Cell Cycle

Cell Cycle: The series of events that take place in a eukaryotic cell from its formation to its division into two daughter cells.

Phases: The cycle consists of two main phases: Interphase (non-dividing) and Mitotic Phase (M-Phase, dividing).

Lasts for ~90% of the cell cycle. The cell carries out normal functions and prepares for division. Divided into:

• G1 Phase (First Gap): Cell grows, makes proteins/organelles.
• S Phase (Synthesis): DNA replication occurs. Chromosomes duplicate (consist of two sister chromatids).
• G2 Phase (Second Gap): Cell prepares for mitosis, makes necessary proteins, and checks for DNA errors.
• G0 Phase: A resting phase where cells stop dividing (e.g., neurons permanently, liver cells temporarily).

Discovered by Walther Flemming (1880s). Division of a parent cell into two genetically identical daughter cells.

• Karyokinesis: Division of nucleus. Phases: Prophase, Metaphase, Anaphase, Telophase.
• Cytokinesis: Division of cytoplasm. In animals (cleavage furrow) and plants (phragmoplast/cell plate).

Phases of Mitosis:

• Prophase: Chromatin condenses into chromosomes. Nucleolus/envelope disappear. Spindle fibers form from centrioles.
• Metaphase: Chromosomes align at the equator forming the Metaphase Plate.
• Anaphase: Spindle fibers pull sister chromatids apart to opposite poles.
• Telophase: Nuclear envelope reforms. Chromosomes uncoil back to chromatin.

Significance: Essential for Growth, Cell Replacement (healing wounds, RBCs), Regeneration (starfish arms), and Asexual Reproduction (budding in Hydra).

Errors (Cancer): Uncontrolled cell division leads to tumors. Benign: Localized. Malignant: Spreads to other tissues (Metastasis/Cancer).

Discovered by Oscar Hertwig (1876). Division reducing chromosome number by half (Diploid -> Haploid). Occurs in germ cells to produce gametes/spores.

Meiosis I: Homologous chromosomes separate. Key events: Synapsis (pairing), Tetrad formation, Crossing Over (exchange of genes usually in Prophase I - genetic variation).

Meiosis II: Similar to Mitosis. Sister chromatids separate.

Significance: Maintains constant chromosome number across generations. Introduces genetic variation favoring evolution.

Mitosis: One division, 2 diploid daughter cells, occurs in somatic cells, no crossing over.

Meiosis: Two divisions, 4 haploid daughter cells, occurs in germ cells, involves crossing over.