Overview
This lecture covers the phases and significance of meiosis II, contrasts meiosis with mitosis, and explains the sources of genetic variation in sexually reproducing organisms.
Meiosis II Process
- Some cells enter a short interphase called interkinesis between meiosis I and II, but no DNA replication occurs.
- Prophase II involves chromosome condensation and spindle formation in both daughter cells.
- During prometaphase II, nuclear envelopes break down and spindle fibers attach to kinetochores on sister chromatids.
- In metaphase II, sister chromatids align at the metaphase plate in both haploid cells.
- Anaphase II separates sister chromatids, pulling them to opposite poles.
- Telophase II and cytokinesis result in four unique haploid cells due to prior crossing over and independent assortment.
Comparison of Meiosis and Mitosis
- Both processes divide the eukaryotic nucleus, but mitosis produces 2 identical cells, while meiosis produces 4 genetically unique haploid cells.
- Homologous chromosomes pair and undergo crossing over in meiosis I, which does not occur in mitosis.
- Independent assortment of chromosomes in metaphase I increases genetic variation, unlike in mitosis.
- Meiosis reduces chromosome number (diploid to haploid); mitosis preserves chromosome number.
Sources of Genetic Variation
- Independent assortment during metaphase I allows for many chromosome combinations.
- Crossing over in prophase I creates new combinations of genes on chromosomes.
- Random fertilization (any sperm can fuse with any egg) further multiplies genetic diversity.
- The number of possible human zygote combinations is about 70 trillion.
Key Terms & Definitions
- Meiosis II — the division that separates sister chromatids to create four haploid cells.
- Interkinesis — a brief interphase-like period between meiosis I and II without DNA replication.
- Crossing Over — exchange of genetic material between homologous chromosomes during prophase I.
- Independent Assortment — random alignment of homologous chromosomes at the metaphase I plate.
- Random Fertilization — process by which any sperm can fertilize any egg, increasing diversity.
- Haploid (n) — a cell with one set of chromosomes.
- Diploid (2n) — a cell with two sets of chromosomes.
- Gametes — reproductive cells (sperm and egg) produced by meiosis.
Action Items / Next Steps
- Review and test yourself on the phases and outcomes of meiosis I and II.
- Compare and contrast features of meiosis and mitosis for exam preparation.
- Read section 11.2 on life cycles for the next class.