Module 10.3 · Cell Biology
Cell Cycle Checkpoints: G1/S, G2/M, Spindle, and p53 DNA Damage Response
Complete guide for CSIR-NET, GATE, and university exams — covering checkpoint mechanisms, key proteins, and cancer relevance.
Contents
- What are cell cycle checkpoints?
- G1/S checkpoint and the Rb protein
- G2/M checkpoint — Wee1, CDC25, and MPF
- Spindle assembly checkpoint and kinetochore control
- p53 tumor suppressor and DNA damage response
- Quick-reference summary table
- High-yield exam questions
1. What are cell cycle checkpoints?
Cell cycle checkpoints are built-in surveillance mechanisms that monitor whether each phase of the cell cycle has been completed accurately before allowing progression to the next phase. First described by Leland Hartwell (Nobel Prize, 2001), these molecular quality controls halt the cycle when errors — such as DNA damage, incomplete replication, or misaligned chromosomes — are detected.
There are three canonical checkpoints in eukaryotic cells: the G1/S checkpoint, the G2/M checkpoint, and the Spindle Assembly Checkpoint (M checkpoint). Checkpoint failure is a hallmark of cancer — when surveillance breaks down, cells with damaged or unequal DNA divide unchecked.
2. G1/S checkpoint and the Rb protein
The G1/S checkpoint — also called the restriction point in mammals — is the primary DNA integrity checkpoint. It ensures the cell’s DNA is undamaged and conditions are favourable before committing to DNA replication in S phase.
The restriction point is the molecular point of no return. Once crossed, the cell is mitogen-independent and committed to completing division.
The central enforcer is the Retinoblastoma protein (Rb), known as the “cellular gatekeeper.” Here is how the Rb–E2F switch works:
G1 → S phase: the Rb–E2F switch
Mitogens arrive
↓
Cyclin D – CDK4/6 activated
↓
Rb phosphorylated
↓
E2F transcription factor released
↓
S-phase genes switched ON
↓
Cell enters S phase
In early G1, hypophosphorylated Rb binds E2F transcription factors, blocking expression of S-phase genes (Cyclin E, DNA polymerase, PCNA).
Cyclin D–CDK4/6 initiates Rb phosphorylation; Cyclin E–CDK2 then hyperphosphorylates Rb, fully releasing E2F — this is the molecular definition of crossing the restriction point.
Rb does not directly inhibit CDK2 or p21. It works exclusively through E2F sequestration, recruiting HDACs to silence promoters.
INK4 family inhibitors (p16, p15, p18, p19) block CDK4/6, keeping Rb hypophosphorylated and E2F trapped — causing G1 arrest. p16 (CDKN2A) is among the most commonly mutated tumour suppressors in cancer.
Viral oncoproteins — HPV E7, adenovirus E1A, SV40 Large T antigen — inactivate Rb to force S-phase entry, exploiting cell machinery for viral replication.
3. G2/M checkpoint — Wee1, CDC25, and MPF control
The G2/M checkpoint verifies that DNA replication is complete and error-free before the cell enters mitosis. Its molecular core is the regulation of MPF (Maturation Promoting Factor) — the Cyclin B–CDK1 (Cdc2) complex that drives mitotic entry.
Wee1 kinase — the brake
Phosphorylates CDK1 at Tyr15 and Thr14 (inhibitory sites), keeping MPF inactive during G2
Named for “wee” (small) yeast phenotype when mutated — cells enter mitosis too early
CDC25 phosphatase — the accelerator
Removes inhibitory phosphates from CDK1 Tyr15/Thr14, activating MPF
Triggers explosive, irreversible mitotic entry via positive feedback with active MPF
MPF activation pathway (G2 → M)
Cyclin B accumulates in G2
↓
CAK phosphorylates Thr161 (activating)
↓
CDC25 removes Tyr15/Thr14-P (inhibitory)
↓
Active MPF (Cyclin B–CDK1)
↓
Mitosis begins
Wee1 and CDC25 form an antagonistic bistable switch: active MPF feeds back to inhibit Wee1 while activating CDC25, making mitotic entry explosive and irreversible.
CAK (CDK7–Cyclin H) phosphorylates the T-loop at Thr161 — an activating phosphorylation essential for full CDK1 activity. Both CAK activity AND CDC25 dephosphorylation are required to convert pre-MPF to active MPF.
DNA damage extends G2 by activating ATM/ATR → Chk1/Chk2, which phosphorylate and inhibit CDC25 while upregulating Wee1 — keeping CDK1 inhibited and blocking mitotic entry.
4. Spindle assembly checkpoint and kinetochore control
The Spindle Assembly Checkpoint (SAC) — also called the M checkpoint — operates during metaphase to ensure every pair of sister chromatids is properly attached to spindle microtubules before anaphase proceeds. Even a single unattached kinetochore is sufficient to halt the entire process.
SAC mechanism: the MCC inhibitory cascade
Unattached kinetochore detected
↓
Mad2 (C-form) generated at kinetochore
↓
MCC assembles (Mad2–BubR1–Bub3–CDC20)
↓
APC/C–CDC20 blocked
↓
Securin & Cyclin B remain stable
↓
Metaphase arrest maintained
Mad2 is the core SAC sensor. At unattached kinetochores, it converts from open (O-Mad2) to closed (C-Mad2) conformation, sequestering CDC20 and preventing APC/C-CDC20 activation.
Securin holds separase inactive. When all kinetochores attach → MCC dissolves → APC/C–CDC20 ubiquitinates Securin → Separase released → Cohesin cleaved → Anaphase begins.
APC/C is an E3 ubiquitin ligase — its sole enzymatic activity. It polyubiquitinates Securin and Cyclin B, targeting them for 26S proteasomal degradation, making anaphase irreversible.
CDC20 is the mitotic co-activator of APC/C. Cdh1 replaces CDC20 in G1 to keep cyclin levels low. Mad2-deficient cells are viable normally but die with benomyl — classic conditional lethality.
APC/C substrate recognition: CDC20 recognizes the D-box (RXXL motif); Cdh1 recognizes both D-box and KEN-box. The RING domain subunit APC11 catalyses ubiquitin transfer from E2 to substrate.
5. p53 tumor suppressor and the DNA damage response
p53 — universally known as the “Guardian of the Genome” — is the most frequently mutated gene in human cancer (more than 50% of all cases). It is a sequence-specific transcription factor and master tumour suppressor that acts at both the G1/S and G2/M checkpoints in response to DNA damage, oncogene activation, and hypoxia.
p53 activation cascade after DNA damage
DNA damage occurs
↓
ATM/ATR kinases activated
↓
p53 phosphorylated at Ser15
↓
MDM2 binding blocked
↓
p53 stabilised and active
↓
p21, 14-3-3σ, Bax transcribed
p53 at G1/S checkpoint
Activates p21 → inhibits Cyclin E–CDK2 → Rb stays hypophosphorylated → E2F trapped → no S-phase entry
p53 at G2/M checkpoint
Activates p21 → inhibits Cyclin B–CDC2 (MPF) → 14-3-3σ sequesters CDC25C → mitotic entry blocked
p53 does NOT act at the spindle assembly (M/A) checkpoint — that is controlled entirely by Mad2/BubR1 sensing kinetochore attachment, independent of p53.
MDM2 regulation: In undamaged cells, MDM2 continuously ubiquitinates p53 for proteasomal degradation. ATM-mediated phosphorylation of p53 at Ser15 blocks MDM2 binding, stabilising p53.
Apoptosis switch: If damage is irreparable, p53 switches from arrest to apoptosis — activating Bax, PUMA, Noxa (pro-apoptotic) and repressing Bcl-2 (anti-apoptotic).
p21 vs p27: Both are Cip/Kip-family CKIs. p21 is the primary p53 target induced by DNA damage. p27 suppresses G1 CDKs during quiescence and is degraded by SCF–Skp2 at the G1/S boundary.
Li–Fraumeni syndrome: Germline TP53 mutations cause early-onset cancers across multiple tissue types — direct evidence that p53’s tumour suppressor role is indispensable.
6. Quick-reference summary table
| Protein |
Checkpoint |
Key function |
| Rb (pRb) | G1/S | Sequesters E2F; “cellular gatekeeper.” Hypophosphorylated = active; hyperphosphorylated = inactive |
| E2F | G1/S | Transcription factor for S-phase genes; released when Rb is phosphorylated by Cyclin D–CDK4/6 then Cyclin E–CDK2 |
| p16 (INK4a) | G1/S | Inhibits CDK4/6; keeps Rb hypophosphorylated; frequently deleted in cancer |
| Wee1 | G2/M | Phosphorylates CDK1 Tyr15/Thr14 (inhibitory); keeps MPF off during G2 |
| CDC25C | G2/M | Phosphatase; removes Tyr15/Thr14-P from CDK1; activates MPF for mitotic entry |
| CAK (CDK7) | G2/M | Activating phosphorylation of CDK1 at Thr161 (T-loop); essential for MPF activation |
| Mad2 | M (SAC) | Sequesters CDC20 at unattached kinetochores; core component of MCC; prevents APC/C activation |
| APC/C | M (SAC) | E3 ubiquitin ligase; degrades Securin and Cyclin B; activated by CDC20 (mitosis) or Cdh1 (G1) |
| Securin | M (SAC) | Inhibits separase; degraded by APC/C–CDC20 to allow cohesin cleavage and anaphase onset |
| p53 | G1/S G2/M | “Guardian of the genome”; transcription factor; activates p21, GADD45, 14-3-3σ, Bax; mutated in >50% of cancers |
| p21 (CDKN1A) | G1/S G2/M | Primary p53 effector; broad CKI; inhibits Cyclin E/A–CDK2 and Cyclin B–CDK1 |
| MDM2 | All | E3 ubiquitin ligase; degrades p53 in undamaged cells; blocked by ATM phosphorylation of p53 at Ser15 |
| p27 (Kip1) | G1/S | Cip/Kip CKI; suppresses G1 CDKs during quiescence; degraded by SCF–Skp2 at G1/S boundary |
7. High-yield exam questions
Frequently tested — CSIR-NET / GATE / University exams
Q: Which protein is the “guardian of the genome”?
→ p53
Q: What does Rb inhibit at the G1 checkpoint?
→ E2F transcription factor (not CDK2 or p21 directly)
Q: Which kinase phosphorylates CDK1 at Tyr15 to inhibit MPF?
→ Wee1 kinase
Q: APC/C co-activator at metaphase-to-anaphase transition?
→ CDC20 (Cdh1 operates in G1)
Q: Enzymatic activity of APC/C?
→ E3 ubiquitin ligase (NOT kinase, NOT phosphatase)
Q: What is found in cells that just crossed the restriction point?
→ Highly phosphorylated (hyperphosphorylated) Rb protein
Q: What happens when Mad2-deficient yeast are treated with benomyl?
→ They die — no SAC to halt anaphase; chromosomes segregate catastrophically
Q: Does p53 act at the spindle assembly checkpoint?
→ No. SAC is controlled by Mad2/BubR1 — entirely p53-independent
Remember: p53 does NOT act at the Spindle Assembly (M/A) checkpoint. That is controlled by Mad2, BubR1, and Bub3 — entirely p53-independent.
