Ch+5+pt+2+FP-module 4

Chapter 5 Part 2: Meiosis and Genetic Variation

Overview of Topics

• Meiosis: Process for sexually reproducing organisms that reduces the number of chromosomes.

• Meiotic Accidents: Errors that occur during meiosis.

• Genetic Variation: The uniqueness of offspring due to genetic shuffling.

• Gametogenesis: The process of forming gametes (sperms and eggs).

Homologous Chromosomes

• Definition: Homologous chromosomes are pairs of chromosomes in body (somatic) cells that have the same length and centromere position.

• Composition: Each homologous chromosome carries genes at the same locus.

• Sex Chromosomes: X and Y are not homologous; all other chromosomes are known as autosomes.

Meiosis

• Purpose: Primary method for sexually reproducing organisms to produce gametes.

• Chromosome Reduction: Reduces the chromosome number from diploid (2n) to haploid (n).

• Mechanism: Involves the separation of paired homologous chromosomes through two rounds of cell division: meiosis I and meiosis II.

• Genetic Shuffling: Genes are shuffled, resulting in genetic variation among offspring.

• Output: Produces four daughter cells, each genetically distinct from one another.

Meiosis I

• Phases: Includes prophase I, metaphase I, anaphase I, and telophase I; has several unique features:

  • Prophase I: Homologous chromosomes pair up and exchange segments (crossing-over).

  • Metaphase I: Tetrads (paired homologs) line up at the metaphase plate.

  • Anaphase I: Independent assortment occurs, leading to chromosome separation.

  • Telophase I: Nuclei may reform; cytokinesis may or may not occur.

• Interkinesis: A period of rest may occur between meiosis I and meiosis II.

Meiosis II

• Mechanism: Functions similarly to mitosis.

• Chromosome Count: Begins with cells that contain haploid number of chromosomes (n).

• Phases: Includes prophase II, metaphase II, anaphase II, and telophase II.

• Outcome: Results in four genetically diverse cells, each carrying a mix of genes from both parents.

Meiotic Accidents

• Nondisjunction: Occurs when homologous chromosomes fail to separate properly during meiosis I or II.

  • Meiosis I: All gametes produced have an abnormal number of chromosomes.

  • Meiosis II: Half of the resulting gametes have the normal chromosome number.

• Occurrence: More common in egg production; may involve issues during the prophase stage.

Consequences of Nondisjunction

• Monosomy: Condition where an organism is missing one chromosome from a pair.

• Trisomy: Condition with three copies of a particular chromosome.

• Impacts on Development: Most monosomies and trisomies are lethal, leading to spontaneous abortion.

  • Example: Trisomy 21 results in Down syndrome.

• Sex Chromosomes: Extra copies of sex chromosomes tend to be less harmful compared to autosomal trisomies.

Variation in Offspring

• Evolutionary Advantage: Sexual reproduction leads to genetic diversity, enhancing adaptation.

• Mechanisms contributing to variation:

  • Crossing Over: Exchange of genetic material between homologous chromosomes increases genetic diversity.

  • Independent Assortment: Random arrangement of homologous chromosomes during meiosis adds to genetic variation.

  • Random Fertilization: Each sperm and egg combination created can lead to various offspring.

Probability and Independent Assortment

• Gamete Combinations:

  • Chromosomes assort independently, leading to a calculation of combinations: 2^23 (over 8 million combinations) for humans.

  • Includes fertilization variability leading up to 70 trillion different potential offspring.

• Additional Variation Through Crossing Over: Further enhances genetic diversity beyond independent assortment.

Mitosis vs. Meiosis

• Mitosis: Produces 2 identical daughter cells through one division cycle (2n to 2n).

• Meiosis: Produces 4 non-identical daughter cells through two division cycles (2n to n).

• Organ Type: Mitosis occurs in somatic cells, whereas meiosis occurs in reproductive cells.

• Chromosome Line Up: Chromosomes in mitosis align in single lines; in meiosis I, homologous chromosomes pair up.

Human Life Cycle

• Formation of Zygote: Fertilization merges an egg (n) and sperm (n) to create a zygote (2n).

• Development: Mitotic divisions occur for growth and development.

• Gametogenesis: At sexual maturity, spermatogenesis (sperm formation) and oogenesis (egg formation) produce gametes, each with n chromosomes.

Spermatogenesis

• Timeline: Begins at puberty and continues throughout life.

• Process: Millions of sperm produced daily; primary spermatocytes (2n) enter meiosis I and secondary spermatocytes (n) enter meiosis II, resulting in matured sperm.

Oogenesis

• Over Lifecycle: Begins in the fetus but pauses at prophase I until puberty.

• Monthly Cycle: At each menstrual cycle, one primary oocyte (2n) resumes meiosis.

• Outcome: Cytokinesis during oogenesis is unequal, producing one secondary oocyte (functional) and one polar body. Meiosis II completes only after fertilization, yielding one egg and an additional polar body.