Chapter 9: Meiosis

Meiosis Overview

• Purpose: Produces gametes (haploid cells) for sexual reproduction.

• Zygote Formation: Fusion of gametes forms a diploid zygote (2n).

Meiosis Process

• Stages: Two divisions (Meiosis I and Meiosis II).

  • Meiosis I: Reduction division (2n → n).

  • Homologous chromosomes separate.

  • Meiosis II: Similar to mitosis; sister chromatids separate in haploid cells.

Key Phases of Meiosis I

1. Prophase I: Homologous chromosomes pair; crossing over occurs.

2. Metaphase I: Chromosomes align randomly.

3. Anaphase I: Homologous chromosomes separate.

4. Telophase I: Nuclear division occurs; results in two haploid cells.

Meiosis II Highlights

• No DNA replication before Meiosis II.

• Similar steps to mitosis: Prophase II, Metaphase II, Anaphase II, Telophase II.

• Four haploid cells produced as the end result.

Unique Features of Meiosis

• Synapsis: Pairing of homologous chromosomes allows for crossing over.

• Reduction Division: Halves chromosome number, ensuring genetic variation in gametes.

Genetic Diversity Mechanisms

• Independent Assortment: Random orientation leads to varied gametes (over 8 million combinations).

• Crossing Over: Exchange of genetic material increases variation.

• Random Fertilization: Any two gametes can fuse, adding to diversity.

Consequences of Meiosis Errors

• Aneuploidy: Incorrect chromosome number due to misalignment or failure to separate homologous chromosomes or sister chromatids.