Cell Cycle, Growth,
and Repair
Explore how cells divide, grow, and repair through the tightly regulated stages of the cell cycle: from interphase through mitosis and cytokinesis.
Anatomy & Physiology
What Is the Cell Cycle?
The cell cycle is the ordered series of events a cell undergoes from its formation until it divides into two daughter cells. It is the fundamental process that enables growth, tissue repair, and maintenance of every organ system in the body.
Key Vocabulary
Interphase
The longest phase: the cell grows, copies its DNA, and prepares to divide.
Mitosis
Nuclear division that distributes duplicated chromosomes equally into two nuclei.
Cytokinesis
Division of the cytoplasm, producing two genetically identical daughter cells.
The Cell Cycle Overview
Interphase: G1, S, and G2
1
G1 (Gap 1): The cell grows in size, synthesizes proteins and organelles, and performs its normal functions. This is the longest sub-phase of interphase.
2
S (Synthesis): The cell replicates its entire DNA so each chromosome now consists of two identical sister chromatids joined at the centromere.
3
G2 (Gap 2): Final preparations occur: additional proteins are made, centrioles are duplicated, and the cell checks for DNA errors before entering mitosis.
The Four Stages of Mitosis
Prophase
Chromatin condenses into visible chromosomes. The nuclear envelope begins to break down and the mitotic spindle forms.
Metaphase
Chromosomes align at the cell's equator (metaphase plate). Spindle fibers attach to each chromatid's centromere.
Anaphase
Sister chromatids are pulled apart toward opposite poles of the cell by shortening spindle fibers.
Telophase
Chromosomes de-condense, nuclear envelopes re-form around each set, and the spindle disassembles.
Visualizing Mitosis
Mitosis vs Meiosis
| Mitosis | Meiosis | |
|---|---|---|
| Purpose | Growth & repair | Gamete production |
| Divisions | 1 | 2 |
| Daughter cells | 2 identical | 4 unique |
| Chromosome number | Diploid \(\left(2n\right)\) | Haploid \(\left(n\right)\) |
| Crossing over? | No | Yes |
| Cell type | Somatic cells | Sex cells only |
Regulating the Cycle: Checkpoints
Signals That Drive Division
Growth Factors
External proteins (e.g., EGF, PDGF) bind receptors and trigger signaling cascades that push the cell past the G1 checkpoint.
Contact Inhibition
Normal cells stop dividing when they touch neighboring cells, preventing overcrowding. Cancer cells lose this control.
When Regulation Fails: Cancer
Cancer arises when mutations disable checkpoints or overactivate growth signals. Two gene families are central:
Proto-oncogenes
Normal genes promoting cell growth. When mutated into oncogenes, they act like a stuck accelerator, driving uncontrolled division.
Tumor Suppressors
Genes like p53 and Rb act as brakes. Loss-of-function mutations remove these brakes, allowing damaged cells to proliferate.
Growth and Tissue Repair
1
Labile cells divide continuously throughout life (e.g., skin epithelium, blood cells, gut lining). They replace themselves every few days.
2
Stable cells are quiescent in G0 but can re-enter the cycle when stimulated (e.g., hepatocytes after liver damage, fibroblasts during wound healing).
3
Permanent cells rarely or never divide after maturation (e.g., neurons, cardiac muscle cells). Damage here is largely irreversible.
Example: Wound Healing
How to Analyze the Cell Cycle
1
Identify the phase: Is the cell growing (interphase), dividing its nucleus (mitosis), or splitting its cytoplasm (cytokinesis)?
2
Check the DNA amount: In G1 the cell has \(2n\) DNA. After S phase it has \(4n\). After mitosis each daughter returns to \(2n\).
3
Look at chromosome behavior: Are chromosomes condensing, aligning, separating, or de-condensing? This tells you the mitotic sub-stage.
4
Consider regulation: Is a checkpoint active? Is a growth factor present? Could a mutation in p53 or Rb explain abnormal division?
Example 1: DNA Content Graph
Example 2: Counting Daughter Cells
A single skin cell undergoes 5 rounds of mitosis. How many cells result?
Common Mistakes
✕ Confusing Mitosis with the Whole Cycle
Mitosis is only nuclear division. The cell cycle also includes interphase (G1, S, G2) and cytokinesis.
✕ Mixing Mitosis and Meiosis
Mitosis produces 2 diploid cells for growth/repair. Meiosis produces 4 haploid gametes for reproduction.
✕ Ignoring G0 Phase
Many cells exit the cycle into G0 (quiescence). Neurons and muscle cells live in G0 indefinitely and do not divide.
Summary Table
| Phase | Key Event | DNA Content |
|---|---|---|
| G1 | Cell growth, organelle production | \(2n\) |
| S | DNA replication | \(2n \to 4n\) |
| G2 | Final growth, error checking | \(4n\) |
| Prophase | Chromosomes condense, spindle forms | \(4n\) |
| Metaphase | Chromosomes align at equator | \(4n\) |
| Anaphase | Sister chromatids move to opposite poles | \(4n\) |
| Telophase | Nuclear envelopes re-form | \(4n\) |
| Cytokinesis | Cytoplasm divides | \(2n\) each cell |
Question 1 of 5
During which phase of the cell cycle does DNA replication occur?
The S (Synthesis) phase is when the cell copies all of its DNA so that each chromosome consists of two sister chromatids joined at the centromere.
Question 2 of 5
A cell with 46 chromosomes undergoes mitosis. How many chromosomes will each daughter cell have?
Mitosis produces two genetically identical daughter cells. Each retains the full diploid chromosome number: 46. Only meiosis halves the count to 23.
Question 3 of 5
Which stage of mitosis is characterized by chromosomes lining up along the metaphase plate?
During metaphase, chromosomes align at the cell's equator (metaphase plate). Remember: Metaphase = Middle. Spindle fibers from opposite poles attach to each chromatid's centromere.
Question 4 of 5
A mutation inactivates the p53 tumor suppressor gene. What is the most likely consequence?
p53 normally detects DNA damage and halts the cell cycle for repair or triggers apoptosis. Without functional p53, damaged cells bypass checkpoints and keep dividing, which can lead to cancer.
Question 5 of 5
A single cell undergoes 4 rounds of mitosis. How many cells are present at the end?
Each round of mitosis doubles the number of cells. After \(n\) rounds: \(2^n\) cells. So after 4 rounds: \(2^4 = 16\) cells.