SLO 10.2 Mitosis

10.2.1 Describe the Events Through Which Mitotic Apparatus Is Formed in Prophase in Animal and Plant Cells

During prophase, the first stage of mitosis, the mitotic apparatus (spindle fibers and associated structures) forms to facilitate chromosome segregation. The key events are:

In Animal Cells:

• Chromatin condenses into distinct chromosomes, each consisting of two sister chromatids joined at the centromere.
• The nucleolus disappears, and the nuclear envelope breaks down, allowing spindle fibers to interact with chromosomes.
• Centrosomes (each containing two centrioles) move to opposite poles of the cell, establishing the poles of the mitotic spindle.
• Microtubules extend from the centrosomes, forming the mitotic spindle, a structure that will align and separate chromosomes.
• Some microtubules (kinetochore microtubules) attach to the kinetochores (protein structures on the centromeres of chromosomes), while others (polar microtubules) extend toward the cell’s center to stabilize the spindle.

In Plant Cells:

• Similar to animal cells, chromatin condenses into chromosomes, and the nucleolus and nuclear envelope disappear.
• Plant cells lack centrioles, so the mitotic spindle forms without centrosomes. Instead, microtubules organize around a diffuse region called the polar organizing material or microtubule-organizing centers (MTOCs) at opposite poles.
• The spindle fibers form and function similarly, with kinetochore microtubules attaching to chromosomes and polar microtubules stabilizing the spindle structure.

Key Difference: Animal cells use centrioles within centrosomes to organize the spindle, while plant cells rely on MTOCs without centrioles.

10.2.2 Describe Formation of Metaphase Plate and the Division of Centromere During Metaphase

Formation of Metaphase Plate:

• During metaphase, spindle fibers align the chromosomes at the cell’s equatorial plane, forming the metaphase plate.
• Kinetochore microtubules from opposite poles attach to the kinetochores of each chromosome’s sister chromatids, ensuring proper alignment.
• The chromosomes are pulled into a single plane at the cell’s center, with tension from opposing microtubules ensuring stability.
• A checkpoint (spindle assembly checkpoint) ensures all chromosomes are properly attached to the spindle before proceeding.

Division of Centromere:

• Although the centromere does not divide during metaphase, it prepares for division. The kinetochores on each sister chromatid are fully attached to spindle fibers, and the cohesin proteins holding sister chromatids together at the centromere are poised for cleavage in the next phase (anaphase).
• The alignment at the metaphase plate ensures that each daughter cell will receive one copy of each chromosome.

10.2.3 Describe Separation of Chromatids During Anaphase

• During anaphase, sister chromatids are separated and pulled to opposite poles of the cell:
• The enzyme separase cleaves cohesin proteins at the centromere, allowing sister chromatids to separate.
• Kinetochore microtubules shorten, pulling each chromatid (now considered an individual chromosome) toward the spindle pole it is attached to.
• Polar microtubules elongate, further pushing the poles apart, increasing the distance between the separating chromosomes.
• This ensures that each pole (and future daughter cell) receives an identical set of chromosomes.
• The process is rapid and highly coordinated to prevent errors in chromosome segregation.

10.2.4 Describe Reformation of Nuclei During Telophase

• During telophase, the cell prepares to complete division by reforming nuclei:
• Chromosomes at each pole begin to decondense, returning to their diffuse chromatin form.
• The mitotic spindle disassembles as microtubules are depolymerized.
• A new nuclear envelope forms around each set of chromosomes, derived from fragments of the original nuclear envelope or endoplasmic reticulum.
• The nucleolus reappears in each daughter nucleus as ribosomal RNA synthesis resumes.
• By the end of telophase, two distinct nuclei are formed, each containing a complete set of genetic material identical to the parent cell.

10.2.5 Describe Physical Division of Cytoplasm During Cytokinesis in Animal and Plant Cells

In Animal Cells:

• Cytokinesis begins during late anaphase or telophase with the formation of a cleavage furrow.
• A contractile ring of actin and myosin filaments forms beneath the plasma membrane, constricting the cell at its equator.
• The ring tightens, pinching the cell into two, dividing the cytoplasm and organelles between the daughter cells.
• The process completes with two separate cells, each with its own nucleus and cytoplasm.

In Plant Cells:

• Plant cells have a rigid cell wall, so they cannot form a cleavage furrow.
• Instead, a cell plate forms at the center of the cell during telophase.
• Vesicles from the Golgi apparatus deliver materials (e.g., pectins and other polysaccharides) to the cell’s midline, fusing to form a new cell membrane and cell wall.
• The cell plate expands outward until it fuses with the existing cell wall, dividing the cytoplasm and forming two daughter cells.

Key Difference: Animal cells use a contractile ring for cytokinesis, while plant cells form a cell plate due to their rigid cell walls.

10.2.6 Compare Details of Events During Mitosis in Animal and Plant Cells

Stage	Animal Cells	Plant Cells
Prophase	Centrosomes with centrioles organize the spindle; nuclear envelope breaks down.	No centrioles; spindle forms via MTOCs; nuclear envelope breaks down.
Metaphase	Chromosomes align at metaphase plate; spindle fibers attach via kinetochores.	Same as animal cells, but spindle is organized by MTOCs.
Anaphase	Sister chromatids separate and move to poles via shortening microtubules.	Identical to animal cells.
Telophase	Nuclear envelope reforms; chromosomes decondense; spindle disassembles.	Same as animal cells; cell plate begins to form for cytokinesis.
Cytokinesis	Cleavage furrow forms via contractile ring, pinching cell into two.	Cell plate forms from Golgi vesicles, building a new cell wall to divide cells.

Key Differences:

• Animal cells rely on centrioles for spindle formation; plant cells use MTOCs.
• Cytokinesis involves a cleavage furrow in animal cells and a cell plate in plant cells due to the presence of a rigid cell wall in plants.

10.2.7 Recognize Significance of Mitosis

1. Growth: Increases cell number for organism development (e.g., growth of a seedling or human embryo).
2. Repair of Damaged Tissues: Replaces damaged cells (e.g., skin cells after a cut or liver cells after injury).
3. Replacement of Worn-Out Cells: Maintains tissues by replacing cells that die naturally (e.g., red blood cells replaced every 120 days).
4. Asexual Reproduction: Enables organisms like yeast, hydra, or plants (via cuttings) to reproduce by producing genetically identical offspring.
5. Mitosis ensures genetic consistency, maintaining the same chromosome number and genetic information in daughter cells, which is essential for organism stability and function.