Mitosis and the Cell Cycle

Overview of the Mitotic Phase

The mitotic phase (M-phase) is a critical part of the cell cycle during which the nucleus and cytoplasm divide, resulting in two genetically identical daughter cells. This phase consists of mitosis (nuclear division) and cytokinesis (cytoplasmic division).

• Mitosis: Division of the cell's nucleus.

• Cytokinesis: Division of the cell's cytoplasm.

• Result: Two identical diploid daughter cells.

Phases of Mitosis

Mitosis is a continuous process divided into four distinct phases, each characterized by specific structural changes within the cell.

• Prophase: Chromosomes condense and become visible; centrioles move to opposite poles; the mitotic spindle forms; the nuclear membrane dissolves.

• Metaphase: Chromosomes align along the metaphase plate (center of the cell); spindle fibers attach to centromeres.

• Anaphase: Sister chromatids separate at the centromere and are pulled to opposite poles by the spindle apparatus; requires ATP.

• Telophase: Chromosomes arrive at poles; spindle disassembles; nuclear membranes reform; chromosomes decondense into chromatin.

Cytokinesis

Following mitosis, the cytoplasm divides through the formation of a contractile ring, resulting in two separate daughter cells. The process involves the formation of a cleavage furrow that pinches the cell in two.

• Daughter cells: Genetically identical to each other and to the parent cell.

Chromosome Number and Diploidy

• Human somatic cells contain 46 chromosomes (23 pairs).

• Cells with two sets of chromosomes are termed diploid (2n).

• 22 pairs are autosomes; 1 pair are sex chromosomes (X and Y).

Biological Importance of Mitosis

• Growth: Increases the number of cells in an organism.

• Tissue Repair: Replaces damaged or dead cells (e.g., epithelial cells, stomach lining, red blood cells).

Abnormal Cell Division and Cancer

Malignant Cell Division

Uncontrolled cell division can lead to cancer. Malignant cells divide uncontrollably and may invade other tissues.

• Causes: Mutations from carcinogens, viruses, or genetic errors.

• Treatments: Surgical removal, radiation therapy, chemotherapy (inhibits cell division).

Meiosis: Formation of Gametes

Overview of Meiosis

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing haploid gametes (sperm and egg cells). It consists of two successive divisions: Meiosis I and Meiosis II.

• Haploid cells (n): Contain one set of 23 chromosomes.

• Gametes: Sperm (male), ova (female).

• Fertilization: Fusion of gametes restores diploid number (46 chromosomes) in the zygote.

Stages of Meiosis

• Meiosis I: Homologous chromosomes separate, reducing chromosome number by half.

• Meiosis II: Sister chromatids separate, similar to mitosis.

Meiosis I Phases

• Prophase I: Homologous chromosomes pair up (tetrads); crossing over occurs, exchanging genetic material and increasing genetic diversity.

• Metaphase I: Tetrads align at the cell equator; independent assortment occurs.

• Anaphase I: Homologous chromosomes (not sister chromatids) separate to opposite poles.

• Telophase I and Cytokinesis: Two haploid cells form, each with duplicated chromosomes.

Meiosis II Phases

• Prophase II: Chromosomes condense in each haploid cell.

• Metaphase II: Chromosomes align at the equator.

• Anaphase II: Sister chromatids separate to opposite poles.

• Telophase II and Cytokinesis: Four non-identical haploid cells are produced.

Comparison of Mitosis and Meiosis

The following table summarizes the key differences between mitosis and meiosis:

Meiosis in Males vs. Females

• Males (Spermatogenesis): Meiosis produces four equal-sized sperm cells, each genetically unique.

• Females (Oogenesis): Meiosis produces one large ovum and smaller polar bodies (which may degenerate); more cytoplasm is allocated to the ovum.

• Timing: In females, meiosis II is completed only after fertilization.

Regulation of Cell Reproduction

Cell Cycle Control and Checkpoints

Cell division is tightly regulated by internal and external mechanisms to ensure proper growth and development.

• Cyclins: Proteins that activate regulatory proteins to initiate cell cycle events.

• Checkpoints: Control points at which the cell assesses readiness to proceed (G1, G2, and M phases).

• External factors: Nutrient and hormone availability can influence cell division.

Cell Differentiation and Environmental Influences

Cell Differentiation

Differentiation is the process by which a cell becomes specialized in structure and function, often due to selective gene expression. This can occur at any stage of development and is influenced by both genetic and environmental factors.

• Gene expression: Determines cell type and function.

• Environmental influences: Local signals and conditions can affect differentiation.

Factors Affecting Fetal Development

Exposure to certain environmental agents during development can cause abnormalities or affect survival.

• Cigarettes: Increased cancer risk in offspring.

• Alcohol: Fetal alcohol syndrome.

• Drugs: Thalidomide, marijuana, heroin, cocaine, etc.

• Chemicals: DDT, formaldehyde, PCB's, lead.

• Radiation: Ionizing and non-ionizing radiation.

• Infections: HIV, syphilis, rubella.

Key Terms and Concepts

• Diploid (2n): Cell with two sets of chromosomes.

• Haploid (n): Cell with one set of chromosomes.

• Chromatid: One of two identical halves of a duplicated chromosome.

• Centromere: Region where sister chromatids are joined.

• Spindle apparatus: Structure that separates chromosomes during cell division.

• Tetrad: Pair of homologous chromosomes during meiosis I.

• Crossing over: Exchange of genetic material between homologous chromosomes.

• Polar body: Small cell produced during oogenesis that usually degenerates.

Relevant Equations and Diagrams

• Chromosome number after mitosis:

• Chromosome number after meiosis:

Additional info: The process of crossing over during Prophase I of meiosis increases genetic diversity, which is essential for evolution and adaptation. Cell cycle checkpoints are crucial for preventing the propagation of damaged DNA, thereby reducing the risk of cancer.