Overview
This lecture explains the process and phases of meiosis, focusing on how it creates four genetically unique gametes through two rounds of division.
Purpose and Key Differences from Mitosis
- Meiosis produces four genetically different cells (gametes), while mitosis produces two identical cells.
- Meiosis introduces genetic variation; mitosis maintains genetic consistency.
- Gametes produced by meiosis are used in sexual reproduction.
Key Structures in Meiosis
- Chromosomes exist as homologous pairs (one from each parent).
- Homologous chromosomes contain the same genes but may have different alleles.
- Sister chromatids are exact copies formed after chromosome duplication.
- Centrosomes organize mitotic spindles; centrioles are present in animal cells.
Stages of Meiosis
- Mnemonic: PMAT × 2 (Prophase, Metaphase, Anaphase, Telophase, twice).
Interphase
- DNA and centrosomes duplicate during S phase, forming sister chromatids.
- DNA is loose and not fully condensed.
Meiosis I
- Prophase I: Homologous chromosomes pair up (synapsis) and exchange genetic material (crossing over), increasing variation.
- Metaphase I: Homologous pairs align at the metaphase plate with random orientation (independent assortment), creating more variation.
- Anaphase I: Homologous chromosomes are pulled apart to opposite poles.
- Telophase I and Cytokinesis: Two new nuclei and cells form, each with half the original chromosome number.
Meiosis II
- Prophase II: Chromosomes condense again; no crossing over occurs.
- Metaphase II: Chromosomes align at the cell center.
- Anaphase II: Sister chromatids are separated to opposite poles.
- Telophase II and Cytokinesis: Four unique haploid cells are produced.
Sources of Genetic Variation
- Crossing over during prophase I combines maternal and paternal genetic material.
- Independent assortment in metaphase I offers multiple possible chromosome arrangements (2^n combinations).
Meiosis in Humans
- Humans have 23 chromosome pairs, enabling over 8 million chromosomal combinations in gametes.
- Fertilization combines two gametes, leading to over 64 trillion possible genetic outcomes.
Egg Versus Sperm Production
- All four sperm cells are commonly used, but in oogenesis (egg formation), only one cell is used as the egg; the others are discarded to preserve cytoplasmic contents.
Key Terms & Definitions
- Homologous chromosomes — Chromosome pairs, one from each parent, containing similar genes.
- Sister chromatids — Identical copies of a chromosome joined together after DNA replication.
- Synapsis — Pairing of homologous chromosomes during prophase I.
- Crossing over — Exchange of genetic material between homologous chromosomes.
- Independent assortment — Random arrangement of chromosome pairs during metaphase I.
- Centrosome — Organelle organizing the spindle apparatus.
- Centromere — Region joining sister chromatids or homologous chromosomes.
Action Items / Next Steps
- Review diagrams of meiosis stages.
- Memorize the order of meiosis phases (PMAT × 2).
- Understand sources of genetic variation in meiosis.