G1/S Checkpoint Event

Q.70 In normal cells, progress through the cell cycle is tightly regulated and each step must be completed before the next step can begin. There are at least three distinct points in the cell cycle at which the cell monitors external signals and internal equilibrium before proceeding to the next stage. These are the G1/S, the G2/M and M checkpoints. In addition to regulating the cell cycle at the checkpoints, the cell controls progress through the cell cycle by means of two classes of proteins: cyclins and cyclin-dependent kinases (CDKs). Based on the above context, answer the following questions: Event that occurs at G1/S checkpoint is: Cell monitors DNA synthesis and damage Cell monitors spindle formation Attachment to kinetochores Cell monitors size and DNA integrity

Q.70 In normal cells, progress through the cell cycle is tightly regulated and each step must be

completed before the next step can begin. There are at least three distinct points in the cell
cycle at which the cell monitors external signals and internal equilibrium before proceeding
to the next stage. These are the G1/S, the G2/M and M checkpoints.

In addition to regulating the cell cycle at the checkpoints, the cell controls progress through
the cell cycle by means of two classes of proteins: cyclins and cyclin-dependent kinases (CDKs).

Based on the above context, answer the following questions:

Event that occurs at G1/S checkpoint is:

  1. Cell monitors DNA synthesis and damage
  2. Cell monitors spindle formation
  3. Attachment to kinetochores
  4. Cell monitors size and DNA integrity

    Cell monitors size and DNA integrity occurs at G1/S checkpoint.

    The G1/S transition represents the restriction point where cells assess growth status, nutrient availability, and genomic integrity before committing to DNA replication, preventing propagation of damaged genomes.

    Option Analysis

    Cell monitors DNA synthesis and damage
    DNA synthesis occurs during S phase, post-G1/S checkpoint. G2/M checkpoint monitors replication completion/damage. G1/S assesses pre-replicative DNA integrity. Incorrect.​

    Cell monitors spindle formation
    Spindle assembly checkpoint operates at metaphase-anaphase M phase transition, ensuring microtubule-kinetochore attachments. G1/S unrelated. Incorrect.​

    Attachment to kinetochores
    Kinetochore-microtubule attachment verification occurs during prometaphase M phase via SAC (Mad2/BubR1). G1 cells lack condensed chromosomes/kinetochores. Incorrect.​

    Cell monitors size and DNA integrity
    Restriction point: Rb hyperphosphorylation (CyclinD-CDK4/6 → CyclinE-CDK2) releases E2F for S-phase gene transcription. ATM/ATR-p53-p21 pathway halts progression if DNA damage detected pre-replication. Cell growth to critical mass also required. Correct.

    Event that occurs at G1/S checkpoint—the primary restriction point—determines eukaryotic cell fate: proliferation, quiescence (G0), or apoptosis based on extracellular mitogens and intracellular homeostasis.

    Checkpoint Sensing Mechanisms

    Size control: Ribosome biogenesis, protein synthesis during G1 increase cell mass. CyclinE-CDK2 threshold requires ~1.8-2.2 pg protein/cell in mammalian fibroblasts. Myc-driven biomass accumulation essential.​

    DNA integrity: ATM/ATR kinases detect double/single-strand breaks. p53 stabilization → p21(CDKN1A) inhibits CDK2, maintaining Rb-E2F repression. Unrepaired lesions trigger permanent G1 arrest or senescence.​

    Molecular Circuitry

    text
    Growth factors → Ras → CyclinD ↑ → CDK4/6 phosphorylates Rb → Partial E2F release
    ↓
    CyclinE ↑ → CDK2 fully phosphorylates Rb → E2F activation → S-phase genes (PCNA, MCM, DNA Pol α)
    ↓ [if DNA damage detected]
    ATM/ATR → Chk1/Chk2 → p53 → p21 → CDK2 inhibition → G1 arrest

    Checkpoint Failure Consequences

    p53/Rb pathway mutations (90% human cancers) abrogate G1/S control, forcing reliance on G2/M checkpoint. HPV E7/E1A, Adenovirus E1B inactivate Rb/p53 respectively, enabling viral replication in cycling cells.​

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