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 without the fusion of gametes, producing offspring genetically identical to the parent.

• Sexual Reproduction: Involves the fusion of gametes, resulting in genetically diverse offspring.

• 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: A 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 connected by a centromere.

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

Functions of Mitosis:

• In unicellular organisms: asexual reproduction.

• In multicellular organisms: growth, development, and tissue repair.

Example: Skin cells divide to replace those lost due to injury.

The Cell Cycle: Phases and Events

Phases of the Cell Cycle

The cell cycle consists of two major stages: Interphase and the Mitotic (M) Phase. Most of the cell's life is spent in interphase, which is subdivided into three phases:

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

• S Phase (Synthesis): DNA replication occurs; chromosomes are duplicated, each consisting of two sister chromatids.

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

The M Phase includes both mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis: Stages and Key Events

Mitosis is divided into five stages, ensuring equal distribution of genetic material:

1. Prophase: Chromatin condenses into visible chromosomes; nucleolus disappears; mitotic spindle begins to form.

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

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

4. Anaphase: Sister chromatids separate and move toward opposite poles; cell elongates.

5. Telophase: Nuclear envelope reforms; chromosomes decondense; nucleolus reappears; spindle disappears.

Cytokinesis usually begins during telophase, dividing the cytoplasm and completing cell division.

Mitotic Spindle and Chromosome Movement

• Mitotic Spindle: A structure made of microtubules that segregates chromosomes during mitosis.

• Centrosome: The microtubule organizing center in animal cells; duplicates during interphase.

• Aster: A radial array of short microtubules extending from each centrosome.

• Kinetochores: Protein complexes at the centromere where spindle fibers attach.

• Metaphase Plate: The plane where chromosomes align during metaphase.

Functions of Spindle Microtubules:

• Kinetochore Microtubules: Attach to kinetochores and pull chromatids apart.

• Non-kinetochore Microtubules: Overlap and push against each other, elongating the cell.

Cytokinesis: Animal vs. Plant Cells

• 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 from vesicles, which develops into a new cell wall.

Binary Fission in Prokaryotes

Prokaryotes (bacteria and archaea) divide by binary fission:

1. DNA replicates at the origin of replication.

2. Two DNA molecules move to opposite ends.

3. Plasma membrane pinches inward, dividing the cell.

Regulation of the Cell Cycle

Cell Cycle Control System

The cell cycle is regulated by a molecular control system, ensuring proper timing and order of events. This system acts like a clock, with checkpoints at critical stages:

• G1 Checkpoint: Determines if the cell will divide; also called the restriction point.

• 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.

Regulatory Molecules: Cyclins and Cdks

• Protein Kinases: Enzymes that activate or deactivate other proteins by phosphorylation.

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

• Cyclin-dependent Kinases (Cdks): Kinases that are active only when bound to a cyclin.

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

Key Regulatory Mechanisms:

• Cyclin levels rise during S and G2 phases, forming active MPF with Cdk.

• MPF promotes mitosis by phosphorylating target proteins.

• Cyclin is degraded during M phase, inactivating MPF and allowing exit from mitosis.

• Cdks are present at constant levels but are only active when bound to cyclin.

Equation (Cdk activation):

Checkpoints and Cell Fate

• If a cell does not pass the G1 checkpoint, it may enter the G0 phase (a non-dividing state) or undergo apoptosis (programmed cell death).

• Cells such as mature nerve and muscle cells often remain in G0 and do not divide.

• Unattached kinetochores at the M checkpoint delay anaphase until all chromosomes are properly attached.

External Regulation of Cell Division

• Growth Factors: Proteins released by certain cells that stimulate others to divide (e.g., 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 substrate to divide.

Cancer and Loss of Cell Cycle Control

Cancer results from the loss of normal cell cycle regulation, leading to uncontrolled cell division.

• Transformation: The process by which a normal cell becomes cancerous.

• Tumor: A mass of abnormal cells; benign tumors remain localized, while malignant tumors invade tissues and can metastasize.

• Metastasis: The spread of cancer cells to distant body sites.

• Characteristics of Cancer Cells: Lack differentiation, abnormal nuclei, do not undergo apoptosis, and can divide indefinitely.

• Cancer Treatments: Include radiation and chemotherapy, which target rapidly dividing cells but can also harm normal dividing cells (e.g., skin, hair, intestinal lining).

• Advances in understanding cell cycle regulation have led to more personalized cancer therapies.

Summary Table: Comparison of Mitosis and Meiosis

Key Terms and Definitions

Additional info: The cell cycle is tightly regulated to prevent uncontrolled cell growth, which can lead to cancer. Understanding the molecular mechanisms of cell cycle control is crucial for developing targeted cancer therapies.