Cell Cycle Control

7 min read Updated Mar 26, 2026

Imagine an airport with three security checkpoints. Before you board, you must pass through all three. At each one, guards check your passport, your ticket, and your bags. If anything is wrong, you are stopped until the problem is resolved. If the problem cannot be fixed, you are removed from the airport entirely.

The cell cycle works the same way. There are three major checkpoints where the cell pauses to verify that everything is in order before proceeding to the next phase. If conditions are not right, the cell stops. If the damage is unrepairable, the cell destroys itself (apoptosis). Cancer is what happens when someone sneaks past every checkpoint.

Circular diagram of the animal cell cycle. Interphase covers Gap 1 (cell grows), S phase (cell duplicates its DNA and its centrosome), and Gap 2 (cell grows further). The mitotic phase runs from prophase and prometaphase through metaphase, anaphase, and telophase with cytokinesis, ending in two daughter cells, and an arrow branches off to the resting phase G0. Inset panels label the chromosome with its sister chromatids, centromere and kinetochore, the centrosome with its centrioles, and the nucleus with its membrane, chromatin and nucleolus — The cell cycle end to end: G1, S and G2 make up interphase, prophase through telophase and cytokinesis divide the cell, and cells that stop dividing exit to G0. The diagram labels the phases, not the control points. The three checkpoints sit at boundaries you can see here: late G1 (is the cell ready to replicate DNA?), the G2 to M transition (was replication error-free?), and within metaphase (are all chromosomes attached to the spindle?). Cyclin-CDK complexes drive progression across each of those boundaries. *Credit: Wikimedia Commons, CC0 Public Domain*

The Three Major Checkpoints

G1 Checkpoint (Restriction Point) - The most important decision point. The cell asks:

Is the cell large enough to divide?
Are there sufficient nutrients and energy?
Is the DNA undamaged?
Are growth factors present?

If conditions are favorable, the cell commits to division and enters S phase. If not, it enters G0 (quiescence). Once past the G1 checkpoint, the cell is committed - it will complete the cycle without needing additional external signals.

G2 Checkpoint - Before entering mitosis, the cell verifies:

Was DNA replication completed successfully?
Is there any remaining DNA damage?
Are mitosis-promoting factors (MPF) present?

M Checkpoint (Spindle Assembly Checkpoint) - During metaphase, the cell checks:

Are all chromosomes attached to spindle fibers from both poles?
Is there proper tension on each kinetochore?

If even one chromosome is unattached, the checkpoint halts division. Failure at this checkpoint leads to nondisjunction - unequal chromosome distribution that causes conditions like Down syndrome (trisomy 21).

Cyclins and CDKs - The Molecular Engine

The cell cycle is driven forward by a partnership between two types of proteins:

Cyclins - regulatory proteins whose concentrations rise and fall in a predictable pattern as the cell progresses through the cycle (hence the name “cyclin”)
CDKs (cyclin-dependent kinases) - enzyme proteins that are always present in the cell but are inactive on their own

Here is the key: CDKs only become active when a cyclin binds to them. The cyclin-CDK complex then phosphorylates (adds phosphate groups to) target proteins that push the cell into the next phase.

Think of it this way: the CDK is the car engine (always there, always ready) and the cyclin is the ignition key (comes and goes). No key, no ignition. No cyclin, no cell cycle progression.

Phase Transition	Cyclin	CDK Partner	Function
G1 to S	Cyclin D	CDK $\frac{4}{6}$	Passes the restriction point
S phase entry	Cyclin E	CDK2	Initiates DNA replication
S to G2	Cyclin A	CDK2	Completes DNA replication
G2 to M	Cyclin B	CDK1	Triggers mitosis (MPF)

The Cyclin B-CDK1 complex is also known as MPF (Maturation Promoting Factor) or M-phase promoting factor. It triggers the cascade of events that initiates mitosis: chromosome condensation, nuclear envelope breakdown, and spindle formation.

Growth Factors and External Signals

Cells do not just decide to divide on their own. They need permission from the body in the form of growth factors - signaling molecules that bind to cell surface receptors and activate signal transduction pathways leading to the production of cyclins.

Without growth factors, most normal cells remain in G0. This is why:

Wound healing is localized (growth factors are released at the injury site)
Cells in a dish stop dividing when they run out of growth factor in the media

Two additional controls on cell growth:

Density-dependent inhibition (contact inhibition) - cells stop dividing when they become too crowded and physically touch neighboring cells
Anchorage dependence - normal cells must be attached to a surface (extracellular matrix) to divide

Cancer cells ignore both of these signals.

Tumor Suppressors - The Brakes

Tumor suppressor genes encode proteins that slow down or stop cell division. They are the brakes on the cell cycle.

p53 - “The Guardian of the Genome”:

Monitors DNA integrity
If damage is detected, p53 halts the cell cycle at the G1 checkpoint to allow time for repair
If repair fails, p53 triggers apoptosis (programmed cell death)
p53 is the most commonly mutated gene in human cancers - when p53 is lost, damaged cells keep dividing instead of self-destructing

Rb (Retinoblastoma protein):

Controls the G1 checkpoint by binding to and inhibiting the transcription factor E2F
When Rb is phosphorylated by Cyclin D-CDK $\frac{4}{6}$ , it releases E2F, allowing the cell to enter S phase
Loss of Rb removes the G1 brake, leading to uncontrolled entry into S phase

Proto-Oncogenes and Oncogenes - The Gas Pedal

Proto-oncogenes are normal genes that encode proteins promoting cell division (growth factors, receptors, signal transduction proteins, transcription factors). They are the gas pedal.

When a proto-oncogene is mutated into an oncogene, it becomes a stuck gas pedal - always on, driving cell division even without the proper signals. Unlike tumor suppressors, only one copy of a proto-oncogene needs to be mutated to cause problems (it is a gain-of-function mutation).

Feature	Tumor Suppressors	Oncogenes
Normal function	Inhibit cell division (brakes)	Promote cell division (gas pedal)
Mutation type	Loss-of-function	Gain-of-function
Copies needed to cause cancer	Both (two-hit)	One (dominant)
Examples	p53, Rb	Ras

How Cancer Develops

Cancer is not a single mutation - it is an accumulation of mutations in multiple regulatory genes. A typical cancer cell might have:

Activated oncogenes (stuck gas pedal)
Inactivated tumor suppressors (broken brakes)
Loss of contact inhibition
Loss of anchorage dependence
Telomerase reactivation (immortality - can divide indefinitely)
Ability to stimulate angiogenesis (growing new blood vessels to feed the tumor)

What happens when cyclin B binds to CDK1?

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It forms the MPF (Maturation/M-phase Promoting Factor) complex, which triggers entry into mitosis. MPF phosphorylates proteins that cause chromosome condensation, nuclear envelope breakdown, and spindle assembly. Without this complex, the cell cannot enter M phase.

Why does a mutation in p53 increase cancer risk?

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p53 is the "guardian of the genome." It monitors DNA for damage, halts the cell cycle at G1 to allow repairs, and triggers apoptosis if damage is irreparable. Without functional p53, cells with damaged DNA continue to divide and accumulate further mutations, leading to cancer. It is the most commonly mutated gene in human cancers.