Chapter 12: The Cell Cycle

Introduction to the Cell Cycle

The cell cycle is a series of events that cells go through as they grow and divide. It is fundamental to the continuity of life, enabling organisms to grow, develop, and repair tissues. Most cell division results in genetically identical daughter cells, except for meiosis, which produces gametes for sexual reproduction.

• Cell Division: The process by which a parent cell divides into two or more daughter cells.

• Asexual Reproduction: Reproduction involving a single parent, producing genetically identical offspring (e.g., binary fission in bacteria, regeneration in sea stars).

• Sexual Reproduction: Involves two parents and produces offspring with genetic variation.

• Genome: The complete set of genetic material in a cell.

• Chromatin: The complex of DNA and proteins that makes up chromosomes in eukaryotic cells.

• Chromosome: A threadlike structure of nucleic acids and proteins carrying genetic information.

• Somatic Cell: Any cell of a living organism other than the reproductive cells; diploid (2n).

• Gamete: Reproductive cell (sperm or egg); haploid (n).

• Diploid (2n): Cells with two sets of chromosomes (e.g., human somatic cells have 46 chromosomes).

• Haploid (n): Cells with one set of chromosomes (e.g., human gametes have 23 chromosomes).

• Sister Chromatids: Two identical copies of a chromosome, attached at the centromere.

• Centromere: The region where sister chromatids are most closely attached.

Example: In humans, somatic cells are diploid (46 chromosomes), while gametes are haploid (23 chromosomes).

Phases of the Cell Cycle and Mitosis

The cell cycle consists of two major stages: Interphase and the Mitotic (M) Phase. Interphase is subdivided into G1, S, and G2 phases, during which the cell grows, replicates its DNA, and prepares for division. The M phase includes mitosis and cytokinesis.

• Interphase: Accounts for about 90% of the cell cycle.

  • G1 Phase (First Gap): Cell grows, recovers from previous division, and accumulates materials for DNA synthesis.

  • S Phase (Synthesis): DNA replication occurs; chromosomes duplicate to form sister chromatids.

  • G2 Phase (Second Gap): Cell prepares for mitosis by synthesizing proteins needed for division.

• M (Mitotic) Phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Key Point: Chromosomes are duplicated during the S phase of interphase.

Stages of Mitosis

• Prophase: Chromatin condenses into visible chromosomes; nuclear envelope begins to fragment; centrosomes move to opposite poles; spindle apparatus begins to form.

• Prometaphase: Nuclear envelope fragments completely; spindle microtubules attach to kinetochores on chromosomes.

• Metaphase: Chromosomes align at the metaphase plate (cell equator).

• Anaphase: Sister chromatids separate and move toward opposite poles; kinetochore microtubules shorten; cell elongates.

• Telophase: Nuclear envelope reforms around each set of chromosomes; chromosomes decondense; spindle disappears; cytokinesis begins.

Example: During metaphase, all chromosomes are aligned at the metaphase plate, ensuring equal distribution to daughter cells.

Cytokinesis

• Animal Cells: Cytokinesis occurs by cleavage, forming a cleavage furrow that pinches the cell in two.

• Plant Cells: Cytokinesis occurs by formation of a cell plate, which develops into a new cell wall.

Comparison Table: Cytokinesis in Animal vs. Plant Cells

Binary Fission in Prokaryotes

• Prokaryotes reproduce by binary fission, a process in which the chromosome replicates and the cell divides into two genetically identical cells.

• Replication begins at the origin of replication and proceeds bidirectionally.

• The plasma membrane pinches inward to separate the cells.

Cell Cycle Regulation

The eukaryotic cell cycle is tightly regulated by a molecular control system to ensure proper division and prevent uncontrolled growth (cancer).

• Cell Cycle Control System: A set of molecules that triggers and coordinates key events in the cell cycle, functioning like a clock.

• Checkpoints: Critical control points where stop and go-ahead signals regulate the cycle (G1, G2, and M checkpoints).

• G1 Checkpoint: The most important checkpoint; if a cell does not receive a go-ahead signal, it enters a non-dividing state called G0 phase or undergoes apoptosis (programmed cell death).

• G2 Checkpoint: Ensures DNA replication is complete and undamaged before mitosis.

• M Checkpoint (Spindle Checkpoint): Ensures all chromosomes are properly attached to the spindle before anaphase begins.

Regulatory Molecules

• Cyclins: Proteins whose concentrations fluctuate cyclically in the cell.

• Cyclin-dependent kinases (Cdks): Protein kinases that are active only when bound to a cyclin; regulate transitions between cell cycle phases.

• MPF (Maturation-Promoting Factor): A cyclin-Cdk complex that triggers passage from G2 to M phase by phosphorylating target proteins.

Key Equation:

• MPF activity rises as cyclin accumulates during S and G2 phases and falls as cyclin is degraded during M phase.

External Controls

• Growth Factors: Proteins released by certain cells that stimulate other cells to divide (e.g., Platelet-Derived Growth Factor (PDGF) stimulates fibroblast division during wound healing).

• Density-Dependent Inhibition: Crowded cells stop dividing when they contact each other.

• Anchorage Dependence: Most animal cells must be attached to a substratum to divide.

Cancer and the Cell Cycle

• Cancer: Uncontrolled cell division due to loss of cell cycle regulation.

• Transformation: Process by which a normal cell becomes cancerous.

• Tumor: Mass of abnormal cells; benign tumors remain at the original site, while malignant tumors invade surrounding tissues and can metastasize (spread to other parts of the body).

• Characteristics of Cancer Cells:

  • Lack differentiation (non-specialized, can divide indefinitely)

  • Abnormal nuclei (enlarged, abnormal chromosome numbers)

  • Do not undergo apoptosis

  • Do not exhibit density-dependent inhibition or anchorage dependence

• Cancer Treatments: Include radiation and chemotherapy, which target rapidly dividing cells. Side effects occur because normal dividing cells (e.g., skin, hair, intestinal lining) are also affected.

• Personalized Medicine: Advances in understanding cell cycle regulation and tumor genetics are leading to more targeted cancer therapies.

Summary Table: Key Terms and Concepts

Additional info: The above notes synthesize and expand upon the provided outline and study guide, incorporating definitions, explanations, and comparisons to create a comprehensive mini-textbook section suitable for exam preparation in General Biology.