1. Core Definition and Phases of the Cell Cycle★★☆☆☆⏱ 4 min
The cell cycle is the ordered, regulated sequence of growth, DNA replication, and division that eukaryotic somatic cells undergo to produce genetically identical daughter cells. It makes up roughly a third of AP Biology Unit 4, which contributes 10-15% of your total AP exam score, appearing regularly in both multiple-choice and free-response sections.
**G1 (Gap 1)**: Cell grows, produces replication machinery, passes the G1 checkpoint; exits to G0 if no division signal is received. G0 can be temporary (liver cells) or permanent (neurons, skeletal muscle).
**S (Synthesis)**: All nuclear DNA is replicated; each chromosome now has two identical sister chromatids connected at a single centromere.
**G2 (Gap 2)**: Cell continues growing, produces mitosis machinery, passes the G2 checkpoint that confirms error-free DNA replication.
**Mitotic (M) Phase**: Includes mitosis (nuclear segregation) and cytokinesis (cytoplasmic division) producing two daughter cells.
Exam tip: If an exam question asks for chromosome number instead of DNA content, remember that the number of centromeres (not chromatids) determines chromosome number, so chromosome number does not change after S phase.
2. Cell Cycle Checkpoints and Regulation★★★☆☆⏱ 5 min
Cell cycle checkpoints are regulatory control points where the cell halts progression until conditions are favorable to continue. Three main checkpoints control progression through the cycle:
**G1 (Restriction) Checkpoint**: End of G1, checks for adequate cell size, nutrients, growth signals, and undamaged DNA before S phase entry.
**G2 Checkpoint**: End of G2, confirms all DNA is fully and accurately replicated before M phase entry.
**Spindle (M) Checkpoint**: End of metaphase, checks that all chromosome kinetochores are correctly attached to spindle microtubules before anaphase.
Regulation of checkpoints relies on two key protein groups: *cyclins* (regulatory proteins whose concentration oscillates throughout the cell cycle) and *cyclin-dependent kinases (CDKs)* (protein kinases that are always present in the cell in an inactive form). CDKs only become active when bound to a specific cyclin, and active cyclin-CDK complexes phosphorylate target proteins to push the cell through the checkpoint.
Exam tip: AP FRQs almost always ask you to interpret a graph of cyclin concentration vs MPF activity – always explicitly state that CDK concentration is constant, only cyclin concentration cycles, so activity depends on cyclin binding.
3. Cell Cycle Dysregulation and Cancer★★★☆☆⏱ 4 min
Cancer is defined as uncontrolled cell division caused by mutations that disrupt cell cycle regulation. For cancer to develop, mutations typically disrupt two classes of cell cycle regulatory genes:
**Proto-oncogenes**: Normal genes that code for proteins that stimulate cell division (e.g., growth factor receptors, cyclins). A gain-of-function mutation converts a proto-oncogene to an oncogene, causing constant, excessive stimulation of cell division even without growth signals. Only one mutated copy is needed for this effect.
**Tumor suppressor genes**: Normal genes that code for proteins that inhibit cell division, repair DNA errors, or trigger apoptosis (programmed cell death) for damaged cells (e.g., p53, Rb, BRCA1). Loss-of-function mutations that inactivate both copies of the gene remove the "brake" on cell division, allowing damaged cells to continue dividing and accumulate more mutations.
Normal cells follow density-dependent inhibition (stop dividing when they form a single layer) and anchorage dependence (must attach to a substrate to divide), while cancer cells ignore both rules.
Exam tip: Always remember the difference between mutation types: gain-of-function for proto-oncogenes/oncogenes (one mutation is enough) vs loss-of-function for tumor suppressors (two hits required). AP exam writers love to test this distinction.
4. AP-Style Practice Problem: Flow Cytometry Calculation★★★★☆⏱ 5 min
Common Pitfalls
Why: Students confuse DNA content (mass) with chromosome number, which is defined by the number of centromeres.
Why: Students mix up the names and roles of the two regulatory complex components.
Why: Students mix up the roles of proto-oncogenes and tumor suppressor genes.
Why: Textbooks often describe G0 as a "resting phase", leading students to assume it is only a temporary pause.
Why: Students misremember the order of M phase events and checkpoint function.