BIO105 4-4

Mitosis & Meiosis Overview

  • Chapter 17 Sections 17.3 - 17.5

Lecture Objectives

  • By the end of the class, you should be able to:

    • Use appropriate terminology to explain what is occurring during each stage of mitosis.

    • Discuss some causes and treatments of abnormal cell division.

    • Explain processes occurring in each phase of meiosis related to eukaryotic cells.

    • Compare differences between mitosis and meiosis.

    • List differences between male and female gametes during development.

    • List checkpoints related to the regulation of cell reproduction.

    • Discuss the influence of environmental factors on cell differentiation.

Mitotic Phase

  • The nucleus and cell cytoplasm divide:

    1. Mitosis: Process of nuclear division.

    2. Cytokinesis: Process of cytoplasm division.

  • End result: Two identical daughter cells.

Mitosis Process

  • Defined by structural changes inside the cell.

  • Four Phases:

    1. Prophase

    2. Metaphase

    3. Anaphase

    4. Telophase

  • Complete process takes approximately 60 minutes.

Mitosis - Prophase

  • Prophase marks the beginning:

    • Centrioles move to opposite poles of the cell.

    • Mitotic spindle reforms from microtubules.

    • Nuclear membrane disappears.

    • Centromeres develop into structures for each duplicated DNA molecule.

Mitosis - Metaphase

  • The “position-changing phase”:

    • Takes about 20 minutes.

    • Duplicated chromosomes align across the center of the cell (metaphasic plate).

    • Mitotic spindle attaches to centromeres.

Mitosis - Anaphase

  • The “apart phase”:

    • Takes approximately 1 minute.

    • Centromeres split, detaching sister chromatids.

    • Chromatids (now daughter chromosomes) move to opposite poles, requiring ATP for movement.

    • Results in two identical sets of chromosomes at opposite ends of the cell.

Mitosis - Telophase

  • The “end phase”:

    • Begins once chromosomes reach the poles.

    • Reverses prophase:

      • Mitotic spindle disassembles.

      • Nuclear membranes form around chromosomes.

      • Chromosomes uncoil to chromatin.

Cytokinesis

  • Cytoplasm divides:

    • Contractile ring of protein filaments forms.

    • Cleavage furrow forms to pinch the cell into two.

    • New cells (daughter cells) are identical to each other and to the parent cell.

Daughter Cells of Mitosis

  • In humans, somatic cells (except sperm and egg):

    • 46 chromosomes (diploid).

    • 22 pairs of autosomes and 1 pair of sex chromosomes (X & Y).

Mitosis & Cytokinesis in the Body

  • Constant cellular division in certain areas requires:

    • Replacement of epithelial cells.

    • Restoration of stomach lining.

    • Production of red blood cells at a rate of 2 million per second.

Abnormal Cell Division

  • Malignant cancer cells:

    • Uncontrolled division, spreading throughout tissues.

    • Causes include mutations from carcinogens and viruses.

    • Treatments involve removal of malignant cells or chemotherapy.

Meiosis

  • Also known as “Reduction-Division”:

    • Two successive divisions reducing chromosome number by half.

    • Resulting cells (gametes) have one set of chromosomes (haploid).

Gametes Created by Meiosis

  • Ova (egg) in females:

    • Occurs in ovaries (oogenesis).

  • Sperm in males:

    • Occurs in testicles (spermatogenesis).

  • After fertilization, the zygote has a diploid number of chromosomes.

Process of Meiosis

  • Prior to meiosis, cells replicate DNA during S-phase.

  • Meiosis consists of:

    1. Meiosis I

    2. Meiosis II

  • Each stage has four phases: Prophase, Metaphase, Anaphase, Telophase.

Meiosis I - Prophase I

  • Proceeds similarly to mitosis prophase with a crucial difference:

    • Crossing over occurs:

      • Homologous chromosomes form tetrads and exchange DNA.

      • Results in genetic recombination.

Meiosis I - Remaining Phases

  • Metaphase I: Tetrads align along the equator independently.

  • Anaphase I: Homologous pairs separate to opposite poles.

  • Telophase I: Cytokinesis occurs, resulting in two non-identical haploid daughter cells.

Meiosis II

  • Process resembles mitosis:

    • Sister chromatids separate.

  • End result: 4 non-identical daughter cells, each haploid (23 chromosomes).

Comparison: Mitosis vs. Meiosis

Feature

Mitosis

Meiosis

Crossover

No crossover

Crossover occurs

Results

Diploid cells (46 chromosomes)

Haploid cells (23 chromosomes)

Daughter Cells

2 identical daughter cells

4 non-identical daughters

Occurrence

Occurs in somatic cells

Occurs in sex cells

Meiosis in Males

  • Produces four equal-sized sperm that are genetically diverse.

Meiosis in Females

  • Eggs are precious in females; one daughter cell receives more cytoplasm (secondary oocyte).

    • Polar body may or may not divide further.

  • Secondary oocyte arrested at Metaphase II until fertilization.

Regulation of Cell Reproduction

  • Not all cells divide at the same rate, controlled by:

    • Internal mechanisms (cyclins activate proteins).

    • Checkpoints at G1, G2, and M phases.

    • External influences such as nutrient or hormone availability.

Differentiation

  • The process whereby a cell becomes specialized:

    • Influenced by gene expression and environmental factors.

Factors Affecting Fetal Development

  • External factors can disrupt embryogenesis:

    • Cigarettes, alcohol, legal/illegal drugs, chemical exposures, radiation, intrauterine infections.

References

  • Johnson, M.D. (2008). Human biology: Concepts and current issues (5th ed). Pearson Education Inc.

  • Johnson, M.D. (2012). Human biology: Concepts and current issues (6th ed). Pearson Education Inc.

  • Johnson, M.D. (2017). Human biology: Concepts and current issues (8th ed). Pearson Education Inc.