PPT 8

Cell Division and Meiosis

Advantages of Sexual Reproduction

• Provides genetic variation through the creation of egg and sperm.

• Particularly advantageous in variable environments.

Internal Fertilization

• Allows sperm to fertilize egg in a dry environment.

• Reduces dependency on water for reproduction.

• More efficient compared to external fertilization.

Meiosis Overview

Reproductive System

• Involves males and females; cyclical process.

• Diploid (2n) vs. Haploid (n) Concept:

  • Diploid: Full set of chromosomes (e.g., Humans: 2n = 46).

  • Haploid: Half set of chromosomes (n = 23).

Key Processes

• Cells undergo meiosis must first replicate (similar to mitosis).

• Two rounds of division:

  • Meiosis I: Homologous chromosomes separate.

  • Meiosis II: Sister chromatids separate.

• Results in 4 daughter cells.

Chromosome Structure in Meiosis

Chromatid Packing

• Chromosomes undergo several levels of packing.

  • 10-nm fibre coils to form 30-nm fibre (looped domains).

  • Involves histones and interaction with linker DNA.

  • Final structure observed during metaphase, chromatid width: 700 nm.

Homologous Chromosomes

• Maternal and paternal chromosome sets involved.

• Sister chromatids are identical copies of a chromosome.

Phases of Meiosis I

Interphase

• Similar to mitosis, chromosomes replicate.

Prophase I

• Chromosomes condense, homologous pairing occurs (synapsis).

• Tetrads form; crossing over happens at chiasmata—this is a key source of genetic variation.

• Spindle apparatus begins to form.

Metaphase I

• Homologous chromosomes line up at the metaphase plate.

• Kinetochore microtubules attach to one chromosome of each tetrad.

Anaphase I

• Spindle fibers guide whole chromosomes toward poles (disjunction).

• Non-disjunction can result in conditions like Down syndrome, Turner syndrome, or Klinefelter syndrome.

Phases of Meiosis II

Overview of Meiosis II

• Another round of division: separates sister chromatids.

  • Prophase II: Chromosomes reform and prepare to line up.

  • Metaphase II: Chromatids align at the metaphase plate.

  • Anaphase II: Sister chromatids move toward opposite poles.

  • Telophase II / Cytokinesis: Four haploid cells formed, genetically unique.

Sources of Variation

• Crossing over during Prophase I.

• Independent assortment during Metaphase I.

• Fertilization introduces further genetic diversity.

Key Terminology

Aneuploidy Examples

• Down Syndrome (Trisomy 21).

• Other conditions include Turner syndrome and Klinefelter syndrome.

Synapsis

• Process during Prophase I where homologous chromosomes pair.

• Chiasmata form where crossing over occurs.

Male vs. Female Meiosis Differences

• Differences in timing, cellular structures, and products.

• Male meiosis: Continuous process, leads to four viable sperm.

• Female meiosis: Discontinuous, typically produces one viable egg and polar bodies.

Practical Application

• Practice labelling key structures in meiosis diagrams (Fig 13.9).

• Understand terms associated with meiosis and their diagrams.