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The G1 Checkpoint and Cell Cycle Regulation

Exploring the G1 checkpoint's role in cell cycle regulation, this overview highlights how cells decide to divide or repair DNA. It delves into the functions of E2F transcription factors, retinoblastoma protein, and cyclin-CDK complexes in controlling cell division. The text also examines the mechanisms of cell cycle arrest and the transition to mitosis, including the G2 checkpoint and the concept of hysteresis in cell cycle transitions.

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1

G1 Checkpoint Function

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Determines cell division commitment; integrates signals for repair, rest, or replication.

2

Role of DNA Integrity in G1 Phase

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Activates repair pathways; prevents cycle progression if damage detected.

3

Transition from G1 to S Phase Mechanism

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CDKs activation; initiates transcription for DNA replication genes.

4

The ______ checkpoint is controlled by E2F transcription factors and the ______ protein family.

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G1 retinoblastoma

5

E2F proteins are crucial for activating genes that are essential for ______ progression, such as those for ______ and ______.

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cell cycle cyclins CDKs

6

Dysregulation of E2F activity is commonly linked to ______, highlighting its importance in cell cycle control.

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tumorigenesis

7

The Rb protein and its kin, ______ and ______, regulate E2F activity by binding to it.

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p107 p130

8

Initial phosphorylation of Rb: involved cyclins and CDKs?

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Cyclin D associates with CDK4 or CDK6, initiating Rb phosphorylation, reducing its affinity for E2F.

9

Consequence of Rb phosphorylation for E2F activity?

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Phosphorylated Rb releases E2F, leading to partial activation of E2F target genes.

10

Role of cyclin E-CDK2 complex in cell cycle progression?

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Cyclin E binds to CDK2, further phosphorylating Rb, fully activating E2F, and promoting transcription of genes for S phase entry.

11

The kinase inhibitor known as ______ binds to cyclin E-CDK2 to inhibit cell cycle progression.

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p27^Kip1 (CDKN1B)

12

Cyclin A joining CDK2 leads to the destruction of ______, enabling the cyclin A-CDK2 complex's full activation.

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p27^Kip1

13

The activated cyclin A-CDK2 complex phosphorylates ______, stopping the transcription of S phase genes.

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E2F

14

In the event of DNA damage, ______ and ______ kinases activate Chk1 and Chk2 to degrade Cdc25A.

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ATM ATR

15

Chk1 and Chk2, once activated, target ______ for degradation to halt the cell cycle in the G1 phase.

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Cdc25A

16

Key event for mitosis transition

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Cyclin B-Cdk1 complex activation triggers transition to mitosis.

17

Role of ATR and ATM kinases in G2 checkpoint

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ATR/ATM kinases activate Chk1/Chk2 to arrest cell cycle if DNA damage or replication errors occur.

18

Cyclin B-Cdk1 regulation mechanism

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Positive feedback loop: Plk1 kinase activates Cdc25, degrades Wee1, promoting mitotic entry.

19

The shift from ______ to M phase in cell division is clearly demonstrated by the development of ______ oocytes.

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G2 Xenopus

20

______ triggers a series of reactions that lead to the activation of the ______ complex, propelling the cell into mitosis.

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Progesterone Cyclin B-Cdk1

21

The transition into mitosis is further secured by a ______ involving Mos and ______, ensuring a firm shift to the mitotic phase.

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positive feedback loop MAPK

22

The model posits that the presence of ______ DNA raises the threshold for ______ activation, thus blocking early mitotic entry.

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unreplicated Cdc2 (Cdk1)

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The G1 Checkpoint and Cell Cycle Regulation

The G1 checkpoint, a pivotal control mechanism in the cell cycle, determines a cell's fate regarding division. In this phase, cells integrate various signals to decide whether to pause and enter a resting state (G0), repair DNA damage, or commit to cell division by passing the restriction point. This decision is influenced by the cell's size, nutrient availability, growth factors, and the integrity of the genome. DNA damage, in particular, activates pathways that halt cell cycle progression, ensuring that only cells with intact DNA replicate. The transition from G1 to S phase, where DNA synthesis occurs, is driven by the activation of specific cyclin-dependent kinases (CDKs) that initiate the transcription of genes required for DNA replication.
Modern microscope with prepared slide and pink and purple stained cells on laboratory bench with blurred equipment background.

The Role of E2F Transcription Factors and Retinoblastoma Protein

The G1 checkpoint is intricately regulated by the interplay between E2F transcription factors and the retinoblastoma protein (Rb) family, known as pocket proteins. E2F proteins are key in activating genes necessary for cell cycle progression, including those encoding cyclins and CDKs. Dysregulation of E2F activity is often implicated in tumorigenesis, underscoring its role in cell cycle governance. The Rb protein, along with its relatives p107 and p130, binds to E2Fs, controlling their activity. Hypophosphorylated Rb inhibits E2F-mediated transcription, while phosphorylation of Rb by cyclin-CDK complexes during late G1 releases E2F, allowing the transcription of S phase genes.

Positive Feedback and Cyclin/CDK Complex Activation

Positive feedback loops are crucial for the G1-to-S phase transition, particularly through the phosphorylation of Rb by specific cyclin-CDK complexes. Growth factor signaling leads to the synthesis of cyclin D, which associates with CDK4 or CDK6, resulting in the initial phosphorylation of Rb. This phosphorylation reduces Rb's affinity for E2F, allowing for partial activation of E2F target genes. Subsequently, cyclin E binds to CDK2, further phosphorylating Rb and fully activating E2F. This culminates in the transcription of a suite of genes that propel the cell into the S phase.

Negative Feedback and Cell Cycle Arrest

Negative feedback loops and cell cycle arrest mechanisms are essential for maintaining the fidelity of cell division. The kinase inhibitor p27^Kip1 (CDKN1B) binds to and inhibits cyclin E-CDK2 complexes, providing a brake on cell cycle progression. As cells progress, cyclin A associates with CDK2, leading to the degradation of p27^Kip1 and the full activation of the cyclin A-CDK2 complex. This complex phosphorylates and inactivates E2F, thus halting transcription of S phase genes. In response to DNA damage, the ATM and ATR kinases activate Chk1 and Chk2, which target Cdc25A for degradation, preventing the activation of cyclin E-CDK2 and arresting the cell cycle in G1.

The G2 Checkpoint and Mitotic Entry

The G2 checkpoint serves as a pre-mitotic phase where cells prepare for division by synthesizing mitotic proteins and ensuring DNA integrity. Activation of the cyclin B-Cdk1 complex is a key event for the transition to mitosis. The checkpoint mechanisms involve ATR and ATM kinases, which can activate Chk1 and Chk2 to induce cell cycle arrest if DNA damage or replication errors are detected. The regulation of cyclin B-Cdk1 involves a positive feedback loop, with Plk1 kinase facilitating the activation of Cdc25 phosphatase and the degradation of the Wee1 kinase, promoting the entry into mitosis.

Mitotic Entry and Hysteresis in Cell Cycle Transitions

The transition from G2 to M phase is a robust, all-or-nothing event, exemplified by the maturation of Xenopus oocytes. Progesterone stimulation initiates a signaling cascade that activates the Cyclin B-Cdk1 complex, driving the cell into mitosis. This transition is reinforced by a positive feedback loop involving Mos and MAPK, which ensures a decisive switch to the mitotic state. The concept of hysteresis in cell cycle transitions, as described by the Novak–Tyson model, indicates that the threshold for activating mitotic entry is higher than that for maintaining mitosis. This model also suggests that the presence of unreplicated DNA increases the threshold for Cdc2 (Cdk1) activation, thereby preventing premature entry into mitosis.