Cell Cycle Checkpoints, Cyclins and CDKs

Cell Cycle Checkpoints: Cyclins, CDKs, p53 and Rb

By StudyStore · Published 2026-11-05 · 1 min read · 328 words

The cell cycle moves a cell through G1, S, G2 and M. Checkpoints stop it when something is wrong. Cyclin-dependent kinases (CDKs) drive each transition, and p53 and Rb are the brakes whose loss gives cancer.

Phases and checkpoints

Phase	What happens	Checkpoint
G1	Growth, normal function	G1/S: size, nutrients, DNA damage
S	DNA replication	Replication completeness
G2	Growth, prepare for mitosis	G2/M: DNA intact and fully copied
M	Mitosis and cytokinesis	Spindle checkpoint: all chromosomes attached
G0	Resting or nondividing	Exit from the cycle

The G1/S checkpoint is the main commitment point. Many differentiated cells, such as neurons, sit in G0.

Cyclins and CDKs

A CDK is always present but inactive. It needs to bind its cyclin, whose concentration rises and falls through the cycle, to become an active kinase that phosphorylates target proteins. Different cyclin-CDK pairs act at different stages, and MPF (cyclin B with CDK1) triggers entry into mitosis.

Tumor suppressors and oncogenes

Type	Role	Effect of mutation
Tumor suppressor (p53, Rb)	Brakes	Loss of function causes cancer, both alleles must be lost
Proto-oncogene (Ras, cyclins)	Accelerators	Gain of function in a single allele gives an oncogene

Rb holds the transcription factor E2F in check, so the cell cannot enter S phase. Phosphorylation of Rb by cyclin-CDK releases E2F. DNA damage raises p53, which causes arrest through p21 to allow repair, and triggers apoptosis if the damage cannot be fixed.

Common traps

Saying cyclin levels are constant. They oscillate, CDK levels do not.
Mixing up the mutation type. Tumor suppressors need loss of function, oncogenes need gain.
Placing the spindle checkpoint in G2. It is within M, before anaphase.
Calling G0 a phase of the cycle. It is an exit from it.

One-minute recap

G1, S, G2, M with checkpoints. Cyclin binds CDK to activate it. Rb blocks E2F until phosphorylated. p53 arrests the cycle or triggers apoptosis. Tumor suppressors: loss of function. Oncogenes: gain of function.

Check yourself: 5-question mini-quiz

Try each question first, then click it to reveal the answer.

1. What activates a CDK?

Binding to its cyclin, whose levels rise and fall through the cycle.

2. What does p53 do after DNA damage?

It halts the cycle for repair, and triggers apoptosis if repair fails.

3. What does Rb do in its unphosphorylated form?

It binds E2F and blocks entry into S phase.

4. Which checkpoint ensures all chromosomes are attached to spindle fibers?

The spindle checkpoint in M phase, before anaphase.

5. Which type of mutation activates a proto-oncogene?

A gain of function, and one mutated allele is enough.

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Written by StudyStore from standard scientific facts. Study aid only. Always check values against the reference your exam provides.