Study Notes: The Cell Cycle

Chapter 12: The Cell Cycle

Concept 12.1: Most Cell Division Results in Genetically Identical Daughter Cells

The cell cycle is a fundamental process by which cells reproduce, ensuring growth, repair, and maintenance in multicellular organisms. Most cell divisions produce two genetically identical daughter cells.

• Cell Division Functions: Cell division is essential for reproduction (in unicellular organisms), growth (in multicellular organisms), and repair (replacement of damaged or dead cells).

• Chromosome Replication and Division: Before a cell divides, it replicates its DNA so that each daughter cell receives an identical set of chromosomes. This ensures genetic continuity across generations of cells.

• Example: Skin cells divide to replace those lost from the surface, maintaining tissue integrity.

Concept 12.2: The Mitotic Phase Alternates with Interphase in the Cell Cycle

The eukaryotic cell cycle consists of a series of phases, including interphase (G1, S, and G2) and the mitotic (M) phase. Mitosis ensures equal distribution of chromosomes to daughter cells.

• Phases of the Eukaryotic Cell Cycle:

  • G1 phase: Cell grows and carries out normal functions.

  • S phase: DNA is replicated.

  • G2 phase: Cell prepares for division.

  • M phase (Mitosis and Cytokinesis): Division of the nucleus and cytoplasm.

• Major Events in Each Phase: Each phase is characterized by specific events, such as DNA synthesis in S phase and chromosome segregation in M phase.

• Mitosis Stages:

  • Prophase: Chromosomes condense, spindle forms.

  • Metaphase: Chromosomes align at the cell's equator.

  • Anaphase: Sister chromatids separate and move to opposite poles.

  • Telophase: Nuclear envelopes reform around chromosomes.

  • Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

• Chromosome Structure: Chromosomes consist of two sister chromatids joined at a centromere. Homologous chromosomes are pairs with the same genes but possibly different alleles.

• Haploid and Diploid: Haploid cells have one set of chromosomes (n), while diploid cells have two sets (2n).

• Cytokinesis in Animals vs. Plants: Animal cells form a cleavage furrow; plant cells form a cell plate.

• Example: In human somatic cells (diploid, 2n=46), mitosis produces two identical diploid cells.

Concept 12.3: The Eukaryotic Cell Cycle is Regulated by a Molecular Control System

The progression of the cell cycle is tightly regulated by a complex system of molecular checkpoints and regulatory proteins to ensure proper division and prevent errors.

• Checkpoints: Critical control points (G1, G2, and M checkpoints) where the cell assesses whether to proceed with division.

• Key Regulatory Proteins:

  • Cyclins: Proteins whose levels fluctuate during the cell cycle.

  • Cyclin-dependent kinases (Cdks): Enzymes activated by cyclins to phosphorylate target proteins, driving cell cycle progression.

  • MPF (Maturation-Promoting Factor): A complex of cyclin and Cdk that triggers the cell's entry into mitosis.

• Internal and External Factors: Internal signals (e.g., DNA damage) and external signals (e.g., growth factors) influence the cell cycle control system.

• Example: If DNA is damaged, the cell may halt at the G1 checkpoint to allow for repair or trigger apoptosis if the damage is irreparable.

Additional info:

• Equation for DNA content during the cell cycle:

$\text{DNA content after S phase} = 2 \times \text{DNA content before S phase}$