CELS191 Lecture 15: Eukaryotic Cell Division - Meiosis

Lecture Overview

• Course Title: CELS191 2025 Molecular Biology & Genetics
• Lecture Title: Eukaryotic Cell Division - Meiosis
• Professor: Hamish G. Spencer, Te Tari Mātai Kararehe, Department of Zoology

Learning Objectives

• Sexual Life Cycle: Understand the definition and significance.
• Meiosis Structures and Events: Identify and summarize the processes involved in meiosis.
• Comparison with Mitosis: Analyze the similarities and differences between meiosis and mitosis.
• Diversity in Gametic and Zygotic Formations: Explain the importance of this diversity for evolution.

Definitions

• Clonal Reproduction: Asexual reproduction leading to offspring that are genetically identical to the parent, seen in organisms like Hydra.
• Gametes: Reproductive cells (sperm and egg) that carry half the genetic information.
• Zygote: The diploid cell resulting from the fusion of two gametes.

Meiosis Overview

• Purpose: To produce haploid gametes from a diploid organism.
• Structure: Involves two rounds of cell division (Meiosis I and Meiosis II) and one round of DNA replication.
• Outcome: Four genetically distinct haploid cells.

Meiosis Stages

1. Meiosis I:

   • Prophase I: Homologous chromosomes pair and undergo crossing over at chiasmata.
   • Metaphase I: Homologous chromosomes align independently.
   • Anaphase I: Homologous chromosomes separate; sister chromatids remain attached.
   • Telophase I & Cytokinesis: Two haploid cells form, each with duplicated chromosomes.

2. Meiosis II:

   • Prophase II: Spindles form; chromosomes, still duplicated, prepare to align at the metaphase plate.
   • Metaphase II: Duplicated chromosomes align independently.
   • Anaphase II: Sister chromatids separate into individual daughter chromosomes.
   • Telophase II & Cytokinesis: Four genetically distinct haploid cells are formed.

Key Differences: Meiosis vs. Mitosis

• Chromosome Pairing: Meiosis involves synapsis and crossing over, while mitosis does not.
• Genetic Diversity: Meiosis produces genetically varied gametes; mitosis produces identical daughter cells.
• End Result:
  • Mitosis: 2 diploid cells (2n to 2n)
  • Meiosis: 4 haploid cells (2n to n)

Genetic Diversity Mechanisms

• Independent Assortment: Random distribution of chromosomes during gamete formation.
• Crossing Over: Exchange of genetic material between homologous chromosomes during Prophase I, increasing variation.
• Random Fertilization: Variety in sperm and egg combinations contribute to genetic diversity.

Importance of Genetic Diversity

• Provides adaptive advantages in changing environments.
• Enhances survival against diseases through variation in genes.
• Facilitates evolutionary flexibility and adaptability of species.

Evaluation Questions

1. What is the difference between haploid and diploid cells?
2. Explain the process of crossing over and its significance.
3. Order and describe the four phases of Meiosis I, including characteristics of chromosomes.
4. How many distinct gametes can be produced by an organism with 5 chromosome pairs through independent assortment?
5. Are cells at the end of Meiosis I and II genetically identical?
6. Describe the structures that separate during anaphase I vs. anaphase II and mitotic anaphase.