Cell Growth and Division

Core Concepts

The Cell Cycle at a Glance

The cycle is divided into interphase and the mitotic phase. Interphase is subdivided into G₁ (first gap), S (synthesis), and G₂ (second gap); the mitotic phase consists of mitosis followed by cytokinesis. A living cell that is not preparing to divide leaves the cycle into a state called G₀.

Interphase: Preparation, Not Resting

Interphase is often called "resting," but it is the busiest part of the cycle. In G₁ the cell grows, produces proteins and organelles, and monitors its size and DNA health. In S phase it replicates its DNA, so each chromosome now consists of two identical copies — sister chromatids — joined at the centromere. In G₂ it makes the proteins and spindle components needed for division and checks that replication was accurate. Cells that will never divide again — many neurons and mature muscle cells, as commonly taught — remain in G₀.

Mitosis: Dividing the Nucleus

Mitosis is continuous but is described in four stages, remembered as PMAT:

• Prophase: Chromatin condenses into visible chromosomes. Centrosomes move to opposite poles and build the mitotic spindle, a network of microtubules. The nuclear envelope breaks down, and spindle fibers attach to kinetochores on each sister chromatid's centromere.
• Metaphase: Chromosomes align along the metaphase plate at the cell's equator, ensuring each daughter cell will receive one copy of every chromosome.
• Anaphase: Sister chromatids are pulled apart — centromeres split, and each chromatid (now a daughter chromosome) is drawn to a pole by shortening spindle fibers. The shortest stage.
• Telophase: Chromosomes arrive at the poles and decondense, new nuclear envelopes form around each set, nucleoli reappear, and the spindle breaks down.

Cytokinesis: Splitting the Cell

Cytokinesis overlaps telophase. In animal cells, a ring of actin and myosin filaments pinches the membrane inward, forming a cleavage furrow that deepens until the cell splits in two. (Plant cells instead build a cell plate.) The result is two daughter cells, each diploid and genetically identical to the parent.

Checkpoints: Quality Control

The cycle does not run on autopilot. Checkpoints are verification gates:

• G₁ checkpoint: Is the cell large enough, are nutrients available, and is the DNA undamaged? If not, the cell may enter G₀.
• G₂ checkpoint: Was DNA replication completed correctly?
• M (spindle) checkpoint: Are all chromosomes attached to spindle fibers before anaphase begins?

Progression is driven by cyclins and cyclin-dependent kinases (CDKs), regulatory proteins that activate in waves. If a checkpoint finds damage that cannot be repaired, the cell can trigger apoptosis — programmed cell death — rather than pass on faulty DNA.

When Control Fails: Cancer

Cancer is what happens when the controls break. Cancer cells typically ignore contact inhibition (the normal stop signal when cells touch neighbors), keep dividing despite DNA damage, and evade apoptosis. Proto-oncogenes normally promote growth; mutated into oncogenes, they push growth too hard. Tumor suppressor genes (such as p53, the commonly taught "guardian of the genome") normally apply the brakes; when inactivated, the brakes are lost. Benign tumors stay confined; malignant tumors invade surrounding tissue and can spread (metastasize).

Common Confusions