TheCell7e Ch17 Lecture

17 The Cell Cycle

Introduction

• Fundamental characteristic of cells is self-reproduction.

• Cell division leads to two daughter cells from one parental cell.

• Regulation and coordination of cell division are crucial.

Eukaryotic Cell Cycle Overview

• Progression controlled by protein kinases conserved from yeasts to mammals.

• Defective regulation links to cancer cell proliferation.

Key Phases of the Eukaryotic Cell Cycle

1. Four Coordinated Processes:

   • Cell growth

   • DNA replication

   • Distribution of duplicated chromosomes

   • Cell division

2. Phases:

   • M Phase: Mitosis and cell division (cytokinesis).

   • Interphase: Prepares the cell for mitosis, comprising three subphases:

     • G1 phase: Cell growth after mitosis.

     • S phase: DNA replication occurs here.

     • G2 phase: Final preparations for mitosis, including protein synthesis.

Phases of Eukaryotic Cell Cycle

• G1 phase: Active cell growth between mitosis and DNA replication.

• S phase: DNA content is duplicated (2n to 4n).

• G2 phase: Growth continues and proteins for mitosis are synthesized.

Variation in Cycle Duration

• Cycle phases vary in duration based on cell type.

• For example, budding yeasts complete all phases in approximately 90 minutes.

Cell Cycle Phases Identification

• Identification via biochemical methods and DNA content analysis.

• G1: Cells are diploid (2n).

• S phase: Increase in DNA content (4n).

Regulation of Cell Cycle Progression

1. Control Points:

   • Critical stages determine the progression through cell cycle based on nutrient availability and cell size.

   • START point: Commitment step defined in yeast.

2. Cell Size and Nutrient Regulation:

   • Yeast must reach minimum size to pass START. Smaller cells grow longer in G1 than larger cells.

3. Animal Cells:

   • Restriction Point: Similar to START, regulated by growth factors.

   • If growth factors are absent, cells enter G0 stage (resting phase).

Checkpoints in Cell Cycle

• DNA damage checkpoints prevent replication of damaged DNA.

• Spindle assembly checkpoint halts mitosis if chromosomes are misaligned.

Major Regulators of Cell Cycle Progression

1. Maturation Promoting Factor (MPF):

   • Induces entry into M phase and regulates transitions.

   • Composed of Cyclin B and Cdk1.

2. Cyclins and Cdks:

   • Cyclins accumulate in interphase, degrade at the end of mitosis.

   • Cyclin-dependent kinases (Cdks) must associate with cyclins to become active.

   • Regulation through phosphorylation and protein-inhibitor interactions.

Tumor Suppressor Genes

• Retinoblastoma (Rb):

  • Inhibitory role in cell cycle progression; often mutated in cancers.

• Rb binds to E2F transcription factors and suppresses genes for progression until phosphorylated.

Events of M Phase

• Major reorganization occurs, including:

  • Chromosome condensation and spindle formation.

  • Cycle includes Prophase, Metaphase, Anaphase, and Telophase.

Meiosis Overview

1. Reduction Division:

   • Reduces chromosome number by half, resulting in haploid cells.

   • Takes place in germ cells for multicellular organisms; unicellular can undergo meiosis and mitosis.

2. Two Rounds of Division:

   • Meiosis I: Homologous chromosomes segregate.

   • Meiosis II: Resembles mitosis where sister chromatids separate.

Fertilization Process

• Engagement of sperm with egg introduces calcium ion increase in egg cytoplasm, crucial for development and completion of meiosis.

Control in Oocyte Meiosis

• Regulatory mechanisms manage meiotic entry and maintenance of synaptic features.

• Oocytes may arrest in different meiosis stages, with hormonal stimulation leading to meiotic progression.

Key Figures

• Diagrams showcase the detailed processes of the cell cycle, mitosis stages, checkpoint systems, etc. for visual understanding.