Overview
This lecture explains the stages and regulation of the cell cycle in somatic (non-reproductive) cells, focusing on the phases of interphase and mitosis.
The Cell Cycle Overview
- The cell cycle is the series of events cells undergo from formation until division into two identical daughter cells.
- Somatic cells (all body cells except reproductive cells) undergo the cell cycle.
- The duration of the cell cycle varies by cell type (e.g., skin cells: <1 day; liver cells: years).
Interphase
- Interphase is the longest phase and prepares the cell for division.
- Consists of three sub-phases: G1, S, and G2.
G1 Phase (Gap 1)
- G1 is primarily for cell growth and normal functions like protein synthesis and energy production.
- Chromosomes are single chromatids during G1.
- G1 checkpoint checks for DNA damage and sufficient protein synthesis.
- If issues are detected, the cell enters G0 (non-dividing phase) or undergoes apoptosis (cell self-destruction).
S Phase (Synthesis)
- DNA is replicated so each daughter cell can receive identical genetic material.
- Chromosome number stays the same (46 in humans), but DNA content doubles.
- Two identical sister chromatids are joined at the centromere.
G2 Phase (Gap 2)
- The cell duplicates organelles in preparation for division.
- G2 checkpoint ensures no DNA damage after replication before entering mitosis.
Mitosis and Cytokinesis
- Mitosis is the process of dividing replicated DNA into two nuclei, followed by cytokinesis (separation into two cells).
- Four subphases of mitosis: Prophase, Metaphase, Anaphase, Telophase.
Prophase
- Nuclear envelope breaks down, chromosomes condense and become visible.
- Centrosomes move to opposite poles.
Metaphase
- Chromosomes align at the cell's middle (metaphase plate).
- Spindle fibers attach to centromeres.
Anaphase
- Sister chromatids are pulled apart to opposite poles by spindle fibers.
Telophase
- Nuclear membranes form around the separated chromosomes.
Cytokinesis
- Cell membrane pinches in, dividing the cell into two daughter cells.
G0 Phase
- Cells in G0 are alive but not dividing or preparing to divide (e.g., neurons always, hepatocytes unless tissue repair is needed).
Key Terms & Definitions
- Somatic Cells — Body cells excluding reproductive (sperm/egg) cells.
- Interphase — Preparation phase before cell division, including G1, S, and G2.
- Mitosis — Process of nuclear division, produces two identical nuclei.
- G1 Checkpoint — Control point assessing DNA and cell readiness to divide.
- Apoptosis — Programmed cell death.
- Sister Chromatids — Identical copies of a chromosome after DNA replication.
- Centromere — Region where sister chromatids are joined.
- Cytokinesis — Division of the cytoplasm to form two separate cells.
- G0 Phase — Non-dividing cell state.
Action Items / Next Steps
- Review the stages and checkpoints of the cell cycle.
- Learn the order and main events of the mitosis subphases.
- Understand key cell cycle terms and their definitions.