Cell Division: Mitosis and Meiosis

Mitosis

  • Definition: Mitosis is a type of cell division crucial for various functions including:

    • Embryonic development

    • Promoting tissue growth after birth

    • Replacing damaged or dying cells

  • Process Overview:

    • In mitosis, there is one division of the parental cell.

    • The result is two daughter cells.

    • Each daughter cell contains the same number of chromosomes as the parental cell (diploid).

  • Chromosome Configuration:

    • Human diploid cells have 23 pairs of chromosomes.

    • Each pair consists of one maternal and one paternal chromosome.

  • Phases of Mitosis:

    1. Prophase:

    • Chromatin condenses into chromosomes.

    • Each chromosome is composed of two identical sister chromatids.

    • Nuclear envelope dissolves.

    • Spindle fibers begin to grow from the cell's centrosomes.

    1. Metaphase:

    • Spindle fibers align chromosomes in the center of the cell.

    1. Anaphase:

    • Each chromosome splits into two genetically identical chromatids.

    • Each chromatid, now a single-stranded daughter chromosome, migrates to opposite ends of the cell.

    1. Telophase:

    • Nuclear envelopes reform around the chromosomes as cell division completes.

Meiosis

  • Definition: Meiosis is a specialized type of cell division aimed at producing gametes (eggs and sperm).

  • Process Overview:

    • Meiosis involves two successive divisions.

    • Results in four daughter cells, each with half the chromosome number of the parental cell (haploid).

  • Chromosome Configuration:

    • Human haploid cells contain 23 unpaired chromosomes.

  • Phases of Meiosis:

    1. Meiosis I (First Division):

    • Prophase I:

      • Chromosomes condense.

      • Late in prophase I, crossing over occurs where chromatids exchange corresponding sections of DNA, leading to genetic diversity.

    • Metaphase I:

      • Homologous chromosome pairs line up in the center of the cell.

      • Chromosome pairs can align randomly, a process called independent assortment.

    • Anaphase I:

      • Homologous chromosomes separate.

    • Telophase I:

      • Cell divides, producing two haploid daughter cells.

    1. Meiosis II (Second Division):

    • Prophase II:

      • Cell prepares to divide again.

    • Metaphase II:

      • Chromosomes line up in the center of the cell.

    • Anaphase II:

      • Each chromosome is pulled apart into two sister chromatids (now single-stranded).

    • Telophase II:

      • Cells divide, resulting in a total of four haploid daughter cells.

  • Outcome:

    • Once meiosis is completed, the resulting male and female gametes contain 23 unique single-stranded chromosomes.

    • This gamete combination contributes to genetic variability in humans.

  • Fertilization:

    • When gametes meet, they form a single fertilized cell called a zygote.

    • The zygote has 46 chromosomes and develops further through mitosis.