Overview
This lecture explains why children and siblings do not look identical by describing the process of meiosis, which generates genetically diverse gametes for sexual reproduction.
The Process of Meiosis
- Meiosis produces haploid gametes (sperm or eggs) from diploid germ cells in the gonads.
- Two sequential cell divisions, meiosis I and meiosis II, reduce chromosome number from diploid (2n) to haploid (n).
- Meiosis begins after the cell completes G1, S, and G2 phases of interphase, with DNA replication occurring in S phase.
Key Differences: Mitosis vs. Meiosis
- Mitosis produces two diploid cells; meiosis produces four haploid cells.
- Homologous chromosomes pair (synapsis) and form tetrads in meiosis I but not in mitosis.
- Genetic diversity arises in meiosis via unique events: crossing over and independent assortment.
Genetic Diversity in Meiosis
- In prophase I, synapsis aligns homologous chromosomes, forming tetrads.
- Crossing over exchanges DNA segments between non-sister chromatids, resulting in new allele combinations.
- During metaphase I, independent assortment randomly aligns tetrads along the metaphase plate, further increasing variation.
- The combination of crossing over and independent assortment results in millions of possible gamete combinations in humans.
Stages of Meiosis
- Meiosis I: Homologous chromosomes separate, creating two haploid cells.
- Meiosis II: Sister chromatids separate, resulting in four genetically distinct haploid gametes.
- Gametes fuse during fertilization to form a diploid zygote.
Why Siblings Differ Genetically
- Crossing over and independent assortment in meiosis produce unique gametes.
- Random fusion of gametes during fertilization adds further genetic diversity.
- The formula 2^n (n = number of chromosome pairs) determines the number of possible unique gametes.
Key Terms & Definitions
- Meiosis — Cell division that produces haploid gametes from diploid cells.
- Haploid (n) — Cell with one set of chromosomes.
- Diploid (2n) — Cell with two sets of chromosomes.
- Synapsis — Pairing of homologous chromosomes.
- Tetrad/Bivalent — Structure formed by synapsed homologous chromosomes.
- Crossing over — Exchange of DNA between non-sister chromatids.
- Independent assortment — Random alignment and separation of chromosomes in meiosis.
- Gamete — Haploid reproductive cell (sperm or egg).
- Zygote — Fertilized egg cell.
Action Items / Next Steps
- Review the stages of meiosis and compare them with mitosis.
- Practice explaining genetic variation sources: crossing over, independent assortment, and random fertilization.