Compare cytokinesis in animal and plant cells, describe how checkpoints regulate the cell cycle, and explain cancer as a failure of that regulation.
Cytokinesis, the physical division of the cytoplasm into two cells, looks different in animals and plants because of one structural fact from Unit 3: plant cells have a rigid cell wall and animal cells do not. In an animal cell, a ring of protein fibers just inside the membrane contracts like a drawstring, pinching the cell into two along a cleavage furrow. A plant cell cannot pinch inward against its own rigid wall, so instead it builds a new dividing wall from the inside out -- vesicles from the Golgi apparatus gather along the cell's center and fuse to form a cell plate, which grows outward until it splits the cell into two, each now needing its own new wall.
The cell cycle does not run on autopilot -- checkpoints at key transition points (notably at the end of G1, the end of G2, and during mitosis itself) act as quality-control inspections, checking that DNA is undamaged and fully copied and that conditions favor division before allowing the cycle to proceed to the next stage. Cancer results when mutations (Unit 8 covers how these arise) disable the genes responsible for these checkpoints or for regulating growth, allowing a cell to divide uncontrollably even when it should stop. The resulting mass of abnormally dividing cells is a tumor; a benign tumor stays contained in one place, while a malignant tumor can invade nearby tissue and spread to distant parts of the body, a process called metastasis. This is also why many cancer treatments, such as radiation and certain chemotherapy drugs, specifically target rapidly dividing cells -- and why they often cause side effects in the body's other fast-dividing, healthy tissues, like hair follicles and the lining of the digestive tract.
Mastery looks like: They can compare cytokinesis in animal and plant cells, explain what a cell cycle checkpoint does, and describe cancer as a checkpoint-regulation failure rather than simply "cells dividing too fast."
Common stumbles: Believing cancer cells divide at an impossibly fast individual speed rather than dividing inappropriately often because checkpoints failed; confusing benign (contained) with malignant (invasive/spreading).