Cell Division and the Cell Cycle

Overview of Cell Division

Cell division is the process by which a parent cell divides into two or more daughter cells. It is essential for growth, development, tissue repair, and reproduction in living organisms.

• Cell Division: The process of producing new cells from a parent cell.

• Genome: The complete set of genetic material in an organism.

• Chromosomes: Structures composed of DNA and proteins that carry genetic information.

• Chromatin: The complex of DNA and proteins (mainly histones) that forms chromosomes within the nucleus.

• Histones: Proteins that help package DNA into chromatin.

• Chromosome Condensation: The process by which chromatin fibers become tightly coiled to form visible chromosomes during cell division.

Types of Cells

• Somatic Cells: All body cells except reproductive cells; typically diploid.

• Germ Line Cells: Cells that give rise to gametes (sperm and eggs).

• Gametes: Reproductive cells (sperm and eggs) that are haploid.

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

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

• Polyploid: Cells with more than two sets of chromosomes (common in plants).

The Cell Cycle

Phases of the Cell Cycle

The cell cycle is a series of events that cells go through as they grow and divide. It consists of interphase and the mitotic (M) phase.

• Interphase: The period of cell growth and DNA replication; includes G1, S, and G2 phases.

• G1 Phase: First gap phase; cell grows and carries out normal functions.

• S Phase: Synthesis phase; DNA is replicated.

• G2 Phase: Second gap phase; cell prepares for division.

• M Phase: Mitotic phase; includes mitosis and cytokinesis.

Checkpoints in the Cell Cycle

Checkpoints are control mechanisms that ensure the cell cycle proceeds correctly.

• G1 Checkpoint: Checks for cell size, nutrients, and DNA damage before DNA synthesis.

• S Phase Checkpoint: Ensures DNA is replicating properly.

• G2 Checkpoint: Checks for DNA damage after replication and before mitosis.

• Metaphase Checkpoint: Ensures all chromosomes are properly attached to the spindle before separation.

Mitosis and Cytokinesis

Stages of Mitosis

Mitosis is the division of the nucleus, resulting in two genetically identical daughter cells. It is divided into several stages:

• Prophase: Chromosomes condense, spindle apparatus begins to form, nuclear envelope starts to break down.

• Prometaphase: Nuclear envelope fragments, spindle microtubules attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move toward opposite poles.

• Telophase: Chromosomes decondense, nuclear envelopes reform around daughter nuclei.

Key Structures in Mitosis

• Centromere: Region where sister chromatids are joined.

• Sister Chromatids: Identical copies of a chromosome connected at the centromere.

• Cohesin Proteins: Hold sister chromatids together until anaphase.

• Centrosome: Microtubule-organizing center; duplicates before mitosis.

• Centriole: Structure within the centrosome (in animal cells) that helps organize microtubules.

• Mitotic Spindle: Structure made of microtubules that separates chromosomes.

• Asters: Radial arrays of microtubules extending from centrosomes.

• Microtubules: Protein filaments that form the spindle apparatus.

• Kinetochores: Protein complexes on chromosomes where spindle fibers attach.

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

Cytokinesis

Cytokinesis is the division of the cytoplasm, resulting in two separate daughter cells.

• Cleavage Furrow: Indentation that begins the process of cytokinesis in animal cells.

• Contractile Ring: Structure of actin and myosin that pinches the cell in two.

• Cell Plate: Structure that forms in plant cells to divide the cytoplasm.

Binary Fission

Binary fission is the method by which prokaryotes (bacteria and archaea) reproduce, involving the replication and division of their single, circular chromosome.

Meiosis

Overview of Meiosis

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing four genetically unique gametes. It is essential for sexual reproduction.

• Meiosis: Cell division that produces haploid gametes from diploid cells.

Cancer and Cell Cycle Regulation

Cancer and Tumor Formation

Cancer is the uncontrolled division of abnormal cells. It often results from mutations in genes that regulate the cell cycle.

• Cancer: Disease caused by uncontrolled cell division.

• Benign Tumor: Non-cancerous growth that does not invade nearby tissues.

• Malignant Tumor: Cancerous growth that can invade and destroy surrounding tissue.

• Metastatic Tumor: Tumor cells that have spread from the original site to other parts of the body.

Genetic Regulation of the Cell Cycle

• Oncogenes: Mutated genes that drive uncontrolled cell division.

• Proto-oncogenes: Normal genes that can become oncogenes when mutated.

• Tumor-Suppressor Genes: Genes that inhibit cell division; mutations can lead to cancer.

• P53: A tumor-suppressor gene that plays a key role in preventing cancer by halting the cell cycle or initiating apoptosis if DNA is damaged.

Cancer Treatments

• Radiation Therapy: Uses high-energy radiation to kill cancer cells or shrink tumors.

• Chemotherapy: Uses drugs to kill or inhibit the growth of cancer cells.

• Angiogenesis: The formation of new blood vessels; tumors can stimulate angiogenesis to supply nutrients for growth.

Summary Table: Key Terms in Cell Division

Example: The Role of P53 in Cell Cycle Regulation

If DNA damage is detected at the G1 checkpoint, the p53 protein can halt the cell cycle and initiate DNA repair or trigger apoptosis (programmed cell death) if the damage is irreparable. Loss of p53 function is associated with many cancers.

Additional info:

• Cell cycle checkpoints are crucial for maintaining genetic stability and preventing the propagation of damaged DNA.

• Binary fission is simpler than mitosis and does not involve spindle formation or complex chromosome condensation.