3.3 Cell Cycle and Cell Division

The Cell Cycle

• Interphase: The preparatory phase.
  • G₁ Phase: Cell growth.
  • S Phase: DNA replication.
  • G₂ Phase: Preparation for mitosis.
• M Phase (Mitosis): Actual cell division.

Mitosis (Equational Division)

• Prophase: Chromosome condensation, spindle formation.
• Metaphase: Chromosomes align at the metaphase plate.
• Anaphase: Sister chromatids separate and move to opposite poles.
• Telophase: Chromosomes decondense, nuclear envelope reforms.
• Significance: Growth, repair, asexual reproduction.

Meiosis I (Reductional Division)

• Prophase I:
  • Synapsis: Pairing of homologous chromosomes.
  • Crossing Over: Exchange of genetic material.
• Metaphase I: Bivalents align at the equator.
• Anaphase I: Homologous chromosomes separate.
• Telophase I: Two haploid cells are formed.

Meiosis II (Equational Division)

• Similar to mitosis.
• Prophase II: Spindle formation.
• Metaphase II: Chromosomes align at the equator.
• Anaphase II: Sister chromatids separate.
• Telophase II: Four haploid daughter cells are formed.
• Significance: Gamete formation, genetic variation.

Mitosis vs. Meiosis

Cell Cycle Regulation

• Cyclins and Cyclin-Dependent Kinases (CDKs):
  • The "engine" of the cell cycle.
  • CDKs are activated by binding to cyclins.
  • Different cyclin-CDK complexes drive different phases of the cycle.

Cell Cycle Checkpoints

• G₁/S Checkpoint: "Restriction point"; checks for DNA damage, nutrients, growth factors.
• G₂/M Checkpoint: Ensures DNA replication is complete before mitosis.
• Spindle Assembly Checkpoint (SAC): Ensures all chromosomes are properly attached to the spindle before anaphase.