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:

  • Centrioles move to opposite poles.

  • Mitotic spindle forms from microtubules.

  • Nuclear membrane dissolves.

  • Centromeres on sister chromatids develop into separate structures.

• Metaphase:

  • Duplicated chromosomes align at the cell's equator (metaphase plate).

  • Mitotic spindle attaches to centromeres.

• Anaphase:

  • Centromeres split, separating sister chromatids (now daughter chromosomes).

  • Daughter chromosomes move to opposite poles, powered by ATP and contractile proteins.

• Telophase:

  • Chromosomes arrive at poles.

  • Mitotic spindle disassembles.

  • Nuclear membranes reform around chromosomes.

  • Chromosomes uncoil into chromatin.

Cytokinesis

• Contractile ring forms just inside the plasma membrane.

• Ring tightens, creating a cleavage furrow that pinches the cell into two.

• Results in two identical daughter cells.

Chromosome Number and Diploidy

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

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

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

Biological Importance of Mitosis

• Essential for growth, tissue repair, and replacement of worn-out cells.

• Examples: Epithelial cells, stomach lining, red bone marrow (producing ~2 million RBCs/sec).

Abnormal Cell Division

• Malignant cancer cells: Exhibit uncontrolled division and can spread (metastasize).

• Causes: Mutations from carcinogens or viruses.

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

Meiosis: Reduction Division and Gamete Formation

Overview of Meiosis

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing haploid gametes (sperm and eggs). This process ensures genetic diversity and is essential for sexual reproduction.

• Involves 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.

Stages of Meiosis

• Meiosis I: Homologous chromosomes separate.

  • 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 independently (independent assortment).

  • Anaphase I: Homologous pairs separate to opposite poles.

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

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

  • Results in four haploid cells, each genetically unique.

Key Features of Meiosis

• Crossing Over: Occurs during Prophase I; homologous chromosomes exchange segments, increasing genetic variation.

• Independent Assortment: Random alignment of tetrads during Metaphase I leads to varied combinations of maternal and paternal chromosomes.

• Genetic Diversity: Result of crossing over and independent assortment.

Meiosis in Males vs. Females

• Males (Spermatogenesis):

  • Produces four equal-sized sperm cells per meiosis.

  • All sperm are genetically unique.

• Females (Oogenesis):

  • Produces one large ovum and smaller polar bodies (which may degenerate).

  • More cytoplasm is allocated to the ovum.

  • Meiosis II is completed only after fertilization.

Fertilization and Chromosome Number

• Fusion of haploid sperm and egg restores the diploid chromosome number (46 in humans).

• The resulting cell is called a zygote.

Comparison of Mitosis and Meiosis

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

Regulation of Cell Reproduction

Cell Cycle Control and Checkpoints

• Cell division is regulated by internal and external mechanisms.

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

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

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

Cell Differentiation and Environmental Influences

Cell Differentiation

• Process by which a cell becomes specialized and different from its parent or sister cells.

• Driven by selective gene expression.

• Can occur at any stage of development or during the cell's lifespan.

Environmental Factors Affecting Differentiation and Development

• External factors can influence gene expression and cell fate, especially during early development.

• Examples of harmful influences:

  • Cigarettes (increased cancer risk in offspring)

  • Alcohol (fetal alcohol syndrome)

  • Legal/illegal drugs (e.g., thalidomide, heroin, cocaine)

  • Chemical exposures (DDT, formaldehyde, PCB's, lead)

  • Radiation (ionizing and non-ionizing)

  • Intrauterine infections (HIV, syphilis, rubella)

Key Terms and Definitions

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

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

• Gamete: Mature sex cell (sperm or egg).

• Zygote: Fertilized egg cell.

• Crossing Over: Exchange of genetic material between homologous chromosomes during Prophase I of meiosis.

• Independent Assortment: Random distribution of homologous chromosomes during Metaphase I of meiosis.

• Cytokinesis: Division of the cytoplasm following nuclear division.

• Centromere: Region where sister chromatids are joined.

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

Relevant Equations

• Chromosome Number in Gametes: Where is the haploid number and is the diploid number.

Example: Human Chromosome Numbers

• Somatic cell: chromosomes

• Gamete: chromosomes

• Zygote (after fertilization): chromosomes

Additional info: The process of cell differentiation is crucial for the development of specialized tissues and organs. Environmental factors can disrupt normal development, leading to congenital abnormalities or increased disease risk later in life.