Sexual Reproduction & Meiosis

Chapter 19: Sexual Reproduction and Genetics

Lecture Overview

  • Objectives:

    • Understand the benefits of sex

    • Understand meiosis and fertilization

Asexual vs. Sexual Reproduction

  • Asexual Reproduction:

    • Definition: Gives rise to offspring that are genetically identical to the parent.

    • Example: Budding in hydra.

  • Sexual Reproduction:

    • Definition: Involves the mixing of DNA from two individuals to produce offspring that are genetically distinct from each other and both parents.

Costs and Benefits of Reproduction

  • Sexual Reproduction:

    • Costs:

    • Requires finding a mate.

    • Only passes on ½ of genetic material to offspring.

    • Benefits:

    • Generates genetic diversity, which provides a competitive advantage in changing environments.

  • Asexual Reproduction:

    • Pros:

    • Independent reproduction, passes on all genetic material.

Cellular Basis of Sexual Reproduction

  • Involvement of Diploid and Haploid Cells:

    • Germ-Line Cells:

    • A small number of cells in the embryo are differentiated for germ-line cells.

  • Human Somatic Cells:

    • Diploid (two copies of each chromosome):

    • Composition: One homologous chromosome from the mother and one from the father.

  • Germ Cells:

    • Definition: Haploid (one copy of each chromosome).

    • Example: In humans, there are 46 chromosomes total, which leads to 23 pairs (diploid).

Meiosis vs. Mitosis

  • Mitosis:

    • Description:

    • 1 round of DNA replication.

    • 1 round of nuclear division.

    • Results in 2 daughter cells.

  • Meiosis:

    • Description:

    • 1 round of DNA replication.

    • 2 rounds of nuclear division.

    • Results in 4 daughter cells.

Steps of Meiosis

  • Meiosis I and II:

    • Meiosis I

    • Meiosis II

Meiosis I: Key Steps

  1. Chromosome Duplication:

    • Chromosome duplication occurs before meiosis begins.

  2. Pairing and Recombination:

    • Homologous chromosomes pair and recombine.

  3. Metaphase Alignment:

    • Duplicated homologs line up on the metaphase spindle.

  4. Separation of Homologs:

    • Occurs at anaphase of meiosis I, resulting in two non-identical haploid cells.

Meiosis II: Key Steps

  1. Separation of Sister Chromatids:

    • Occurs at anaphase of meiosis II, separating sister chromatids into distinct cells.

  2. End Results of Meiosis II:

    • 4 haploid cells that are not identical to one another.

Genetic Variation in Meiosis

  • Recombination:

    • Homologous chromosomes can trade genetic material during crossing over.

    • Chiasma:

    • Definition: Point at which homologous chromosomes exchange genetic material during prophase.

Nondisjunction Errors

  • Definition:

    • Nondisjunction: Failure of chromosomes to separate properly during meiosis.

    • Example: Down syndrome is caused by nondisjunction at chromosome 21, resulting in three copies in the embryo.

Mechanisms for Generating Genetic Diversity

  • Homologous Recombination:

    • Involves crossing over during meiotic prophase.

  • Independent Assortment:

    • Independent assortment refers to the distribution of maternal and paternal homologs during meiosis I.

    • This can lead to various combinations of alleles in the gametes produced.

Conclusion

  • The study of meiosis emphasizes the significance of genetic diversity and the mechanisms involved in sexual reproduction, illustrating both the processes and implications for evolution and genetics.