One Cell, Two Identical Daughters: The Cell Cycle and Mitosis
Atlas the guide stands beside a glowing magnified skin cell on a lab bench, tracing a circular timeline of glowing chromosomes splitting into two matching sets
- Sequence the four phases of the cell cycle (G1, S, G2, and M phase) in order
- Describe how DNA replication in S phase ensures each daughter cell receives a complete genome
- Order the four stages of mitosis (prophase, metaphase, anaphase, telophase) and state what happens in each stage
- Explain how checkpoints regulate the cycle and why uncontrolled division is dangerous
- Describe how cell differentiation allows genetically identical cells to become specialised for different functions
Key terms
- Interphase
- The non-dividing portion of the cycle (G1, S, G2) when the cell grows and copies DNA
- Sister chromatids
- Two identical DNA copies joined at the centromere after S-phase replication
- Centromere
- The constricted region holding sister chromatids together until anaphase separates them
- Checkpoint
- A regulatory control point that verifies conditions before the cell proceeds to the next phase
- Differentiation
- The process by which genetically identical cells specialize by expressing different genes
Interphase Sets the Stage
The majority of a cell's life is spent in interphase, divided into G1, S, and G2. In G1 the cell grows and performs its routine functions; in S phase it replicates its entire genome so every chromosome becomes two identical sister chromatids; in G2 it grows further and synthesizes proteins needed for division while proofreading the new DNA. Only a cell that passes G2 commits to mitosis, which guarantees each daughter inherits a complete copy.
Mitosis Distributes the Copies
Mitosis partitions the duplicated chromosomes equally. In prophase chromatin condenses into visible chromosomes and the spindle forms; in metaphase chromosomes align at the cell's equator; in anaphase the centromeres split and sister chromatids are pulled to opposite poles; in telophase nuclear envelopes reform around each chromosome set. Cytokinesis then physically divides the cytoplasm, yielding two daughter cells with identical diploid genomes.
Checkpoints and Cancer
Checkpoints at the G1/S boundary, the G2/M boundary, and the spindle assembly point during M phase pause the cycle until DNA is intact and properly attached. Proteins like p53 halt division and trigger repair or apoptosis when damage is detected. When these regulators mutate and checkpoints fail, cells divide without restraint and accumulate damage, which is the cellular hallmark of cancer.
Worked examples
A human cell in G1 has 46 chromosomes. How many does each daughter cell have after mitosis?
- S phase replicates each chromosome into two sister chromatids, but the chromosome count stays at 46 because joined chromatids count as one chromosome.
- In anaphase the centromeres split and the sister chromatids separate, distributing one complete copy to each pole.
- Cytokinesis divides the cell so each daughter receives the full set of 46 chromosomes, matching the parent.
Answer: 46 chromosomes per daughter cell.
Activity
Drag these events into the correct order a cell follows from growth to splitting in two
Practice
List the stages of mitosis in order and state the key event of each stage.
Explain how two cells with identical DNA can become a neuron and a skin cell.
Common mistakes to avoid
- Mitosis halves the chromosome numberMitosis keeps the chromosome number constant; meiosis is the division that halves it to make gametes.
- Different cell types have different DNAAll somatic cells share identical DNA; differentiation arises from which genes are switched on or off, not from sequence differences.
Check your understanding
During which phase of the cell cycle is the cell's DNA copied?
Why do the two cells produced by mitosis have identical DNA?
A student says mitosis must reduce the chromosome number by half, like making sex cells. Why is this wrong?
Two skin cells and two nerve cells in the same person all have identical DNA. What best explains why they look and behave so differently?
Recap
The cell cycle copies DNA once in S phase, then mitosis splits it once so two daughter cells inherit identical genomes. Checkpoints guard each transition, and after division differentiation lets identical cells specialize by expressing different genes.
Reflect
Why might a tissue that rarely divides be slower to heal after injury?