The Cell Cycle and Mitosis

High-Yield Summary

  • Cell cycle: G1 (growth) → S (DNA replication, sister chromatids form) → G2 (final prep) → M (mitosis + cytokinesis). G1+S+G2 = interphase.
  • G0 is a resting state cells can exit the cycle into (e.g. most neurons spend their life here).
  • Two checkpoints gate progression: G1/S (restriction point, enforced by p53 — checks DNA integrity) and G2/M (checks growth/duplication complete; p53 also suppresses cyclin B if damage detected).
  • Cyclins (levels rise/fall) bind CDKs (always present, inactive alone) — the complex phosphorylates transcription factors to switch on next-phase genes.
  • Mitosis: prophase → metaphase → anaphase → telophase → cytokinesis. One diploid cell → two identical diploid daughter cells.
  • Checkpoint failure (e.g. p53 loss-of-function) lets damaged cells divide unchecked — a hallmark of cancer.

Key Terms

Somatic cell
Diploid (2N) body cell — carries two copies of each chromosome.
Sister chromatids
Two identical copies of a replicated chromosome, joined at the centromere.
p53
"Guardian of the genome" — tumor suppressor enforcing the G1/S checkpoint by halting the cycle when DNA damage is detected.
Cyclin-dependent kinase (CDK)
Enzyme always present but inactive until bound by a cyclin; the active complex drives phase transitions.
Kinetochore
Protein complex on the centromere that spindle fibers attach to during mitosis.
Centrosome
Organelle (built from a pair of centrioles) that organizes the mitotic spindle from each pole.

The 4 Phases of the Cell Cycle

  1. 1G1 (Gap 1) — cell grows, produces organelles; G1/S checkpoint confirms DNA is intact.
  2. 2S (Synthesis) — DNA replicates; each chromosome becomes 2 sister chromatids joined at the centromere.
  3. 3G2 (Gap 2) — cell finishes growing/replicating components; G2/M checkpoint confirms readiness to divide.
  4. 4M (Mitosis + Cytokinesis) — nucleus divides, then the cytoplasm divides.

The 5 Stages of Mitosis

  1. 1Prophase — centrosomes move to opposite poles, spindle forms, kinetochores assemble on centromeres, chromatin condenses into visible X-shaped chromosomes, nuclear membrane breaks down.
  2. 2Metaphase — chromosomes align at the metaphase plate; each kinetochore attached to spindle fibers from both poles.
  3. 3Anaphase — sister chromatids split at the centromere and are pulled to opposite poles (chromosome count doubles here — same DNA, just recounted).
  4. 4Telophase — new nuclear envelopes form around each chromosome set; chromosomes decondense back to chromatin; spindle breaks down.
  5. 5Cytokinesis — cytoplasm/organelles physically divide (overlaps with telophase), yielding 2 identical diploid daughter cells.

Common MCAT Trap

  • Chromatin vs. chromosome vs. sister chromatid: same DNA, different packaging state. Chromatin = loose (interphase). Chromosome = condensed/visible. Sister chromatid = one of 2 identical copies still joined at the centromere (still counted as ONE chromosome until anaphase splits them).
  • Apparent chromosome-count doubling in anaphase is just recounting split sister chromatids as individual chromosomes — no new DNA is made.
  • p53 acts at BOTH checkpoints (primary role at G1/S, contributes to G2/M via cyclin B suppression) — don't restrict it to only one.

Quick Recall

What are the 3 sub-phases that make up interphase?

What enzyme must a CDK bind to become active?

At which mitotic stage are sister chromatids pulled apart?

Which tumor suppressor primarily enforces the G1/S checkpoint?

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