Kami Export - Meiosis Review Guide (Adv.) KEY

Meiosis Overview

• Definition: Meiosis is the process of cell division that produces haploid gametes, reducing the chromosome number by half.

• Purpose: It occurs only in sex cells (gametes), such as egg cells (ova) and sperm cells.

• Difference from Mitosis: Unlike mitosis, which occurs in somatic (body) cells, meiosis results in reproductive cells.

Types of Gametogenesis

• Spermatogenesis: Formation of sperm cells in the testes of biological males.

• Oogenesis: Formation of egg cells in the ovaries of biological females.

Chromosome Basics

• Homologous Chromosomes: Pairs of similar chromosomes carrying the same genes, but may have different versions (alleles).

• Haploid Cells: Result from meiosis; contain one chromosome from each homologous pair.

Meiosis Process

• Two Divisions: Meiosis involves two cell divisions, termed Meiosis I and Meiosis II, culminating in four haploid cells.

• Independent Assortment: During meiosis, homologous chromosomes are randomly distributed to daughter cells, leading to genetic diversity.

• Nondisjunction: Failure of sister chromatids to separate during anaphase can result in gametes with abnormal chromosome numbers.

Phases of Meiosis I

1. Prophase I

• Nuclear envelope begins to dissolve; chromosomes condense.

• Homologous chromosomes pair up.

• Crossing Over: Genetic material is exchanged between chromosomes.

2. Metaphase I

• Spindle fibers attach to paired homologous chromosomes.

• Chromosomes align at the equator of the cell.

3. Anaphase I

• Homologous chromosomes separate and move towards opposite poles.

4. Telophase I and Cytokinesis

• Spindle apparatus breaks down; nuclear membranes reform.

• Cytoplasm divides, resulting in two haploid daughter cells that each contain a random assortment of chromosomes.

Phases of Meiosis II

1. Prophase II

• Nuclear envelope dissolves; spindle apparatus forms in each haploid daughter cell from meiosis I.

2. Metaphase II

• Sister chromatids align along the equator of the cell.

3. Anaphase II

• Sister chromatids separate and move to opposite poles.

4. Telophase II and Cytokinesis

• New nuclear membranes form; cytoplasm divides.

• Results in four unique haploid cells.

Fertilization

• Process: Fusion of a sperm cell and an egg cell to form a diploid zygote.

• In humans, gametes each contain 23 chromosomes; the zygote has 46 chromosomes total, restoring the diploid number.

• The zygote undergoes mitotic divisions to develop into an embryo.

Chromosome Types

• Autosomes: Non-sex chromosomes that control characteristics.

• Sex Chromosomes: Determine sex; males have XY, and females have XX.

Reproductive Strategies

• Asexual Reproduction: Examples include binary fission in bacteria, fragmentation, and budding; results in identical offspring.

Short Answer Concepts

1. Reduction Division: Meiosis reduces the chromosome number to half; restores during fertilization.

2. Oogenesis vs. Spermatogenesis: Oogenesis is egg cell formation; spermatogenesis is sperm cell formation.

3. Haploid Necessity: Gametes must be haploid for successful fertilization.

4. Zygote vs. Gamete: Gametes are haploid, unfertilized cells; zygotes are diploid, fertilized cells.

5. Unique Offspring: Variation arises from independent assortment, crossing-over, and random gamete combination.

6. Meiosis Errors: Errors can result in gametes with abnormal chromosome numbers, leading to genetic disorders like Down Syndrome.

Multiple Choice Questions

• DNA Replication Phase: Interphase

• Crossover Phase: Prophase I

• Nuclear Membrane Reform Phase: Telophase II

• Description of Chromosomal Content in Cell: A human cell containing 22 autosomal chromosomes and a Y chromosome is a sperm cell.

• Homologous Chromosome Movement: Meiosis I

• Stage for Chromatid Separation: Anaphase II

• Haploid Gamete Production: Meiosis

Fill In Chart Example

• Humans: 46 chromosomes (diploid), 23 homologous pairs, 23 haploid cells.

• Bats: 44 chromosomes (diploid), 22 homologous pairs, 22 haploid cells.

Gamete vs Somatic Cell Characteristics

1. Body cells are referred to as somatic cells. (S)

2. Eggs and sperm represent gametes. (G)

3. Homologous chromosome pairing occurs in gametes. (G)

4. Gametes are haploid. (G)

5. In humans, gametes contain 22 autosomes and 1 sex chromosome. (G)

6. Somatic cells are identical after division. (S)

Phases Labeling

• MII: Sister chromatids line up in the middle.

• PI: Crossing over occurs.

• TI: Cytoplasm splits.

• AII: Sister chromatids are separated.

• AI: Homologous chromosomes separate.

• TII: Four individual nuclear membranes form.

• MI: Pairs of chromosomes line up in the middle.