Step by Step
Int
Interphase — 90% of the cell cycle
Interphase has three sub-phases: G1 (cell grows and makes proteins, preparing for DNA replication), S phase (DNA is actually synthesized/replicated, duplicating the chromosomes), and G2 (final growth and organelle duplication, preparing for division). Together, interphase makes up about 90% of the entire cell cycle.
PM
Prophase and metaphase
In prophase, chromatin condenses into visible chromosomes and the mitotic spindle begins forming. In metaphase, the chromosomes align precisely at the cell's middle, along the metaphase plate.
AT
Anaphase and telophase
In anaphase, sister chromatids are pulled apart toward opposite poles of the cell. In telophase, nuclear envelopes reform around each set of chromosomes, which then decondense back into chromatin. Cytokinesis follows, physically dividing the cytoplasm into two genetically identical daughter cells.
Chk
Checkpoints — quality control, and what happens when they fail
G1, G2, and M checkpoints all exist to verify that DNA is undamaged before the cell is allowed to proceed to the next phase. Cancer results specifically when these checkpoints fail, allowing cells with damaged DNA to continue dividing uncontrollably.
A cell with DNA damage that successfully passes through the G1 checkpoint despite that damage may go on to replicate the damaged DNA during S phase and eventually divide into two daughter cells that both carry the same defect — illustrating exactly why checkpoint failure is so closely tied to cancer development.
Applied Walkthrough
1
A researcher studying a tumor sample finds that the cells have lost normal G1 checkpoint function.
2
Ask: what does this checkpoint failure actually allow to happen? Normally, the G1 checkpoint would detect DNA damage and halt the cell cycle before that damaged DNA gets replicated and passed on. Without functioning checkpoint control, cells with damaged DNA are allowed to proceed through S phase, replicate that damaged DNA, and continue dividing.
3
This is exactly the mechanism underlying many cancers — checkpoint genes (like p53 and Rb) become mutated or nonfunctional, removing the cell's normal ability to pause and either repair damage or trigger self-destruction, allowing uncontrolled division to continue instead.
4
Understanding checkpoints this way reframes them from an abstract cell-cycle detail into the actual mechanistic explanation for why cancer specifically involves loss of normal cell cycle control.
Exam Application
Exams test the correct order and content of each phase (G1, S, G2 within interphase; prophase, metaphase, anaphase, telophase during mitosis), the fact that interphase makes up about 90% of the total cell cycle, and the connection between checkpoint failure (particularly G1) and cancer development.
⚠ Common Trap
The most common trap is assuming mitosis (the visible, dramatic division phase) makes up most of the cell cycle, when in fact interphase — the less visually dramatic growth and preparation phase — accounts for roughly 90% of the total time.
✓ Quick Self-Check
1. What are the three sub-phases of interphase, and what happens in each?
G1 (growth, protein synthesis, preparation), S (DNA replication), and G2 (final growth, organelle duplication).
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2. Approximately what percentage of the cell cycle does interphase occupy?
About 90%.
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3. What happens during prophase and metaphase?
Prophase: chromatin condenses into chromosomes, spindle forms. Metaphase: chromosomes align at the metaphase plate.
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4. What happens during anaphase and telophase?
Anaphase: sister chromatids are pulled to opposite poles. Telophase: nuclear envelopes reform and chromosomes decondense.
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5. What is the purpose of cell cycle checkpoints, and what happens when they fail?
Checkpoints verify DNA is undamaged before the cell proceeds to the next phase; when they fail, cells with damaged DNA can continue dividing uncontrollably, which is a key mechanism underlying cancer.
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