Overview
This lecture covers the types and processes of cell division, specifically amitosis, mitosis, and meiosis, as well as their roles in growth, repair, and reproduction across bacteria, animals, and plants.
Types of Cell Division
- Cell division can be direct (amitosis) or indirect (mitosis and meiosis).
- Amitosis is a simple division seen in bacteria, resulting in two identical daughter cells.
Mitosis and Its Phases
- Mitosis is used for growth and repair, creating two identical diploid daughter cells from a diploid parent.
- Mitosis is part of the cell cycle, following interphase (G1, S, G2).
- G1: Cell size increases.
- S: DNA replicates.
- G2: Organelles are formed, cell prepares to divide.
- Mitosis stages:
- Prophase: Chromatin condenses into chromosomes, nuclear membrane disappears.
- Metaphase: Chromosomes line up at the equator.
- Anaphase: Chromatids move to opposite poles.
- Telophase: Chromosomes decondense, nuclear membrane reforms.
- Cytokinesis: Cytoplasm divides, forming two daughter cells.
Meiosis and Its Phases
- Meiosis produces gametes, reducing chromosome number by half (diploid to haploid) and introducing genetic variation.
- Meiosis has two major stages: Meiosis I and II, starting with DNA replication during interphase.
- Meiosis I:
- Prophase I: Five sub-stages (leptotene, zygotene, pachytene, diplotene, diakinesis); genetic recombination occurs.
- Metaphase I: Tetrads align at the equator.
- Anaphase I: Homologous chromosomes separate.
- Telophase and Cytokinesis I: Two haploid cells form.
- Meiosis II resembles mitosis, producing four haploid cells.
Gametogenesis in Animals and Plants
- Spermatogenesis: Spermatogonia undergo mitosis, enlarge to primary spermatocytes (meiosis I), producing secondary spermatocytes (meiosis II), yielding four spermatids that mature into sperm.
- Oogenesis: Oogonia undergo mitosis, become primary oocyte (meiosis I), forming secondary oocyte and polar body; meiosis II forms one ovum and more polar bodies.
- In plants, microsporogenesis produces pollen (male gametes) and megasporogenesis produces the embryo sac (female gametes) in angiosperms.
Chromosome Abnormalities
- Abnormal cell division, such as non-disjunction, can result in conditions like Down syndrome due to failed chromosome separation.
Key Terms & Definitions
- Amitosis — Direct cell division without mitotic stages, seen in bacteria.
- Mitosis — Indirect cell division producing two identical daughter cells.
- Meiosis — Cell division producing four genetically unique gametes with half the chromosome number.
- Interphase — Phase of the cell cycle where growth and DNA replication occur.
- Chromatid — One copy of a duplicated chromosome.
- Tetrad — A paired set of homologous chromosomes in meiosis.
- Gamete — A reproductive cell (sperm or egg) with half the number of chromosomes.
- Gametogenesis — The process of forming gametes.
- Non-disjunction — Failure of chromosomes to separate properly during meiosis.
Action Items / Next Steps
- Answer: Identify which cell division phase fails in Down syndrome.
- Practice more questions on 'Cell division' as recommended.