Overview
This lecture covers the process, purpose, and importance of meiosis, including chromosome structure, terms like haploid/diploid, stages of meiosis, genetic variation, non-disjunction, and differences with mitosis.
Meiosis: Introduction and General Concepts
- Meiosis is cell division that produces four daughter cells, each with half the chromosome number of the parent.
- Meiosis occurs only in reproductive (sex) cells or gametes (sperm, ova in animals; pollen, ovules in plants).
- Gametes are haploid (n), while body (somatic) cells are diploid (2n).
- Meiosis maintains chromosome number across generations and introduces variation.
Chromosome Structure and Terminology
- Chromosomes are thread-like structures in the nucleus containing DNA and genes.
- DNA coils around proteins called histones to form chromosomes.
- Chromosomes exist as single-stranded (unreplicated) or double-stranded (replicated) after DNA replication.
- Homologous chromosomes are pairs with the same structure and gene positions—one from each parent.
- Chromatids are the identical halves of a replicated chromosome, joined at the centromere.
Haploid vs. Diploid
- Haploid (n): one set of chromosomes (e.g., gametes have 23 in humans).
- Diploid (2n): two sets (one from each parent, e.g., most human cells have 46).
- Fertilization restores the diploid number by fusing two haploid gametes.
Human Karyotype and Sex Chromosomes
- Humans have 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes (XX for female, XY for male).
- The male's sperm determines the biological sex of offspring.
Phases of Meiosis
- Meiosis has two main divisions: Meiosis I (reduction division) and Meiosis II (equational division).
- Phases: Prophase, Metaphase, Anaphase, Telophase (repeated in both divisions).
- Prophase I: Chromosomes condense; homologous chromosomes pair and crossing over occurs, leading to variation.
- Metaphase I: Homologous pairs align at the cell’s equator.
- Anaphase I: Homologous chromosomes separate to opposite poles.
- Telophase I/Cytokinesis: Two haploid cells form.
- Meiosis II: Similar to mitosis; sister chromatids of each chromosome separate, resulting in four haploid cells.
Genetic Variation and Importance
- Crossing over during prophase I exchanges genetic material, creating new gene combinations.
- Meiosis ensures genetic diversity and stable chromosome numbers across generations.
- Variation enables adaptation and survival in changing environments.
Non-Disjunction and Down Syndrome
- Non-disjunction: Failure of chromosome pairs or chromatids to separate during meiosis (anaphase I or II).
- Results in gametes with abnormal chromosome numbers (e.g., 24 or 22 instead of 23 in humans).
- Fertilization with these gametes can cause conditions like Down syndrome (trisomy 21: three copies of chromosome 21).
Differences and Similarities: Mitosis vs. Meiosis
- Mitosis produces two identical diploid cells; meiosis produces four genetically unique haploid cells.
- Mitosis occurs in somatic (body) cells; meiosis in sex (germ) cells.
- Crossing over occurs only in meiosis I, not in mitosis.
- Both involve similar phases (prophase, metaphase, anaphase, telophase) and DNA replication during interphase.
Key Terms & Definitions
- Meiosis — cell division in sex cells producing haploid gametes.
- Mitosis — cell division in body cells producing identical diploid cells.
- Chromosome — DNA-containing thread-like structure in the nucleus.
- Haploid (n) — cell with one set of chromosomes.
- Diploid (2n) — cell with two sets of chromosomes.
- Homologous chromosomes — chromosome pairs with the same genes.
- Crossing over — exchange of genetic material between homologous chromatids in prophase I.
- Non-disjunction — failure of chromosomes/chromatids to separate during meiosis.
- Autosome — non-sex chromosome.
- Gonosomes — sex chromosomes (X, Y).
Action Items / Next Steps
- Review diagrams of meiosis and mitosis stages.
- Memorize differences between mitosis and meiosis for exams.
- Prepare for next topic: human and vertebrate reproduction.