Overview
The lecture reviews meiosis, including the roles of homologous chromosomes, sources of genetic variation, key stages in meiosis, and the consequences of nondisjunction.
Homologous Chromosomes & Ploidy
- Homologous chromosomes are chromosome pairs from each parent, with the same genes in the same positions but possibly different alleles.
- Diploid cells (2N) have pairs of homologous chromosomes, found in body (somatic) cells.
- Haploid cells (N) have one set of chromosomes and are gametes (egg or sperm).
Mitosis vs Meiosis
- Mitosis produces diploid daughter cells identical to the parent (46 chromosomes in humans).
- Meiosis produces haploid gametes with half the chromosome number (23 in humans).
- If gametes were made by mitosis, fertilization would double the chromosome number each generation.
Stages of Meiosis
- One round of DNA replication is followed by two cell divisions.
- Meiosis I: Homologous pairs line up and separate, reducing chromosome number by half.
- Meiosis II: Sister chromatids separate, similar to mitosis.
- Results in four genetically unique haploid cells.
Nondisjunction & Chromosome Disorders
- Nondisjunction is the failure of chromosomes/chromatids to separate during meiosis.
- Nondisjunction produces gametes with abnormal chromosome numbers, often leading to cell death.
- Trisomy 21 (three copies of chromosome 21) causes Down syndrome, strongly correlated with maternal age.
Sources of Genetic Variation
- Genetic variation results from mutations, meiosis (crossing over, random orientation), and fertilization.
- Crossing over (prophase I): Exchange of genetic material between nonsister chromatids of homologous chromosomes at chiasmata.
- Random orientation (metaphase I & II): Homologous pairs and sister chromatids align randomly, producing many possible combinations (2^n).
- Humans can produce 2^23 possible gamete combinations from random orientation alone.
Phases and Key Events in Meiosis
- DNA replicates before meiosis begins.
- Prophase I: Chromosomes condense, nuclear membrane dissolves, crossing over occurs.
- Metaphase I: Homologous pairs align at cell center.
- Anaphase I: Homologous pairs separate (not chromatids).
- Telophase I & Cytokinesis: Two haploid cells form.
- Meiosis II: No DNA replication; resembles mitosis; sister chromatids separate.
- Ends with four haploid, genetically unique cells.
Key Terms & Definitions
- Homologous chromosomes — Chromosome pairs with the same genes but possibly different alleles, from each parent.
- Diploid (2N) — Cell with homologous chromosome pairs.
- Haploid (N) — Cell with a single set of chromosomes.
- Mitosis — Cell division producing genetically identical diploid cells.
- Meiosis — Cell division producing haploid gametes with genetic variation.
- Nondisjunction — Failure of chromosome separation in meiosis, causing abnormal gamete chromosome numbers.
- Trisomy 21 — Condition with three copies of chromosome 21, causing Down syndrome.
- Crossing over — Exchange of genetic material between nonsister chromatids of homologous chromosomes.
- Random orientation — Random arrangement of chromosomes during metaphase I & II.
- Chiasma (pl. chiasmata) — Site where crossing over occurs.
Action Items / Next Steps
- Review diagrams of meiosis stages and homologous chromosome behavior.
- Be able to recognize nondisjunction and trisomy 21 in karyotypes.
- Memorize the differences between meiosis I & II, and between mitosis and meiosis.
- Remember sources of genetic variation for future lessons on evolution.