Meiosis and Genetic Variation

Learning Objectives

Explain the purpose of meiosis in sexual reproduction.

Describe the key events of Meiosis I and Meiosis II.

Distinguish between mitosis and meiosis.

Explain how crossing over and independent assortment create genetic variation.

Creating Genetic Diversity: Meiosis

Meiosis is a special type of cell division that reduces the number of chromosomes in the parent cell by half and produces four genetically distinct gamete cells. This process is essential for sexual reproduction.

Purpose of Meiosis

To produce haploid (n) gametes (sperm and egg cells) from a diploid (2n) parent cell.

To introduce genetic variation into the offspring.

The Stages of Meiosis

Meiosis involves two consecutive rounds of division: Meiosis I and Meiosis II.

Meiosis I: Separation of Homologous Chromosomes

The key events that create genetic variation occur here.

Prophase I: Homologous chromosomes (one inherited from each parent) pair up to form a structure called a bivalent. Crossing over occurs, where segments of DNA are exchanged between the homologous chromosomes, creating new combinations of alleles.

Metaphase I: The homologous pairs line up at the metaphase plate. The orientation of each pair is random, a process called independent assortment.

Anaphase I: Homologous chromosomes are pulled to opposite poles. Sister chromatids remain attached.

Telophase I: The cell divides, resulting in two haploid cells, each with a duplicated set of chromosomes.

Meiosis II: Separation of Sister Chromatids

This stage is very similar to mitosis.

Prophase II & Metaphase II: Chromosomes line up individually at the metaphase plate.

Anaphase II: Sister chromatids are pulled apart to opposite poles.

Telophase II: The cells divide, resulting in a total of four genetically unique haploid cells.

Meiosis vs. Mitosis

| Feature | Mitosis | Meiosis |

|------------------------|-----------------------------------------------|-----------------------------------------------|

| Purpose | Growth, repair, asexual reproduction | Produces gametes for sexual reproduction |

| Number of Divisions| One | Two |

| Daughter Cells | Two, diploid (2n), genetically identical | Four, haploid (n), genetically unique |

| Key Event | Separation of sister chromatids | Separation of homologous pairs (Meiosis I) |

| Genetic Variation | No | Yes, via crossing over & independent assortment|

Key Terms

Meiosis: A type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes.
Haploid (n): Having a single set of unpaired chromosomes.
Diploid (2n): Containing two complete sets of chromosomes, one from each parent.
Homologous Chromosomes: A pair of chromosomes (one from each parent) that are similar in shape and size and carry genes for the same traits.
Crossing Over: The exchange of genetic material between homologous chromosomes during Prophase I of meiosis, resulting in recombinant chromosomes.