Mitotic Cell Division Notes

Chapter 16: Mitotic Cell Division

Overview of Mitotic Cell Division

  • Definition: Mitosis is the process where a cell divides to produce two new cells that are genetically identical to the original.

    • Original Cell: Referred to as the mother cell.

    • New Cells: Known as daughter cells.

  • Key Components: Involves both mitosis and cytokinesis.

  • Functions: Used for asexual reproduction, development, and growth of multicellular organisms.

Preparation for Cell Division

  • DNA Replication: Each chromosome is duplicated to produce two sister chromatids.

    • Sister Chromatids: Definition - Two identical copies of a chromosome, associated with proteins.

Structure of Chromatids

  • Centromere: A specific region of DNA where two chromatids are joined.

  • Kinetochores: Specialized proteins located at the centromere.

    • Function: Serve as the attachment sites for microtubule spindle fibers, aiding in chromosome sorting.

Mitotic Spindle

  • Definition: The mitotic spindle apparatus is critical for organizing and sorting chromosomes during mitosis.

    • Composition: Comprised of microtubules that form long spindle fibers.

    • Function: Ensures each daughter cell receives the correct number and type of chromosomes.

Microtubule Types in Mitotic Spindle

  • Formation: Microtubules are formed from tubulin proteins.

  • Types of Microtubules:

    • Astral Microtubules: Position the spindle in the cell.

    • Polar Microtubules: Function to separate the two poles.

    • Kinetochore Microtubules: Attach to kinetochores bound to centromeres.

Centrosomes

  • Definition: Centrosomes, accompanied by a pair of centrioles, are situated at each pole of the cell.

    • Duplication: Centrosomes are duplicated during the interphase phase of the cell cycle.

    • Role in Mitosis: Known as the
      Microtubule Organizing Center (MTOC), where spindle fibers originate and grow from.

    • Note: Animal cells possess centrioles, while other eukaryotic cells do not.

Cell Cycle Stages

  • Interphase: Phase when chromosomes are not compact (long and spread out) and are located in the nucleus. Consists of three sub-phases:

    • G1 Phase: Gap 1, where the cell grows and prepares for DNA synthesis.

    • S Phase: Synthesis phase where DNA is replicated.

    • G2 Phase: Gap 2, where the cell prepares for mitosis.

  • Mitosis: Broken down into several steps - Prophase, Prometaphase, Metaphase, Anaphase, Telophase.

  • Cytokinesis: Final stage where the cell divides into two daughter cells.

Stages of Mitosis

  • Prophase:

    • Events:

    • Chromosomes condense and become visible under a light microscope.

    • Nuclear membranes dissociate into small vesicles.

    • Sister chromatids are joined as pairs.

  • Prometaphase:

    • Events:

    • Nuclear envelope completely fragments.

    • The mitotic spindle is fully assembled.

    • Centrosomes move to opposite poles of the cell.

    • Spindle fibers interact with sister chromatids through kinetochore microtubules.

  • Metaphase:

    • Events:

    • Sister chromatids align along a plane halfway between the poles, known as the metaphase plate.

    • Chromosomes are organized into a single row.

  • Anaphase:

    • Events:

    • Connections between sister chromatids break, allowing them to be pulled apart.

    • Each individual chromatid is attached to one pole by kinetochore microtubules.

    • Kinetochore microtubules shorten, pulling chromosomes toward poles.

    • Two poles move apart due to overlapping polar microtubules lengthening and pushing against each other.

  • Telophase:

    • Events:

    • Chromosomes reach respective poles and begin to decondense.

    • Nuclear membranes re-form, resulting in two separate nuclei.

Cytokinesis

  • Definition: Process that occurs immediately following mitosis, leading to the separation of the mother cell into two daughter cells.

    • Segregation: Separation includes two nuclei and the cytosol.

    • Differences Between Animals and Plants:

    • Animal Cells: Cytokinesis occurs via a cleavage furrow that constricts like a drawstring.

    • Plant Cells: Formation of a cell plate that develops into a new cell wall.

Evolution of Eukaryotic Mitosis

  • Bacterial Mitosis (Binary Fission):

    • Chromosomes are anchored to the membrane.

    • FtsZ Proteins: Form a ring at the separation site and assist in chromosome segregation.

  • Dinoflagellates Mitosis:

    • Chromosomes are attached to the nuclear envelope.

    • The nuclear envelope does not fragment, but the nucleus divides.

  • Diatoms and Yeasts Mitosis:

    • Microtubules form within the nucleus; nuclear envelope remains intact.

  • Complex Eukaryotes:

    • Nuclear envelope disintegrates during mitosis.

    • Spindle apparatus forms, with tubulin (the monomer of microtubules) being evolutionarily related to FtsZ proteins.