Biology Chapter 13: Meiosis and Sexual Life Cycles

Overview

• Meiosis: A special type of cell division producing cells with half the chromosomes of the parent cell, occurring in specialized cells like those in testes and ovaries.
• Objectives:
  1. Offsprings acquire genes from parents by inheriting chromosomes.
  2. Fertilization and meiosis alternate in sexual life cycles.
  3. Meiosis reduces the chromosome set from diploid to haploid.
  4. Genetic variation in sexual life cycles contributes to evolution.

Key Concepts

Genetic Inheritance

• Inheritance: Transmission of traits from parents to offspring.
• Genetics: Study of heredity and inherited variation.
• Offspring inherit genes, which are hereditary units, from parents.
• Genes program traits and are encoded in DNA.

Sexual vs Asexual Reproduction

• Asexual Reproduction: Single parent produces genetically identical offspring.
• Sexual Reproduction: Involves two parents, leading to genetically diverse offspring.

Chromosomes and Genes

• Humans have 46 chromosomes in somatic cells (all cells except gametes).
• Gametes: Reproductive cells (sperm and egg) with 23 chromosomes.
• Autosomes: Chromosomes 1-22.
• Sex Chromosomes: XX in females, XY in males.

Life Cycles

• Life Cycle: Sequence of stages in the reproductive history of an organism.
• Fertilization and meiosis are key events alternating in life cycles.

Meiosis

• Meiosis I and II: Two consecutive cell divisions producing four daughter cells with half the number of chromosomes.

Meiosis I

1. Prophase I: Chromosomes condense, homologous chromosomes pair (synapsis), crossing over occurs.
2. Metaphase I: Tetrads align at the cell's equator.
3. Anaphase I: Homologous chromosomes separate.
4. Telophase I and Cytokinesis: Two haploid cells form.

Meiosis II

1. Prophase II: Spindle apparatus forms.
2. Metaphase II: Chromosomes line up at equator.
3. Anaphase II: Sister chromatids separate.
4. Telophase II and Cytokinesis: Four haploid daughter cells form.

Key Definitions

• Sister Chromatids: Identical copies of a chromosome, connected by cohesion.
• Homologous Chromosomes: Chromosomes from each parent with the same genes at the same loci.
• Crossing Over: Exchange of genetic material between homologous chromosomes, increasing genetic diversity.

Genetic Variation and Evolution

• Mechanisms of Genetic Variation:
  1. Crossing Over: During prophase I, leads to recombination of alleles.
  2. Independent Assortment: Random alignment of homologous chromosome pairs during metaphase I.
  3. Random Fertilization: Random pairing of gametes increases variability.

Comparison: Mitosis vs Meiosis

• Mitosis: Results in two identical diploid cells.
• Meiosis: Results in four genetically unique haploid cells.
• Unique meiosis features: Synapsis and crossing over, alignment of homologous pairs at metaphase, separation of homologs.

Conclusion

• Sexual reproduction and meiosis introduce genetic diversity, which is essential for evolution.
• Understand the processes and differences to grasp how genetic traits are passed and varied across generations.