Mitiosis and Meiosis

Unit 5.2: Mechanisms of Cell Division

Learning Objectives

  1. Identify the stages of M phase/cytokinesis for somatic cells on a cell cycle diagram.

  2. Compare and contrast the stages of M phase in somatic versus germ line cells with respect to:

    • Key events that occur in each phase

    • Ploidy (number of chromosomes) of daughter cells

    • Genetic similarity

  3. Predict the genetic makeup of daughter cells produced via mitosis versus meiosis.

Cell Types

  • Germ cells (haploid): Contains 1 set of chromosomes (1n)

  • Somatic cells (diploid): Contains 2 sets of chromosomes (2n)

Genome after Fertilization

  • Egg (n) and Sperm (n) fuse during fertilization to form:

    • Zygote (2n)

    • The zygote contains 23 pairs of homologous chromosomes.

    • Example of Diploid Offspring: Zygote has a diploid karyotype consisting of chromosomes 1 through 22 plus sex chromosomes (XY or XX).

Overview of Cell Cycle Stages

  • Interphase:

    • G₁ Phase

    • S Phase (DNA replication)

    • G₂ Phase

  • M Phase:

    • Includes Mitosis (nuclear division) and Cytokinesis (cytoplasmic division)

Detailed Stages of Cell Cycle

  1. G₁ Phase: Cell grows and synthesizes proteins.

  2. S Phase:

    • DNA replication occurs.

    • Results in 2 identical sister chromatids, connected at the centromere.

    • Ensures genetic material is accurately distributed in cell division.

  3. G₂ Phase:

    • Cell continues to grow and prepares for division.

    • Checks for DNA replication errors before entering M Phase.

  4. M Phase:

    • Mitosis involves several key stages:

      • Prophase: Chromosomes condense and spindle fibers emerge.

      • Metaphase: Chromosomes align at the cell's equator.

      • Anaphase: Sister chromatids are pulled apart to opposite poles.

      • Telophase: Nuclear membranes reform around each set of chromosomes.

      • Cytokinesis: Cytoplasm divides, resulting in two daughter cells.

Comparison: Mitosis vs. Meiosis

Mitosis

  • Outcomes:

    • Produces 2 diploid daughter cells (2n), genetically identical to the parent cell.

    • Key for growth, tissue repair, and asexual reproduction.

    • Mitosis phases: Prophase, Metaphase, Anaphase, Telophase, Cytokinesis.

  • Cell Type: Somatic cells (e.g., blood cells, muscle cells).

Meiosis

  • Outcomes:

    • Produces 4 haploid daughter cells (1n), each genetically diverse.

    • Essential for sexual reproduction and gamete formation.

    • Meiosis phases include two rounds: Meiosis I (homologous chromosomes separate) and Meiosis II (sister chromatids separate).

  • Cell Type: Germ line cells (e.g., egg and sperm).

Phases of Meiosis

  1. Meiosis I:

    • Prophase I: Homologous chromosomes pair and exchange genetic information (crossing over).

    • Metaphase I: Paired homologous chromosomes align.

    • Anaphase I: Homologous chromosomes are pulled apart.

    • Telophase I and Cytokinesis: Two daughter cells are formed, each haploid but with replicated chromosomes.

  2. Meiosis II:

    • Prophase II: Chromosomes condense and prepare for division again.

    • Metaphase II: Chromosomes align at the equator individually.

    • Anaphase II: Sister chromatids are separated and pulled to opposite poles.

    • Telophase II and Cytokinesis: Results in four genetically distinct haploid cells.

Key Concepts

  • Chromosomal Conditions: Understanding the number of chromosomes in parent and daughter cells:

    • Parent cell may be haploid or diploid, determining the type of division.

    • Distinguishing between chromosomal states in result cells A-D as shown in simplified diagrams.

  • Polymerase Chain Reaction (PCR):

    • Technique for amplifying DNA; involves primers, nucleotides, and a heat-resistant polymerase (Taq polymerase).

    • Example: Used in genetic research/diagnosis, such as detecting mutations in the BRCA gene associated with breast cancer.

Practical Implications and Applications

  • Knowledge of mitosis and meiosis is critical in fields such as genetics, medicine, and biotechnology.

  • Understanding cell division mechanisms aids in comprehending developmental biology, cancer research, and evolutionary biology.