AP Biology / Cell Communication and Cell Cycle
Lesson
Dividing without losing the plot
Mitosis partitions copied chromosomes. Checkpoints decide whether to proceed.
Learning goals
- Order the major events of the mitotic cell cycle.
- Explain why a checkpoint failure is dangerous.
Explanation
Before mitosis, DNA is copied in S phase. Mitosis then lines up sister chromatids and separates them so two nuclei get matching sets. Cytokinesis splits the cytoplasm. The point is two cells with the same genetic information, not “growth” as a vague word.
Checkpoints ask whether DNA is intact, whether the cell is large enough, and whether the spindle is attached. If those questions are ignored, damaged DNA can be passed on. Cancer is many diseases; one shared theme is control systems that no longer halt the cycle when they should.
Key terms
- Mitosis. Nuclear division that separates sister chromatids into two identical nuclei.
- Checkpoint. A control step that can halt the cell cycle if conditions are not met.
Common mistakes
- Confusing mitosis with meiosis or with DNA replication.
- Saying cancer is “uncontrolled growth” without mentioning failed regulation.
Practice
Original Marlow Works items. Check the answer explanation after you try.
A cell’s surface-area-to-volume ratio is important because it affects:
Original Marlow Works item — not a College Board question.
Take your time—this is practice, not a test.
Answer explanation
If sister chromatids fail to attach to the spindle, what should a working checkpoint do?
Answer. Delay anaphase until attachments are correct, so both daughters do not receive broken chromosome sets.
The checkpoint exists to prevent an irreversible, uneven split.
Related resources
External links with reuse status. Marlow Works is independent and does not copy restricted exam or textbook material.
Official / link only
AP Biology course page
Official eight-unit framework, science practices, and lab resources.
College Board · All rights reserved · accessed 2026-10-01
Open sourceReference only — do not copy
OpenStax Biology 2e
Biology reference with commercial-use restrictions on the current publisher page.
OpenStax · CC BY-NC-SA 4.0 with additional restrictions · accessed 2026-10-01
Link only unless permission is obtained.
Open source