Mitosis and the Cell Cycle

Overview of Mitosis

Mitosis is the process by which a eukaryotic cell divides its nucleus and cytoplasm to produce two genetically identical daughter cells. This process is essential for growth, tissue repair, and asexual reproduction in multicellular organisms.

• Mitosis: Division of the nucleus.

• Cytokinesis: Division of the cytoplasm.

• End result: Two diploid daughter cells, each with 46 chromosomes in humans.

Phases of Mitosis

• Prophase: Chromosomes condense and become visible; centrioles move to opposite poles; mitotic spindle forms; nuclear envelope breaks down.

• Metaphase: Chromosomes align at 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; nuclear envelopes reform; chromosomes decondense into chromatin; spindle disassembles.

• Cytokinesis: Contractile ring forms a cleavage furrow, dividing the cytoplasm and producing two identical daughter cells.

Chromosome Number and Types

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

• 22 pairs of autosomes; 1 pair of sex chromosomes (XX or XY).

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

Functions and Examples of Mitosis

• Growth and development (e.g., epithelial cells, stomach lining).

• Tissue repair (e.g., red bone marrow produces new red blood cells).

Abnormal Cell Division

• Malignant cancer cells: Result from uncontrolled cell division, often due to mutations from carcinogens or viruses.

• Treatments: Removal (surgery, radiation), or inhibition of cell division (chemotherapy).

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). This process introduces genetic diversity and is essential for sexual reproduction.

• Two successive divisions: Meiosis I and Meiosis II.

• Produces four non-identical haploid cells (n = 23 in humans).

• Gametes: Ova (eggs) in females, sperm in males.

Phases of Meiosis

• Meiosis I: Homologous chromosomes separate.

  • Prophase I: Homologous chromosomes pair up (synapsis) and exchange genetic material (crossing over), forming tetrads.

  • Metaphase I: Tetrads align at the cell equator.

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

  • Telophase I: Cytokinesis produces two haploid cells, each with duplicated chromosomes.

• Meiosis II: Sister chromatids separate (similar to mitosis).

  • 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: Cytokinesis produces four non-identical haploid cells.

Key Features of Meiosis

• Crossing Over: Exchange of genetic material between homologous chromosomes during Prophase I, increasing genetic variation.

• Independent Assortment: Random alignment of homologous pairs during Metaphase I leads to genetic diversity.

• Fertilization: Fusion of haploid gametes restores diploid chromosome number in the zygote.

Meiosis in Males vs. Females

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

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

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

Comparison of Mitosis and Meiosis

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 specific cell cycle events.

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

• External influences: Nutrient and hormone availability can affect cell division rates.

Cell Differentiation and Environmental Influence

Cell Differentiation

• Process by which a cell becomes specialized in structure and function.

• Results from selective gene expression, influenced by internal and external factors.

• Can occur at any stage of development.

Environmental Factors Affecting Development

• Exposure to harmful substances during development can cause abnormalities or death of the embryo/fetus.

• Examples of teratogens and their effects:

  • Cigarettes: Increased cancer risk in offspring.

  • Alcohol: Fetal alcohol syndrome.

  • Drugs: Thalidomide, marijuana, heroin, cocaine.

  • Chemicals: DDT, formaldehyde, PCBs, lead.

  • Radiation: Ionizing and non-ionizing radiation.

  • Infections: HIV, syphilis, rubella.

Additional info:

• Cell cycle checkpoints are crucial for preventing the propagation of damaged DNA, which can lead to cancer.

• Crossing over and independent assortment during meiosis are the main sources of genetic variation in sexually reproducing organisms.