify protein levels and phosphorylation states of key regulators like RB1 and CDK2. These methods provide a global view of the transition.
CRISPR Screens and Functional Genomics
Genome-wide CRISPR knockout or activation screens are powerful tools to identify genes that regulate the G1/S transition. For example, screens can be designed to find genes whose loss causes cell cycle arrest or whose overexpression drives proliferation. These screens often use reporters or sorting to enrich for cells with altered transition.
Biochemical Assays for Kinase Activity
In vitro kinase assays using recombinant CDK-cyclin complexes and substrates like RB1 can measure catalytic activity. Immunoprecipitation followed by western blotting can assess complex formation and phosphorylation status. These assays are useful for dissecting molecular mechanisms.
How CRISPR Can Be Used to Study GO:0044843 cell cycle G1/S phase transition
Knockout
CRISPR knockout of genes such as CDK2, CDK4, or CCND1 can be used to test their requirement for the G1/S transition. Cells lacking these genes may arrest in G1, which can be measured by flow cytometry. Knockout of negative regulators like CDKN1A or CDKN2A may accelerate the transition. These models are valuable for target validation.
Point Mutation
Point mutations in genes like CDK4 (e.g., R24C) that render the kinase resistant to inhibitors can be introduced using CRISPR knock-in. Such models help study how specific mutations affect G1/S transition and drug response. Point mutations in RB1 that disrupt phosphorylation sites can also be engineered to dissect signaling.
Knock-in
Knock-in of fluorescent tags (e.g., GFP) into endogenous loci such as CCND1 or CDK2 allows real-time visualization of protein dynamics during the G1/S transition. Knock-in of reporter genes under the control of E2F promoters can be used to monitor transcriptional activity. These models are powerful for live-cell imaging.
Overexpression
Overexpression of cyclin D1, CDK4, or E2F1 using CRISPR activation or lentiviral vectors can drive cells through the G1/S transition and is useful for studying oncogenic transformation. Inducible overexpression systems allow precise temporal control. These models help identify sufficiency of a gene for transition.
How EDITGENE Supports cell cycle G1/S phase transition Research
Researchers studying cell cycle G1/S phase transition-related genes often need to determine whether a candidate gene is causally involved in the transition, and to dissect the precise molecular mechanisms. This requires robust genetic models that can knockout, mutate, tag, or overexpress the gene of interest in relevant cell types. EDITGENE provides a comprehensive suite of CRISPR services to accelerate such studies.
Contact EDITGENE today to design your custom CRISPR model for cell cycle G1/S phase transition research.
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Frequently Asked Questions About cell cycle G1/S phase transition
What is the cell cycle G1/S phase transition?
It is the process by which a cell in G1 phase commits to S phase, the point of no return for DNA replication, annotated as GO:0044843.
What genes are involved in the G1/S transition?
Key genes include CCND1, CDK4, CDK6, CCNE1, CDK2, RB1, E2F1, CDKN2A, CDKN1A, and TP53, among others.
How is the G1/S transition regulated?
It is regulated by cyclin-CDK complexes, CDK inhibitors, phosphorylation of RB1, and E2F-mediated transcription, with additional control by signaling pathways and chromatin architecture.
Why is the G1/S transition important in cancer?
Dysregulation leads to uncontrolled proliferation; mutations in RB1, CDKN2A, CCND1, and CDK4 are common in cancers.
What methods are used to study the G1/S transition?
Flow cytometry, live-cell imaging with PIP-FUCCI, RNA-seq, proteomics, and CRISPR screens are commonly used.
What is the role of CDK4/6 in the G1/S transition?
CDK4/6-cyclin D complexes phosphorylate RB1, initiating the release of E2F transcription factors that drive S phase entry.
Can cells undergo G1/S transition without CDC7?
Yes, recent studies show that high CDK activity can drive origin firing independently of CDC7, revealing redundancy.
How does chromatin architecture affect the G1/S transition?
Higher-order chromatin organization and nuclear compartmentalization help coordinate the transcription of cell cycle genes and the timing of replication origin firing.
What are CDK4/6 inhibitors?
They are drugs that block CDK4/6 activity, causing G1 arrest and used in cancer therapy, such as palbociclib for breast cancer.
What is the restriction point?
The restriction point is a late G1 checkpoint after which cells are committed to enter S phase even if growth factors are removed, closely linked to the G1/S transition.
Conclusion
The G1/S phase transition (GO:0044843) is a fundamental cell cycle process that integrates multiple signals to ensure faithful DNA replication. Its dysregulation is central to cancer and other proliferative disorders, making it a prime target for research and therapy. Advances in CRISPR technology and live-cell imaging continue to unravel the complex regulatory networks, including non-coding RNAs and micropeptides, that control this critical decision point. For researchers aiming to dissect the G1/S transition, precise genetic models are indispensable. EDITGENE's comprehensive CRISPR services, from knockout to knock-in and library screening, provide the tools needed to accelerate discoveries in this field.
References
- 1. Bertoli C et al.. 2013. Control of cell cycle transcription during G1 and S phases.. Nat Rev Mol Cell Biol 14(8):518-28 PMID: 23877564
- 2. Grant GD et al.. 2018. Accurate delineation of cell cycle phase transitions in living cells with PIP-FUCCI.. Cell Cycle 17(21-22):2496-2516 PMID: 30421640
- 3. Hume S et al.. 2020. A unified model for the G1/S cell cycle transition.. Nucleic Acids Res 48(22):12483-12501 PMID: 33166394
- 4. Stein GS et al.. 2006. An architectural perspective of cell-cycle control at the G1/S phase cell-cycle transition.. J Cell Physiol 209(3):706-10 PMID: 17001681
- 5. Suski JM et al.. 2022. CDC7-independent G1/S transition revealed by targeted protein degradation.. Nature 605(7909):357-365 PMID: 35508654
- 6. Ghule PN et al.. 2018. Higher order genomic organization and regulatory compartmentalization for cell cycle control at the G1/S-phase transition.. J Cell Physiol 233(10):6406-6413 PMID: 29744889
- 7. Hengstschläger M et al.. 1999. Cyclin-dependent kinases at the G1-S transition of the mammalian cell cycle.. Mutat Res 436(1):1-9 PMID: 9878675
- 8. Yang JE et al.. 2023. LINC00998-encoded micropeptide SMIM30 promotes the G1/S transition of cell cycle by regulating cytosolic calcium level.. Mol Oncol 17(5):901-916 PMID: 36495128
