What the exam asks
- Section I (Multiple Choice): Expect question sets built around tables of root-tip cell counts, diagrams of chromosomes and spindle fibers, or graphs of relative DNA content per cell across time. You may be asked to calculate the percentage of cells in a specific phase or predict the effect of a mitotic inhibitor.
- Section II (Free Response): Frequently appears in Q1/Q2 (Experimental Results) where a chemical treatment alters the mitotic index, or Q4 (Conceptual Analysis) and Q5 (Analyze Model) where you must predict and justify what happens when a step of mitosis or cytokinesis is disrupted.
Core ideas
Interphase and the phase
The cell cycle consists of interphase (, , ) followed by the M phase (mitosis and cytokinesis).
| Phase | Key cellular events | DNA content (diploid cell) | Chromosome structure |
|---|---|---|---|
| (Gap 1) | Cell grows, synthesizes proteins and organelles, and performs normal metabolic roles. | Unduplicated (1 chromatid per chromosome) | |
| (Synthesis) | DNA replication occurs; each chromosome is copied to form two identical sister chromatids joined at the centromere. |
Nondividing cells may exit the active cycle (typically from ) to enter , a metabolically active resting state. Fully differentiated cells like mature human neurons remain in permanently, whereas cells like liver cells (hepatocytes) or lymphocytes can in response to external growth factors or tissue injury.
Mitosis and genetic continuity
Mitosis is a continuous nuclear division process divided into four sequential phases that ensure each of the two daughter cells receives a complete, identical genome:
- Prophase: Chromatin condenses into visible chromosomes (each consisting of two sister chromatids). The nucleolus disappears, the nuclear envelope fragments, and the mitotic spindle begins to form as centrosomes migrate to opposite poles. Spindle microtubules attach to kinetochore proteins at the centromeres.
- Metaphase: Kinetochore microtubules align the chromosomes in a single file along the metaphase plate (cell equator). This alignment ensures symmetrical distribution.
- Anaphase: Sister chromatids separate at their centromeres as motor proteins walk along kinetochore microtubules, which shorten to pull the newly independent daughter chromosomes to opposite poles. Non-kinetochore microtubules lengthen to elongate the cell.
- Telophase: Two genetically identical nuclei reform at opposite poles as nuclear envelopes reassemble, nucleoli reappear, and chromosomes decondense back into chromatin.