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Cell Cycle and Mitosis
Interphase, Prophase, Metaphase, Anaphase, Telophase
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The cell cycle is the ordered sequence of events by which a cell duplicates its contents and divides into two daughter cells. It consists of interphase (G1, S, and G2 phases) and the mitotic phase. During interphase, the cell grows, replicates its DNA, and prepares for division. The S phase is when DNA synthesis doubles the chromosome number from diploid to tetraploid before segregation.
Mitosis itself has four stages: prophase, metaphase, anaphase, and telophase. Chromosomes condense and become visible in prophase, align at the cell equator in metaphase, are pulled to opposite poles in anaphase, and decondense as new nuclei form in telophase. Cytokinesis then splits the cytoplasm to produce two genetically identical daughter cells. Cell cycle checkpoints at G1, G2, and the spindle assembly checkpoint ensure accuracy — errors at these points can lead to cancer.
Key Facts
- Interphase = G1 (growth) + S (DNA replication) + G2 (prep for division)
- Mitosis phases: Prophase, Metaphase, Anaphase, Telophase (PMAT)
- Sister chromatids are joined at the centromere; spindle fibers pull them apart in anaphase
- Mitosis produces 2 genetically identical diploid daughter cells
- G1 checkpoint: checks cell size and DNA integrity before committing to division
- Spindle assembly checkpoint: ensures all chromosomes are attached before anaphase
Vocabulary
- Chromatin
- The complex of DNA and proteins that makes up chromosomes; decondensed during interphase and condensed during mitosis.
- Centromere
- The constricted region of a chromosome where sister chromatids are joined and where spindle fibers attach.
- Spindle fiber
- A microtubule that forms during cell division and moves chromosomes toward opposite poles of the cell.
- Cytokinesis
- The division of the cytoplasm that follows mitosis, producing two separate daughter cells.
- Checkpoint
- A regulatory control point in the cell cycle that ensures conditions are suitable before the cell proceeds to the next phase.
Common Mistakes to Avoid
- Confusing mitosis with meiosis. Mitosis produces 2 identical diploid cells (for growth and repair); meiosis produces 4 genetically diverse haploid cells (for reproduction).
- Thinking DNA replication occurs during prophase. DNA is replicated during S phase of interphase, well before mitosis begins.
- Assuming cytokinesis is part of mitosis. Cytokinesis is a separate process that divides the cytoplasm; mitosis only refers to nuclear division.
- Forgetting that 'tetraploid' is a transient state. After S phase the cell has 4n DNA content, but after mitosis each daughter has 2n again — the ploidy level is restored.
Practice Questions
- 1 A cell begins mitosis with 46 chromosomes (human). How many chromosomes does each daughter cell contain after mitosis and cytokinesis?
- 2 During which phase of the cell cycle does DNA replication occur, and what enzyme is responsible for synthesizing the new DNA strand?
- 3 Explain what would happen if the spindle assembly checkpoint failed and a chromosome did not attach to a spindle fiber before anaphase.