Overview
This lecture covers the essentials of cell division, focusing on mitosis, including reasons for cell division, the process of cytokinesis in animal and plant cells, stages of mitosis, and how to identify them under a microscope.
Purposes and Principles of Cell Division
- Cell division creates new cells for growth, tissue repair, and reproduction.
- New cells originate from pre-existing cells, supporting the cell theory.
- Continuity and change are illustrated by the processes of mitosis (continuity) and meiosis (change).
Cytokinesis
- Cytokinesis divides the cytoplasm after nuclear division.
- In animal cells, a contractile ring (actin and myosin) forms a cleavage furrow to pinch cells apart.
- In plant cells, vesicles form a cell plate that develops into a new cell wall between daughter cells.
- Equal cytokinesis produces two similar cells; unequal cytokinesis can occur, as seen in yeast budding and oogenesis.
Key Examples of Unequal Cytokinesis
- Yeast budding produces a large parent and a small bud with essential organelles and a nucleus.
- Oogenesis (egg formation) involves unequal cytoplasm division, creating one large oocyte and small polar bodies.
Genetic Material Replication and Chromosomes
- Before cytokinesis, cells replicate their DNA to ensure each daughter cell gets a nucleus.
- Chromatin is loose DNA in non-dividing cells; during division, DNA condenses into chromosomes via supercoiling.
- Each chromosome consists of two sister chromatids joined by cohesin proteins at the centromere; microtubules attach to kinetochores for movement.
Mitosis: Process and Phases
- Mitosis ensures genetic continuity by producing two genetically identical, diploid (2n) daughter cells.
- Four mitosis stages: prophase, metaphase, anaphase, telophase.
- Prophase: Chromatin condenses, spindle forms, nuclear envelope dissolves.
- Metaphase: Chromosomes align at the cell's equator, spindle fibers attach.
- Anaphase: Sister chromatids separate as spindle fibers shorten.
- Telophase: Chromosomes decondense, nuclear envelopes reform, cytokinesis begins.
Identifying Mitotic Stages in Micrographs
- Interphase: Evenly spread chromatin, normal cell activity.
- Prophase: Chromatin condenses into visible chromosomes.
- Metaphase: Chromosomes aligned at equatorial plate.
- Anaphase: Chromatids pulled to opposite poles.
- Telophase: Two nuclei form, cell plate or cleavage furrow visible.
Key Terms & Definitions
- Cell division — process of creating new cells from existing ones.
- Cytokinesis — division of the cell’s cytoplasm.
- Chromatin — loose DNA in the nucleus.
- Chromosome — condensed DNA structure, visible during cell division.
- Sister chromatids — identical copies of a chromosome, connected at the centromere.
- Cohesin — protein that binds sister chromatids together.
- Spindle microtubules — fibers that move chromosomes during division.
- Kinetochore — protein structure where spindle fibers attach on the centromere.
- Diploid (2n) — cell with chromosome pairs.
- Haploid (n) — cell with unpaired chromosomes (from meiosis).
Action Items / Next Steps
- Practice drawing and labeling mitosis phases.
- Learn to identify mitosis stages on micrographs (e.g., onion root tip).
- Review differences between mitosis and meiosis.
- Prepare or review microscope slide preparation skills if required.