The Cell Cycle and Cell Division

The Cell Cycle Overview

• Definition: The cell cycle is a sequence of events in eukaryotic cells where the cell contents are duplicated and divided into two daughter cells. This cycle is conserved across all eukaryotes.
• Observation: In cultured animal cells, not all cells divide simultaneously; however, when they do divide, they follow the same stages of mitosis.

Phases of the Cell Cycle

• M Phase:
  • Division of nucleus (mitosis) and cytoplasm (cytokinesis).
• Interphase: Period between cell divisions, subdivided into three phases:
  • G1 phase: Cell growth and preparation for DNA synthesis.
  • S phase: DNA synthesis; chromosomes are replicated.
  • G2 phase: Preparation for mitosis, including further growth and DNA repair.

Cell Cycles in Multicellular Organisms

1. Non-dividing Cells:
   • Examples: terminally differentiated cells such as nerve cells, muscle cells, and red blood cells. They lose the ability to divide.
2. Stimulated Dividing Cells:
   • Example: Liver cells that divide to replace damaged tissue when necessary.
3. Continuously Dividing Cells:
   • Examples: Hematopoietic and epithelial stem cells that regularly divide and replenish tissues.

G0 Phase

• G0 (Gap 0): A resting state where cells are metabolically active but not dividing. Cells in G0 can exit the cycle and are often involved in specific functions without proliferation.

Cell Cycle Control System and Checkpoints

• Major Checkpoints:
  • Start transition (G1→S)
  • G2/M transition (G2→M)
  • Metaphase-to-anaphase transition (Spindle assembly checkpoint)
• Control Mechanism: Regulates progression through the cell cycle using molecular switches such as cyclin-dependent protein kinases (Cdks).
  • Cyclin-Cdk Complex: Activation triggers entry into the next phase; inactivation occurs after the transition.

Events During M Phase

1. Prophase:
   • Chromatin condenses, and mitotic spindle begins to form using microtubules from centrosomes.
   • Cohesins are removed from chromosome arms, but remain at centromeres to hold sister chromatids together.
2. Prometaphase:
   • Nuclear envelope disassembly allows mitotic spindle to attach to kinetochores of chromosomes.
   • Chromosomal movement begins as the spindle assembles completely.
3. Metaphase:
   • All chromosomes align at the metaphase plate; continuous microtubule dynamics maintain tension in the spindle.
4. Anaphase:
   • Separation of sister chromatids occurs, facilitated by the action of cohesins and motor proteins (kinesins and dyneins).
   • Anaphase A and B refer to the pulling of chromatids and separation of spindle poles, respectively.
5. Telophase:
   • Chromosomes decondense; nuclear envelope reassembles; contractile ring initiates cytokinesis.
6. Cytokinesis:
   • Cytoplasm divides via a contractile ring composed of actin and myosin filaments, forming two daughter cells.

Cytokinesis in Plant Cells

• Cytokinesis involves the formation of a cell plate, guided by a phragmoplast structure with microtubules and vesicles, which fuse to create the new cell wall between daughter cells.

Comparison of Mitosis and Meiosis

• Mitosis:
  • Involves one round of DNA replication and one round of cell division, producing two identical diploid cells.
• Meiosis:
  • Involves one round of DNA replication followed by two rounds of cell division, producing four non-identical haploid cells due to homologous chromosome separation and independent assortment.

Important Factors in Cell Division

• Microtubule Dynamics: Essential for the assembly and function of the mitotic spindle facilitating chromosome segregation.
• Motor Proteins: Interact with microtubules to assist in chromosome movement and spindle dynamics.

Summary of Learning Objectives

• Describe phases of the cell cycle, including specific roles of M phase, mitosis, and cytokinesis.
• Understand the cell cycle control system and its checkpoints.
• Distinguish cytokinesis in animal versus plant cells.
• Discuss the dynamics of microtubules and actin filaments in the context of cell division.