Lecture 6 Slides

Lecture Overview

• Title: The Role of Sexual Reproduction in Evolution

• Course: Ch 13 Bio 80

• Graphic: By Kia Simon/KQED

Phylogeny Practice

• Individual Task: Create a phylogenetic tree with provided information.

  • Label derived characters.

• Collaborative Task: Compare your tree with peers and adjust cladograms for agreement.

Lecture Outcomes

By the end of this lecture, you should be able to:

1. Compare benefits and costs of asexual vs sexual reproduction.

2. Describe the alternation of fertilization and meiosis in sexual life cycles.

3. Explain how meiosis reduces chromosome pairs from diploid (2n) to haploid (n).

4. Describe each step of meiosis.

5. Identify drivers of genetic variation in sexual life cycles.

6. Compare meiosis and mitosis.

Understanding Heredity

• Heredity Defined: Transmission of traits across generations.

• Genes and Gametes:

  • Genes passed through reproductive cells (gametes: sperm and eggs).

  • Chromosomes carry most DNA—humans possess 46 chromosomes (somatic cells).

  • Each gene's specific position on a chromosome is termed its locus.

Asexual vs Sexual Reproduction

• Asexual Reproduction: One individual reproduces without gamete fusion, leading to clones.

• Sexual Reproduction: Involves fusion of gametes, resulting in offspring with unique genetic combinations.

• Life Cycle: Sequence of reproductive stages across generations.

The Cell Cycle Overview

• Processes: CSR, Mitosis, Meiosis (Table Not Included)

Chromosomes and Their Inheritance

• Chromosome Sets: Meiosis creates gametes (n) while somatic cells (2n) undergo mitosis.

• Sets in Humans: 23 chromosomes in gametes; 46 in zygotes and somatic cells.

Meiosis Process

Introduction

• Meiosis reduces chromosome sets from diploid to haploid, resulting in four daughter cells.

Stages of Meiosis

1. Meiosis I: Separates homologous chromosomes.

   • Includes Prophase I, Metaphase I, Anaphase I, Telophase I.

2. Meiosis II: Separates sister chromatids.

   • Includes Prophase II, Metaphase II, Anaphase II, Telophase II.

Comparison of Mitosis and Meiosis

• Mitosis: Produces 2 genetically identical diploid cells.

• Meiosis: Produces 4 genetically diverse haploid cells.

• Unique Events in Meiosis I:

  1. Synapsis and crossing over.

  2. Homologous alignment at metaphase plate.

  3. Homolog separation in anaphase I.

Genetic Diversity in Sexual Reproduction

• Mechanisms of Genetic Diversity:

  1. Independent Assortment: Random distribution of chromosomes.

  2. Crossing Over: Exchange of genetic material between homologous chromosomes.

  3. Random Fertilization: Genetic uniqueness of the zygote.

Human Life Cycle

• Gametes: Formed by meiosis; haploid (n = 23).

• Fertilization: Combines haploid gametes to form a diploid zygote (2n = 46).

• Development: Zygote undergoes mitosis, developing into an adult.

Types of Sexual Life Cycles

Overview

• Variations in organismal life cycles during meiosis and fertilization:

  • Animal Life Cycle: Gametes are haploid; no further division before fertilization.

  • Plant/Algal Life Cycle: Alternation of generations between diploid sporophytes and haploid gametophytes.

  • Fungal Life Cycle: Predominantly haploid; zygote is the only diploid stage.

Asexual Reproduction and Its Benefits

• Organisms may reproduce asexually for varied reasons:

  • Energy conservation.

  • Increased likelihood of reproduction under favorable conditions.

Case Studies in Asexual Reproduction

• Coscinasterias acutispina: Can reproduce both sexually and asexually, benefiting from dispersal and energetic efficiencies.

• Bdelloid Rotifers: Asexually reproducing but acquiring DNA from other organisms through horizontal gene transfer (HGT).

Conclusion and Questions

• Takeaway: The interplay of sexual and asexual reproduction shapes genetic diversity and adaptation.

• Open Floor: Ask questions for further clarification.