Overview
This lecture explains the stages and regulation of the cell cycle, including mitosis and cytokinesis, and highlights the consequences of disrupted cell cycle control, such as cancer.
The Cell Cycle
- The cell cycle has two major phases: interphase (cell growth) and the mitotic phase (cell division).
- Interphase includes G1 (growth), S (DNA synthesis), and G2 (preparation for division) phases.
- Most cells spend the majority of time in interphase; G0 is a resting phase for non-dividing cells.
Structure of Chromosomes
- During the S phase, DNA replicates, resulting in sister chromatids held together at the centromere.
- Homologous chromosomes are pairs inherited from each parent.
- After S phase, cells have 92 chromatids (46 chromosomes, each with two chromatids).
Mitosis and Cytokinesis
- Mitosis has four stages:
- Prophase: Chromatin condenses, chromosomes become visible, nuclear envelope disappears, spindle forms.
- Metaphase: Chromosomes align at the metaphase plate, spindle fibers attach to centromeres.
- Anaphase: Sister chromatids separate and move to opposite poles.
- Telophase: New nuclei form, chromosomes decondense, spindle disassembles.
- Cytokinesis follows mitosis, dividing the cytoplasm to form two cells via a cleavage furrow.
- Failure of cytokinesis can result in multinucleated cells, sometimes linked to cancer.
Cell Cycle Control
- Checkpoints (G1, G2, metaphase) ensure proper progression through the cell cycle.
- Cyclins and cyclin-dependent kinases (CDKs) regulate checkpoint transitions.
- The metaphase checkpoint ensures all chromatids are properly attached before separation.
Cell Cycle Dysregulation and Cancer
- Loss of cell cycle control leads to excessive cell division and tumors.
- Cancer can result from mutations in genes regulating the cell cycle, such as proto-oncogenes (promote division) and tumor suppressor genes (inhibit division).
- Proto-oncogenes can become oncogenes, leading to uncontrolled cell division.
- Tumor suppressor gene malfunction removes crucial stop signals.
- Normal cells exhibit contact inhibition, preventing overgrowth.
Key Terms & Definitions
- Somatic cell — any body cell except for gametes.
- Interphase — phase when the cell grows and prepares for division.
- Mitosis — process where the nucleus divides, forming two identical nuclei.
- Cytokinesis — division of the cytoplasm, producing two cells.
- Chromatid — one of two identical halves of a replicated chromosome.
- Centromere — region where sister chromatids attach.
- Checkpoint — control point in the cell cycle for regulating progression.
- Cyclin/CDK — proteins that regulate the cell cycle.
- Proto-oncogene — gene promoting cell cycle progression.
- Tumor suppressor gene — gene that inhibits cell division.
- Oncogene — mutated proto-oncogene causing uncontrolled division.
Action Items / Next Steps
- Review the stages of mitosis and the cell cycle for understanding and recall.
- Visit the suggested interactive link to further explore mitosis and its phases.
- Prepare for potential quiz on key terms and stages of mitosis.