Chapter 11: Meiosis and Sexual Reproduction

Overview

• Sexual reproduction involves combining gametes (haploid cells) through fertilization.
• Two gametes merge to form a zygote, which is typically a diploid cell.
• This process enhances genetic diversity by mixing chromosomes from two individuals.

Gametes

• Gametes are sex cells, typically sperm and egg, which are haploid (one set of chromosomes).
• Meiosis is the process through which gametes are formed, involving reduction division.

Meiosis vs. Mitosis

• Mitosis: Produces two genetically identical diploid daughter cells.
• Meiosis: Involves two divisions, starting with one diploid cell to produce four genetically distinct haploid cells.
  • Meiosis I: Reduces the chromosome number by half, separating homologous chromosomes.
  • Meiosis II: Separates sister chromatids in haploid cells.

Life Cycle (Human Focus)

• Sexual maturity leads to gamete production: females produce eggs; males produce sperm.
• Fertilization results in a zygote, which undergoes mitosis to develop into an embryo and eventually a mature individual.

Twins

• Monozygotic Twins (Identical): Originate from a single zygote that splits.
• Dizygotic Twins (Fraternal): Result from two separate eggs fertilized by two separate sperm.

Sexual vs. Asexual Reproduction

• Many species can reproduce both sexually and asexually.
• Internal and external fertilization are forms of sexual reproduction.

Biological Sex and Intersex Conditions

• Biological sex involves a combination of chromosomes, genes, and environmental factors.
• Humans are not hermaphroditic but may display intersex characteristics.

Meiosis Details

• Stages:
  • Prophase I: Homologous chromosomes pair up and exchange genetic material (crossing over).
  • Metaphase I: Homologous chromosomes line up independently at the metaphase plate (Law of Independent Assortment).
  • Anaphase I: Separation of homologous chromosomes.
  • Telophase I: Formation of two haploid cells.
  • Meiosis II: Similar to mitosis, separates sister chromatids.

Genetic Variation

• Increased by crossing over, independent assortment during meiosis, and random fertilization.

Non-disjunction

• Occurs when chromosomes fail to separate properly.
• Can happen in Meiosis I or II, leading to genetic disorders due to aneuploidy.

Human Gametogenesis

• Spermatogenesis: Continuous production of sperm from puberty onward, resulting in four small sperm cells.
• Oogenesis: Females born with all oocytes; typically one egg matures per cycle, and three polar bodies are formed and often discarded.

Important Concepts

• Homologous Chromosomes: One from each parent; separated in Meiosis I.
• Sister Chromatids: Exact copies of each other; separated in Meiosis II.

Summary

• Meiosis and sexual reproduction are key to genetic diversity.
• Understanding the differences between meiosis and mitosis is crucial for grasping how organisms reproduce and maintain genetic diversity.