5.1/5.2 - Meiosis and Genetic Diversity

Introduction to Genetics

• Genetics: The study of heredity and hereditary variation.

• Heredity: The transmission of traits from one generation to the next, from parents to offspring. Genetics explores how these traits are passed on and expressed. It's important to note that not all traits are solely determined by DNA; environmental factors such as temperature, stress, and other conditions can also influence trait development.

Genes and Chromosomes

• Genes: Segments of DNA that code for the basic units of heredity. These are the instructions for making proteins that carry out various functions in the body.

• Inheritance: Offspring acquire genes from their parents by inheriting chromosomes. Each gamete (sperm or egg) carries a unique combination of genes, ensuring genetic diversity.

Homologous Chromosomes

• Definition: A pair of chromosomes (one from each parent) that are similar in size, shape, and genetic content. They carry the same genes, although the specific versions (alleles) of these genes may differ.

• Inheritance: Each homologous chromosome pair consists of one chromosome from the mother and one from the father. While they contain the same types of genes (such as those for plant height), the specific alleles may vary (one may code for tall plants, the other for short).

• Genetic Variation: During meiosis, homologous chromosomes can exchange segments through a process called crossing over, leading to genetic recombination. This results in four unique gametes, each with a different combination of alleles, increasing genetic diversity in offspring.

Meiosis

Meiosis is a specialized type of cell division that results in four genetically unique haploid cells, crucial for sexual reproduction. It ensures genetic diversity and reduces the chromosome number by half, which is essential for forming gametes (sperm and eggs in animals, pollen and ovules in plants).

Key Stages of Meiosis

1. Interphase:

• Preparation: The cell undergoes G1, S (where DNA replication occurs), and G2 phases. Chromosomes are in the chromatin form.

2. Meiosis I:

• Prophase I:

  • Synapsis: Homologous chromosomes pair up to form tetrads (four sister chromatids).

  • Crossing Over (Recombination): Occurs at the chiasmata, where homologous chromosomes exchange genetic material, increasing genetic diversity.

• Metaphase I:

  • Independent Orientation: Tetrads line up randomly along the metaphase plate. The arrangement is random, which contributes to genetic variation.

• Anaphase I:

  • Homologous Chromosome Separation: Homologous chromosomes are pulled to opposite poles. Sister chromatids remain attached.

• Telophase I and Cytokinesis:

  • Formation of Two Haploid Cells: Each cell now has a haploid set of chromosomes, but each chromosome still consists of two sister chromatids.

Meiosis II:

• Prophase II:

  • Spindle Formation: The spindle apparatus forms in each haploid cell, and chromosomes condense.

• Metaphase II:

  • Chromosome Alignment: Chromosomes line up at the metaphase plate similar to mitosis, but with half the chromosome number.

• Anaphase II:

  • Sister Chromatid Separation: Sister chromatids are separated and move towards opposite poles of the cell.

• Telophase II and Cytokinesis:

  • Formation of Four Haploid Cells: Nuclei reappear around each set of chromosomes, and the cells divide, resulting in four genetically unique haploid cells.

Key Takeaways

• Haploid Cells: Meiosis produces four haploid cells from a single diploid parent cell, each with half the number of chromosomes (e.g., 23 in humans).

• Genetic Diversity:

  • Crossing Over: Occurs in Prophase I, creating genetic variation.

  • Independent Assortment: During Metaphase I, chromosomes are distributed randomly, contributing to genetic diversity.

  • Random Fertilization: The combination of gametes during fertilization adds to genetic variability.

• Nondisjunction: If chromosomes or sister chromatids fail to separate properly during meiosis, it results in gametes with abnormal chromosome numbers, which can lead to genetic disorders.