Meiosis

Chromosome Number

  • Homologous chromosomes: matching pairs of chromosomes.

    • One from each parent (Dad and Mom).

  • Diploid cells (2N): contain both sets of chromosomes (46 in humans).

  • Haploid cells (1N): contain only one set of chromosomes (23 in humans).

Diploid vs. Haploid

  • DIPLOID: Contains both sets of chromosomes (1 from each parent).

  • HAPLOID: Contains one set of chromosomes, which combine during sexual reproduction with another haploid to form a diploid cell.

Phases of Meiosis

  • Meiosis is a type of cell division that produces haploid cells (gametes) from diploid cells.

    • Male gamete: sperm.

    • Female gamete: egg.

Meiosis Stages

  1. Meiosis I

    • Interphase I

    • Prophase I

    • Metaphase I

    • Anaphase I

    • Telophase I and Cytokinesis.

  2. Meiosis II

    • Prophase II

    • Metaphase II

    • Anaphase II

    • Telophase II and Cytokinesis.

Detailed Stages of Meiosis

Interphase I

  • Consists of G1 phase, S phase, G2 phase.

  • The normal diploid cell replicates its DNA, forming sister chromatids.

Prophase I

  • DNA coils tightly into chromosomes.

  • Spindle fibers appear.

  • Nucleus and nucleolus disappear.

  • Chromosomes pair into homologous chromosomes, forming a Tetrad:

    • Synapsis: the pairing of homologous chromosomes.

    • Crossing over happens, resulting in genetic recombination of alleles.

Metaphase I

  • Homologues align at the metaphase plate.

  • Spindle fibers attach to the centromeres of chromosomes.

Anaphase I

  • Homologues are randomly pulled to opposite poles (maternal and paternal chromosomes).

  • This random separation is referred to as independent assortment.

Telophase I & Cytokinesis

  • Nuclear membrane reforms.

  • The cell separates into haploid (1N) cells creating two daughter cells through cytokinesis.

Meiosis II

Prophase II

  • No DNA replication occurs.

  • New centrioles form.

  • Nuclear membrane dissolves.

Metaphase II

  • Chromosomes align at the metaphase plate.

  • Spindle fibers attach to the centromeres of chromosomes.

Anaphase II

  • Sister chromatids separate and move to opposite ends of the cell.

Telophase II & Cytokinesis

  • Results in four haploid (1N) daughter cells.

Gamete Formation

Male Gamete Formation

  • Spermatogenesis: formation of male gametes (sperm).

  • In male animals, haploid gametes are sperm, while in some plants, pollen contains haploid sperm cells.

Female Gamete Formation

  • Oogenesis: formation of female gametes.

  • Typically produces one egg in female animals, with three polar bodies reabsorbed by the body.

  • Egg cell in plants is the female gamete.

Comparing Mitosis and Meiosis

  • Mitosis:

    • Produces two genetically identical diploid cells.

    • Cells are used for growth and replacement.

    • Also used in asexual reproduction.

  • Meiosis:

    • Produces four genetically different haploid cells.

    • Cells are produced for sexual reproduction (gametes).

Summary of Reproduction

  • During sexual reproduction, gametes (sperm or eggs) are produced, containing half the chromosome number found in somatic cells.

  • Upon fertilization, gametes unite to form a zygote with a complete set of chromosomes (diploid).

Asexual Reproduction

  • Asexual reproduction produces genetically identical offspring from one parent.

    • Unicellular organisms: can reproduce through binary fission or mitosis.

    • Multicellular organisms: can reproduce through budding off.

Review Points

  • Body cells (diploid cells, 2N) contain both sets of chromosomes (46 in humans).

  • Gametes (haploid cells, 1N) have one set of chromosomes (23 in humans).

  • Mitosis produces two genetically identical diploid cells; Meiosis produces four genetically different haploid cells.