Overview
This lecture covers the stages and main features of Meiosis II, explaining prophase II, metaphase II, anaphase II, and telophase II, as well as summarizing meiosis as a whole.
Prophase II
- Begins with two cells from meiosis I, each with recombinant chromosomes.
- No crossing over occurs, as homologous chromosomes are now in separate cells.
- Spindle fibers form, and centrioles move to opposite poles.
- Chromosomes are visible as double-stranded recombinant forms.
Metaphase II
- Chromosomes align individually along the equator of each cell.
- Spindle fibers attach to the centromeres of chromosomes.
- Random arrangement of chromosomes occurs, increasing genetic variation.
- Chromosomes are no longer paired with homologous partners.
Anaphase II
- Spindle fibers contract, separating sister chromatids into single-stranded chromosomes.
- Each chromatid, once separated, is considered a chromosome.
- Chromatids/chromosomes move towards opposite poles of the cell.
Telophase II and Cytokinesis
- Nuclear membranes reform around chromosomes at each pole.
- Cytoplasm divides through cytokinesis, producing four non-identical haploid cells.
- Each resulting cell contains single-stranded chromosomes.
Summary: Meiosis I vs. Meiosis II
- Meiosis I reduces chromosome number (diploid to haploid) and introduces variation via crossing over.
- Meiosis II separates recombinant chromatids into individual chromosomes, resulting in four unique haploid cells.
- Chromosomes are called chromatids when joined by a centromere; once separated, each is a chromosome.
Key Terms & Definitions
- Prophase II — Stage where spindle fibers form, centrioles migrate, and chromosomes are visible with no crossing over.
- Metaphase II — Chromosomes line up singly at the equator; spindle fibers attach to centromeres.
- Anaphase II — Sister chromatids split and move toward opposite poles, each now called a chromosome.
- Telophase II — Nuclear membranes reform, and cytokinesis produces four haploid, non-identical cells.
- Random Arrangement — Chromosomes align randomly at the equator, enhancing genetic diversity.
- Cytokinesis — Division of cytoplasm to form separate cells.
- Diploid — Cell with a full set of chromosomes (e.g., 46 in humans).
- Haploid — Cell with half the original chromosome number (e.g., 23 in humans).
- Recombinant Chromosomes — Chromosomes that have exchanged genetic material during crossing over.
- Single-Stranded Chromosomes — Individual chromosomes after chromatids have separated in meiosis II.
Action Items / Next Steps
- Review previous meiosis I notes or video for foundational understanding.
- Practice identifying stages of meiosis II in diagrams.
- Complete related exam questions or worksheets as assigned.