02 — Checkpoints and Mitosis

Core Explanation

Cell-cycle checkpoints are surveillance mechanisms that arrest progression when critical events are incomplete or when damage is detected. They are not "stop and inspect" pauses in the way a human inspector checks a part; rather, they are biochemical pathways that generate a diffusible STOP signal when a problem is sensed, and that signal must be actively overcome for the cycle to continue. Three checkpoints are essential:

G₁/S Checkpoint — Rb, E2F, p53, and p21

The G₁/S checkpoint is the primary gatekeeper of proliferation in mammalian cells. Its central molecular switch is the retinoblastoma protein (Rb) and the E2F family of transcription factors.

The Rb/E2F Switch

In quiescent (G₀) and early G₁ cells, Rb is hypophosphorylated and active. In this state, Rb binds E2F transcription factors, physically blocking their transactivation domain and recruiting histone deacetylases (HDACs) that compact local chromatin and silence E2F target genes. The genes that encode cyclin E, cyclin A, Cdk2, and DNA replication factors are therefore kept off.

When growth factors activate receptor tyrosine kinase → Ras → MAPK signaling cascades, cyclin D is transcribed. Cyclin D–Cdk4/6 complexes phosphorylate Rb at multiple sites, causing it to partially release E2F. E2F then transcribes its own targets — including cyclin E. Cyclin E–Cdk2 further phosphorylates Rb in a positive-feedback loop, leading to full hyperphosphorylation and complete release of E2F. Once this loop engages, the G₁→S transition becomes independent of extracellular growth signals — this is the molecular definition of the restriction point.

The p53–p21 DNA Damage Brake

When DNA double-strand breaks or other lesions are detected during G₁, the kinases ATM and ATR phosphorylate and stabilize the transcription factor p53 (encoded by the TP53 gene). p53 has several context-dependent roles:

• Cell-cycle arrest: p53 transcribes CDKN1A, which encodes p21^Cip1^. p21 binds and inhibits cyclin E–Cdk2 and cyclin A–Cdk2, preventing Rb hyperphosphorylation and freezing the cell in G₁.
• Senescence: If damage is irreparable and chronic, p53 can drive a permanent cell-cycle exit program.
• Apoptosis: If damage is catastrophic, p53 transcribes pro-apoptotic Bcl-2 family members (Bax, Puma, Noxa) to eliminate the cell.

This is why TP53 is the most frequently mutated gene in human cancer — loss of p53 disables the G₁/S DNA-damage checkpoint, allowing genomically compromised cells to replicate.

G₂/M Checkpoint

After S phase, the cell must confirm that DNA replication is complete and that no damage has occurred during or after replication. The G₂/M checkpoint works through ATR and Chk1 kinases (replication stress) and ATM/Chk2 (DNA damage). Active Chk1/Chk2 phosphorylate and inactivate Cdc25 phosphatase, preventing it from removing the inhibitory phosphates on cyclin B–Cdk1. Simultaneously, Chk1/Chk2 phosphorylate and activate Wee1, reinforcing the inhibitory phosphorylation. The result is that cyclin B–Cdk1 complexes accumulate but remain in their inactive, phosphorylated state — the cell is arrested in G₂ and cannot enter mitosis.

Spindle Assembly Checkpoint (SAC)

The SAC is the last checkpoint before anaphase and operates on a fundamentally different principle from the DNA-damage checkpoints. It does not monitor DNA integrity; it monitors kinetochore–microtubule attachment and tension.

Each chromosome has two kinetochores (one per sister chromatid), and each must be attached to microtubules emanating from opposite spindle poles (amphitelic attachment or bi-orientation). Unattached kinetochores, or those lacking tension (monotelic or syntelic attachment), recruit a multi-protein complex — Mad2, BubR1, Bub3, and Cdc20 — that assembles into the mitotic checkpoint complex (MCC). The MCC binds and inhibits Cdc20, the activator of the APC/C.

This means: as long as even one kinetochore is unattached or lacks tension, Cdc20 is sequestered, the APC/C cannot ubiquitinate securin or cyclin B, and anaphase is blocked. Once all kinetochores achieve bi-orientation and come under tension, the SAC is silenced, Cdc20 is released, and APC/C^Cdc20^ triggers anaphase within minutes.

Mitosis — Structural Machinery

The Mitotic Spindle

The spindle is a bipolar array of microtubules organized by two centrosomes (spindle poles). Three microtubule populations serve distinct functions:

• Kinetochore microtubules: Attach to kinetochores at the centromere of each sister chromatid. Their plus-ends are embedded in the kinetochore; depolymerization at these ends generates the pulling force that separates chromatids in anaphase.
• Interpolar microtubules: Extend from opposite poles and overlap at the spindle midzone. Kinesin-5 (Eg5) motors crosslink and slide them apart, generating the pushing force that separates spindle poles.
• Astral microtubules: Radiate from centrosomes toward the cell cortex. Dynein motors anchored at the cortex pull on astral microtubules, positioning the spindle within the cell and determining the cleavage plane.

Cohesion, Securin, and Separase — Why Anaphase Is Irreversible

Sister chromatids are held together from the moment of DNA replication by cohesin, a ring-shaped protein complex that topologically encircles both DNA duplexes. At the metaphase→anaphase transition, APC/C^Cdc20^ ubiquitinates securin, marking it for proteasomal degradation. Securin is a tightly-binding inhibitor of separase, a cysteine protease. Once securin is destroyed:

1. Separase is liberated and immediately cleaves the Scc1 (Rad21) subunit of cohesin rings at centromeres.
2. Cohesin rings open, sister chromatids are released, and kinetochore microtubules pull them to opposite poles (anaphase A).
3. Cohesin cleavage is irreversible — there is no known ligase that can re-circularize the opened cohesin ring in vivo. Combined with the fact that Scc1 is a proteolytic fragment rapidly degraded, sister-chromatid separation cannot be undone.

This irreversibility is why the SAC exists — the cell invests enormous regulatory energy in making absolutely certain that every chromosome is properly attached before separase is unleashed. A single prematurely separated chromatid cannot be repaired; the resulting aneuploidy in daughter cells is permanent.

Questions

Q1: Why does loss of Rb function lead to uncontrolled proliferation even in the absence of growth factors?

A1: Rb is the downstream convergence point of growth-factor signaling cascades. When Rb is functional, it sequesters E2F transcription factors; growth-factor-dependent cyclin D–Cdk4/6 activity is required to phosphorylate Rb and release E2F. Loss of Rb removes this dependency — E2F is constitutively free to transcribe cyclin E, cyclin A, and DNA replication genes, driving the cell past the restriction point regardless of external signals. This is why RB1* is a tumor suppressor gene and why DNA tumor viruses (HPV E7, adenovirus E1A, SV40 large T antigen) all target Rb for inactivation.*

Q2: Explain the apparent paradox: the SAC arrests cells when microtubules are depolymerized by drugs like nocodazole, yet normal anaphase itself depends on microtubule depolymerization at kinetochores.

A2: The SAC detects the absence of attachment and tension at kinetochores, not microtubule depolymerization per se. When nocodazole completely depolymerizes all microtubules, every kinetochore is unattached, the SAC is maximally engaged, and the cell arrests. During normal anaphase, kinetochore microtubules are attached, the SAC is satisfied and silenced, and tubulin depolymerization at the kinetochore plus-ends provides the motive force for chromatid movement. The key distinction is that depolymerization in anaphase occurs at kinetochores that are (a) attached and (b) under tension from both poles — the SAC is already off by that point. The checkpoint responds to the kinetochore state, not to microtubule dynamics.

Q3: How does the G₂/M checkpoint communicate with the mitotic entry machinery?

A3: The G₂/M checkpoint converges on the activation state of cyclin B–Cdk1. The kinases ATM/ATR (sensors) activate Chk1/Chk2 (transducers), which then phosphorylate Cdc25 on Ser216. This phosphorylation creates a binding site for 14-3-3 proteins, which sequester Cdc25 in the cytoplasm, preventing it from dephosphorylating and activating cyclin B–Cdk1 in the nucleus. Simultaneously, Chk1/Chk2 activate Wee1, increasing the pool of inhibitory phosphates on Cdk1. The net effect is that cyclin B–Cdk1 complexes accumulate in G₂ but remain in a doubly phosphorylated, catalytically inactive state. Only when the damage signal resolves do phosphatases remove the inhibitory marks and Cdc25 returns to the nucleus to trigger mitotic entry.