Cell Cycle & Cell Death Overview

Cell Cycle & Cell Death

• Learning Objectives:
  • Define the cell cycle and illustrate the phases.
  • Explain the events occurring in each phase.
  • Recognize the relationship between cell differentiation and the cell cycle.
  • Differentiate between cell death by apoptosis and necrosis.
  • Define apoptosis and its characteristics.

The Cell Cycle

• Definition: A sequence of events during cell division and rest (interphase), producing new cells.
• Periods:
  • Interphase: Gap 1 (G1), Synthesis (S), and Gap 2 (G2) phases.
  • Mitosis: Nucleus (karyokinesis) and cytoplasm (cytokinesis) division.

Stages of Cell Cycle

• G₁ (Gap 1) Phase:

  • Interval between mitosis and synthesis.
  • Chromosomes are s-chromosomes (46 chromosomes).
  • The nucleus shows no increase in DNA content.
  • Active RNA and protein synthesis, growth.
  • Cells either divide again or enter a non-dividing state (G₀).

• S (Synthesis) Phase:

  • DNA molecules and centrioles duplicate.
  • The cell contains 46 identical pairs of chromosomes (often called double chromosomes).

• G₂ (Gap 2) Phase:

  • Period between synthesis and mitosis (relatively short).
  • Cells prepare for mitotic division, synthesize cytoplasmic material (e.g., proteins).
  • Organelles increase in number.
  • Chromosomes are double chromosomes (double the DNA compared to G₁).
  • Most cells transition from G₂ to the next mitosis.

• Mitosis (M):

  • Chromosomes shorten, align in the cell's middle, then separate.
  • Creates two genetically identical daughter cells.
  • Cell cycle completion time is 10-30 hours, and the cycle begins again after mitosis completion.
  • Includes stages Prophase, Metaphase, Anaphase, Telophase.

Cell Cycle Checkpoints

• Checkpoints: Control points to ensure each phase is completed.
• Stages:
  • Start (restriction) checkpoint (before S phase)
  • G₂/M checkpoint (ensuring DNA replication is complete)
  • Metaphase checkpoint (ensuring all chromosomes are correctly attached to the mitotic spindle).
• Control Mechanism: Cyclin-dependent kinases (CDKs) and cyclins regulate the entire cycle.

Cell Development

• Stages:
  • Growth: Increase in mass (size or number of cells).
  • Differentiation: Developing new features and functions.
  • Integration: Growth and differentiation are balanced, creating tissues and organs.

Cell Death

• Necrosis:

  • Cell death due to external factors (e.g., injury, toxins).
  • The cell swells, chromatin clumps, and cytoplasm gets weakly stained.
  • The cell bursts, releasing contents.

• Apoptosis:

  • Normal cell death due to internal factors (programmed cell death).
  • A rapid, highly regulated process that eliminates defective or unneeded cells.
  • The cell shrinks, DNA is condensed and degraded.
  • Forms apoptotic bodies, which are then removed by phagocytes (cells specialized for debris removal).