Prophase
Chromatin condenses into visible chromosomes. The mitotic spindle begins to form as the cell prepares to move chromosomes.
01 · Begin with a person · Chapter 05
When a cell divides, what keeps the story in balance?
Sana has just learned that a biopsy confirmed breast cancer. There is a lot to take in. Before the tests and pathways, she deserves time, clear answers and a chance to ask what matters to her.
As she begins to understand the diagnosis, one question keeps returning: how does a cell know when to grow, copy its DNA and divide?
Sana is a fictional composite for learning. Her experience does not represent every person with breast cancer.
01 · Begin with a person→02 · Ask the question→03 · Trace the mechanism→04 · Connect the systems→05 · Return to the person→06 · Check understanding
02 · Ask the physiology question
What must happen before one cell becomes two?
A cell needs to grow, copy its DNA once, check that the copy is ready, and distribute one complete set of chromosomes to each daughter cell. Select a phase to follow the sequence.
G1 · Grow and assess
The cell grows, performs its usual work and responds to signals. At the G1 checkpoint, it can assess conditions and DNA integrity before committing to DNA synthesis.
Relative DNA amount2C · before replication
Keep the order straight: DNA replication occurs in S phase. The chromosomes then separate during mitosis in M phase. Mitosis is nuclear division; cytokinesis divides the cytoplasm. Together they produce two cells. [1–3]
G0 describes a state outside the active cycle. Some cells can re-enter the cycle; others are highly specialised and rarely divide.
03 · Trace the mechanism
Mitosis: separate the copied chromosomes carefully.
Step through a simplified animal cell. The drawing is schematic; it shows the key movements rather than every chromosome.
Stage 1 of 6
Interphase · prepareBefore mitosis, DNA has already been replicated during S phase. Each chromosome now consists of two sister chromatids joined at a centromere.
Metaphase
Chromosomes line up at the cell’s equator. The spindle-assembly checkpoint monitors whether kinetochores are properly attached to spindle microtubules.
Anaphase → telophase → cytokinesis
Sister chromatids separate and move to opposite poles. New nuclei form; the cytoplasm then divides. The result is two daughter cells with the same chromosome number as the parent cell.
Some textbooks include prometaphase as a separate stage; others group its events with prophase. The chromosome movements are the same.
03 · Trace the mechanism
Checkpoints are pauses with a purpose.
Cell-cycle checkpoints monitor whether key conditions are met. They can delay progression while problems are repaired; when damage cannot be repaired, a cell may enter apoptosis or other fates.
What is the safest first response?
Choose a situation and response to see the reasoning.
Three important control points
G1Conditions and DNA damage
SDNA copying monitored
G2Replication complete?
MChromosomes attached?
DivideOnly when ready
The major internal checkpoints are commonly described at G1, G2/M and metaphase (the spindle-assembly checkpoint). S-phase surveillance also helps detect problems during DNA replication. These controls lower risk; they cannot guarantee that every error is caught. [7–9]
Repair is not the only outcome. A cell may pause, repair damage, enter a non-dividing state, undergo senescence or die. The response depends on the type and extent of damage and on the cell.
04 · Connect the systems
Cells use both accelerators and brakes.
Growth signals and cell-cycle regulators work together. Cancer can arise when changes accumulate in pathways controlling growth, DNA repair, checkpoints or cell death. The details vary between cancers and between people.
A simplified G1-to-S pathway
SignalGrowth cues
Cyclin D+ CDK4/6
RbBrake released
E2FGenes for S phase
S phaseDNA synthesis
In a simplified model, cyclin D–CDK4/6 activity can phosphorylate Rb, changing its restraint on E2F transcription factors and helping the cell progress toward S phase. This pathway is one part of a much larger network. [8, 10]
Regulatory brakes working in this simplified model
Growth signals appropriately controlled
Balanced: signals and regulatory proteins coordinate progression, repair pauses and division.
Where p53 and p21 fit
When DNA damage is detected, p53 can activate genes such as CDKN1A, which encodes p21. p21 can inhibit cyclin–CDK activity, helping pause the cycle while the cell responds to damage. Rb also helps restrain progression from G1 toward S phase.
These proteins are often called tumour suppressors because their normal functions help prevent inappropriate cell proliferation. Alterations in their pathways can remove important restraints, but cancer usually involves a combination of biological changes rather than one simple broken switch. [8, 9]
For Sana: “Cell-cycle control” helps explain one part of cancer biology. It does not describe her whole diagnosis, predict her outlook or determine her treatment. Her care team interprets her individual pathology and discusses options with her.
A careful clinical connection: Some CDK4/6 inhibitors are used with other medicines for specific breast-cancer subtypes, including certain hormone-receptor-positive, HER2-negative disease. That is a treatment connection to the pathway—not a treatment recommendation for Sana or for every person with breast cancer. [10]
04 · Connect the systems
Two kinds of division. Different purposes.
Mitosis supports growth, repair and tissue maintenance. Meiosis makes haploid gametes and contributes to genetic variation. They share some vocabulary, but their outcomes differ.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Where / purpose | Somatic cells; growth, replacement and repair | Germ-line cells; production of gametes |
| DNA replication | Once before division, in S phase | Once before meiosis I; no new S phase between meiosis I and II |
| Number of divisions | One nuclear division | Two successive divisions |
| What separates first? | Sister chromatids | Homologous chromosomes in meiosis I; sister chromatids in meiosis II |
| Typical outcome | Two daughter cells with the same chromosome number as the parent | Four haploid products in the standard teaching model; gamete development differs by sex and species |
| Genetic variation | Daughter cells are usually genetically very similar, barring mutation or division error | Crossing over and independent assortment produce new combinations |
Open the meiosis sequence
Meiosis I · reduce
Homologous chromosome pairs come together. Crossing over can exchange DNA between non-sister chromatids. Homologues then separate, reducing the chromosome set from diploid to haploid.
Meiosis II · separate
Sister chromatids separate in a second division. There is no second round of DNA replication between meiosis I and meiosis II.
Remember: mitosis separates sister chromatids in one division; meiosis I separates homologues, then meiosis II separates sister chromatids. [4–6]
05 · Return to the person
Science should make room for a question.
Sana’s question brought us to the cell cycle. Before the conversation ends, what could a clinician or student say or do to help her feel safe enough to ask what she needs?
This reflection stays in this page while it is open; it is not saved or sent.
Carry these ideas forward
Cell division is not a single switch. It is a sequence of growth, DNA replication, monitoring, chromosome separation and cell division. Control systems help coordinate each step.
For a person facing a diagnosis, physiology is one tool for understanding—not a substitute for listening, informed discussion or individual care.
Return to the cell book roadmap ↗06 · Check understanding
Five questions. Take them at your pace.
Choose one answer for each question, then read the explanations—even if you got it right.
Sources and further reading
The chapter synthesises these learning resources. The short case is fictional; it is not based on an individual patient.
- National Human Genome Research Institute · Mitosis
- TeachMePhysiology · Mitosis
- NCBI Bookshelf · In brief: How do most cells divide (mitosis)?
- NCBI Bookshelf · Genetics, Meiosis
- UNSW Embryology · Cell Division: Meiosis
- TeachMePhysiology · Meiosis
- Alberts et al. · Components of the Cell-Cycle Control System, Molecular Biology of the Cell (NCBI Bookshelf)
- LibreTexts · Control of the Cell Cycle
- MedlinePlus Genetics · How genes control growth and division
- National Cancer Institute · CDK4/6 inhibitor and cell-cycle mechanism
- National Cancer Institute · Ribociclib and specific breast-cancer uses
Educational resource only. It does not diagnose, predict an individual outcome or recommend treatment.
The next question begins here
Open the book roadmap ↗Every pathway belongs to a person.
Keep the mechanism. Keep the human being in view.