Cell Division: Mitosis & Cytokinesis

• Cytokinesis represents the final stage of cell division.
• Mitosis involves replicating and separating chromosomes.
• Cytokinesis involves splitting one cell into two.

Cell Cycle Overview

• The eukaryotic cell cycle consists of four phases.
• Growth Phase 1 (G1-phase).
• Synthesis Phase (S-phase).
• Growth Phase 2 (G2-phase).
• Mitotic Phase (M-phase).

Introduction to Cytokinesis

• During interphase, DNA is replicated, with 2 asters ready for separation.
• Mitosis separates chromosomes through prophase, metaphase, eventually progressing to anaphase.
• Anaphase marks the beginning of cytokinesis.
• Cytokinesis involves separating everything else in the cell like the cytoskeleton and organelles.

Overview of Cytokinesis Structures

• Microtubule asters are visible at the poles of the cell.
• Invagination of the cell membrane occurs.
• Actin and myosin concentrate in an area known as the contractile ring to help the cells pinch apart.
• Microtubules join the two cells to be separated until they disassemble.

Definition of Cytokinesis

• Final step in cell division.
• A single eukaryotic cell's cytoplasm divides to form two daughter cells.

Cleavage Furrow

• The appearance of a cleavage furrow is the initial visible change in cytokinesis.
• The furrow deepens, dividing the cell in two.
• A contractile ring drives the process.

Contractile Ring Composition and Function

• The contractile ring is made of actin and myosin filaments beneath the plasma membrane.
• Membrane vesicles fuse into the plasma membrane to allow contraction without membrane ripping.
• Once contracted, more membrane fusion occurs to ensure both cells have enough membrane material.
• PM stands for plasma membrane.

Force Generation in Cytokinesis

• Actin and Myosin II within the contractile ring are responsible for generating the force in cytokinesis.
• Actin is present in the cortex and contractile ring.
• Myosin is exclusively present in the contractile ring.

Contractile Ring Assembly

• Actin, tubulin, and myosin disassemble in the mother cell.
• Monomeric actin and myosin accumulate at the contractile ring site during anaphase.
• Local actin polymerizes, mediated by formin, establishing unbranched actin nucleation.

Microtubule Role

• Microtubules form mitotic spindles and pull chromosomes apart.
• Microtubules depolymerize to allow the formation of the contractile ring.

Normal Cytokinesis Events

• Chromosomes are pulled apart.
• A cleavage furrow is formed.
• A contractile ring forms.
• Red microtubules extend into both dividing cells and eventually disappear.

Cytokinesis Inhibition

• Adding taxol prevents microtubule depolymerization.
• Prevents the separation of asters, which leads to cell death.

Disassembly of Contractile Ring

• Actin filaments depolymerize and exit the ring to maintain size and volume during contraction.
• The contractile ring is fully disassembled, and the midbody forms.
• The midbody then tethers the two daughter cells together.

Midbody Composition and Function

• The midbody contains the central spindle.
• Densely packed microtubules are in the spindle midzone and remain on the inner plasma membrane.
• Midbody remnants will orient the spindle for subsequent cell division.
• The division of the cell acts as a landmark.

RhoA Role

• Local activation of RhoA starts the assembly and contraction of the contractile ring.
• RhoA is a family of Ras GTPases

RhoA Functions

• Small guanosine triphosphatase (GTPase) of the RAS family.
• Regulates the remodeling of the actin cytoskeleton during cell morphogenesis and motility.
• Promotes actin polymerization.
• Binds and activates formin to promote actin nucleation at the contractile ring.
• Signals to its downstream effectors, Rho Kinases such as ROCK1 and activates it.

ROCK1 Function

• Phosphorylates and deactivates myosin phosphatase.
• Inhibits Myosin phosphatase-mediated deactivation of myosin II.
• ROCK1-mediated activation of Myosin II is the net effect.

RhoA Activation and Impact

• Rho GEF exchanges GDP for GTP, making RhoA active.
• Ultimately, a RhoGAP deactivates it by hydrolyzing GTP back to GDP.
• RhoA activates Formin.
• Which results in filamentous actin forming.
• This also activates ROCK1 that inhibits myosin phosphatase; activating Myosin II.

RhoA and Muscle Contraction

• Rho activates mDia (formin) which activates profilin, promoting actin polymerization.
• Rho simultaneously activates ROCK1, inhibiting myosin phosphatase.
• ROCK1 activates LIM kinase, which phosphorylates cofilin, inhibiting depolymerization.

Regulation of Cytokinesis

• The time and place of cytokinesis is tightly regulated.
• Including the position of the furrow.
• Microtubules of the mitotic spindle determine the plane of animal cell division.
• This is triggered by the complete segregation of chromosomes.

Furrow Formation Signals

• Signals from the spindles during anaphase.
• Ensured equal distribution of chromosomes.
• Astral Stimulation Model describes how microtubules carry signals that focus on a ring in the cell cortex.
• Mutation in signal transducers (such as RhoA) in drosophila, result in failed cytokinesis

Models Behind Spindle Signals

• Asters have separated.
• Microtubules are still extended to the surface of the cell.
• Signals are sent along microtubules between the two dividing cells.
• Evidence supports that it's the MTs that extend toward the PM that are responsible for initiating the cell.

More Complex Cells

• There are other organelles to separate during cytokinesis like membrane bound organelles.
• The ER segregates during interphase.
• The mitochondria doubles in number with each cell division.
• The Golgi fragments during M-Phase, move with spindle poles, and are reconstructed during telophase.

Organelle Positioning

• Mitochondria cluster towards the division site.
• The ER surrounds the nuclear material.
• The Golgi appears more diffuse

Mitochondrial Localization

• Mitochondria localize to the cleavage furrow during cytokinesis.
• This aids ATP supply.

Golgi Positioning

• In mammals, a centralized Golgi cluster disperses into fragments as the cell divides.
• In plants, Golgi vesicles deposit into the cell plate for rigid structure formation.

Spindle Repositioning

• Some cells reposition their spindle to divide asymmetrically
• Cell division can be symmetric, resulting in identical progeny.
• 95% of the events.
• Or asymmetric, resulting in non-identical progeny.
• ~5% of events.
• To produce two different progeny.

Stem Cell Division

• Stem cells self-renew and give rise to progenitor cells.
• Cells then divide symmetrically for cellular increase.
• Asymmetric segregation of cell fate determinants give to two different daughter progeny

Drosophila Development

• Drosophila embryos have RNAs that are labelled from different stages of development.

Mitosis Absence

• Mitosis can occur without cytokinesis
• Leads to nuclear division without cytoplasmic division.
• Early Drosophila embryo.
• Results in large cell with several thousand nuclei
• Cytokinesis later occurs during cellularization.

Cellular Fusion

• Cellular fusion is the opposite of Cytokinesis
• Has embryonic progenitors divide into myocytes.
• Myocytes fuse to form myofibrils present in muscles