Cell-Cycle Control & Cancer

You’ll be able to

State what the G1, G2, and M checkpoints each verify and how cyclin-Cdk complexes drive transitions
Explain the Rb/E2F switch and how p53 enforces the G1 checkpoint
Distinguish proto-oncogenes from tumor suppressor genes and relate their mutations, plus loss of density-dependent inhibition, to cancer

Three checkpoints, three different questions

Each checkpoint interrogates a different condition, and confusing them loses points. The G1 checkpoint (the restriction point) is the master decision: is the cell large enough, are nutrients and growth signals present, and — critically — is the DNA undamaged? Pass it and the cell is committed to divide; fail it and the cell may exit into the resting G0 state. The G2 checkpoint asks a narrower question: was DNA replication completed accurately and without damage during S phase? It guards the entry into mitosis. The M (spindle-assembly) checkpoint, during metaphase, verifies that every chromosome is correctly attached to spindle fibers at the metaphase plate before anaphase separates the sister chromatids — this prevents daughter cells from receiving the wrong chromosome number.

The engine and the Rb/E2F switch

Transitions are driven by cyclin-Cdk complexes: a cyclin-dependent kinase (Cdk) is a constant-level enzyme that is inactive until it binds a cyclin, a regulatory protein whose concentration rises and falls through the cycle. When a specific cyclin accumulates, it activates its Cdk, and the complex phosphorylates targets that push the cell past a checkpoint; then the cyclin is degraded and the complex shuts off. A key target at G1 is the retinoblastoma protein (Rb). Unphosphorylated Rb acts as a brake by binding and holding the transcription factor E2F captive. When a growth signal drives cyclin D-Cdk4/6 to phosphorylate Rb, Rb changes shape and releases E2F, which then switches on the genes for S-phase entry and DNA replication. Rb is therefore a molecular switch: phosphorylated Rb = brake off = go.

The G1 → S decision, in one line

growth signal → cyclin D-Cdk4/6 → Rb phosphorylated → E2F released → S-phase genes ON

Rb *restrains* the cycle when unphosphorylated; phosphorylating it *removes* the brake. Losing Rb removes the brake permanently.

Accelerators, brakes, and the loss of social control

Cancer is driven by two gene classes with opposite logic. Proto-oncogenes encode "go" proteins (growth-factor receptors, Ras, cyclins); a gain-of-function mutation converts one into an oncogene stuck on — like an accelerator jammed down — and a single mutated copy is enough to matter (dominant at the cell level). Tumor suppressor genes encode "stop" proteins; they act as brakes, so it usually takes loss of both copies to remove the brake — one working copy still makes enough protein (the "two-hit" rule; recessive at the cell level). The premier tumor suppressor, p53, the "guardian of the genome," enforces the G1 checkpoint: on sensing DNA damage it halts the cycle for repair or, if damage is irreparable, triggers apoptosis. Normal cells also obey density-dependent inhibition — they stop dividing when they crowd a surface and touch neighbors — and anchorage dependence. Cancer cells lose both: they ignore crowding, pile into a tumor, and can break away to metastasize. Because several such controls must fail together, cancer is a multi-step accumulation of mutations, which is why incidence rises steeply with age.

Worked example

A cell line is engineered so that its Rb protein can no longer be bound by E2F (as if Rb were permanently "off"), while everything else is normal. Predict how this cell will behave at the G1/S boundary, and identify what category of cancer-related defect this mimics.

1.Normally, unphosphorylated Rb holds E2F captive, keeping S-phase genes off until a growth signal drives cyclin D-Cdk4/6 to phosphorylate Rb and release E2F.
2.If Rb can no longer bind E2F, then E2F is free by default — regardless of whether any growth signal or Rb phosphorylation has occurred.
3.Free E2F constitutively switches on the S-phase genes, so the cell crosses the G1 restriction point and enters S phase without waiting for the normal "go" signal.
4.Rb is a tumor suppressor (a brake); losing its function removes G1 restraint, so this mimics a loss-of-function tumor-suppressor mutation, driving inappropriate, signal-independent cell-cycle entry.

Answer: The cell will enter S phase inappropriately — E2F stays active without any growth signal, so the G1 restriction point is bypassed. This reproduces a loss-of-function tumor-suppressor defect (Rb inactivation): the brake is gone, so cells divide without the normal external go-ahead, a hallmark step toward cancer.

Checkpoint

A growth signal leads to phosphorylation of the retinoblastoma (Rb) protein. What is the immediate downstream consequence?

Watch out

Match each checkpoint to its own question. G1: is the cell ready and the DNA undamaged (commit to divide)? G2: was replication finished and error-free? M: is every chromosome attached to the spindle? Writing "checks the DNA" for all three earns nothing — the graders want the specific condition.

Checkpoint

Normal cells grown in a dish divide until they form a single layer and then stop, whereas cancer cells keep dividing and pile up on top of one another. The normal cells’ behavior is called:

Checkpoint

A single activating mutation in one copy of a proto-oncogene can promote cancer, yet a tumor suppressor gene usually must lose BOTH copies before its loss contributes to cancer. Why the difference?

On the exam

Cancer answers should name which control failed and how: an oncogene stuck on (gain of function, one copy) or a tumor suppressor lost (loss of function, two hits — e.g. p53 at G1 or Rb), plus loss of density-dependent inhibition. Stress that cancer needs several mutations accumulating together, not one.

Answer the 3 checkpoints as you read.