L10: Better phrasing for an essay Flashcards
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what is cytokenesis and instances where it isnt needed
Cytokinesis is the final step of cell division, in which the cytoplasm of a parental cell is physically divided to produce two daughter cells. In most eukaryotic cells, this process closely follows mitosis, ensuring that each new cell receives the necessary organelles and cytoplasmic contents.
- Exception — Cytokinesis Without Mitosis: Although typically coupled, cytokinesis does not always occur after mitosis. When mitosis proceeds without cytokinesis, it results in the formation of multinucleated cells. This phenomenon, known as multinucleation, features multiple nuclei sharing a single cytoplasmic space.
- Example — Drosophila Embryo: A well-studied example is the early Drosophila melanogaster (fruit fly) embryo. After fertilization, localized maternal mRNAs (e.g., bicoid and nanos) orchestrate spatially controlled nuclear divisions within a shared cytoplasm, forming a syncytium. Over several rounds of nuclear division, the nuclei migrate toward the cell cortex. Eventually, individual membranes form around each nucleus in a process called cellularization — a delayed cytokinesis that allows for rapid, coordinated development during early embryogenesis.
- Specialized Animal Cells — Multinucleation: Certain specialized cells in animals also undergo mitosis without subsequent cytokinesis. For instance, hepatocytes in the liver may become multinucleated. These cells maintain chromosomal division but bypass the physical cytoplasmic separation, which may be linked to their high metabolic demands and unique functions.
- Cytokinesis Mechanism in Animal Cells: In most animal cells, cytokinesis involves the formation of a cleavage furrow driven by a contractile ring composed of actin filaments and myosin motor proteins. This contractile apparatus tightens around the cell’s equator, pinching it into two separate daughter cells.
- Clarification — No Volume Increase: It is important to note that cytokinesis does not require an increase in cell volume. Instead, it is a division process that redistributes existing cytoplasmic content between daughter cells.
the actin-myosin ring
The formation and function of the actomyosin ring are crucial for cytokinesis, which is the final step of cell division. Here’s a breakdown of the process:
Assembly of the Actomyosin Ring:
The actin filaments and myosin II motors assemble at the inner cortex, which is the inner side of the plasma membrane (PM).
The contractile ring forms here, with actin filaments forming a structure that gets attached to the inner plasma membrane.
Myosin II, as a motor protein, interacts with the actin filaments. Myosin moves along the actin filaments, which causes the filaments to slide past one another. This sliding pulls the actin filaments together, resulting in the constriction of the actomyosin ring.
Contraction of the Ring:
As myosin II functions, it causes the actin filaments to slide towards their barbed ends (the plus ends), resulting in ring constriction.
Myosin’s motor activity causes the actin filaments to become shorter, pulling the ring tighter and driving the cleavage furrow to ingress (invaginate) deeper into the cell’s cortex.
Cleavage Furrow Formation and Ingression:
As the ring contracts, it drags the plasma membrane (PM) with it, resulting in the formation of the cleavage furrow.
The cleavage furrow continues to ingresses until it eventually reaches the nuclear midbody, which is the region between the two daughter cells at the final stages of division.
This ingression leads to the formation of the intercellular bridge, which is the final connection between the two daughter cells, before abscission (the final separation
The actomyosin ring must be attached to the plasma membrane to function correctly. If this attachment doesn’t occur, cytokinesis cannot proceed.
This attachment is facilitated by linker proteins that connect the actin-myosin machinery to the plasma membrane and the medial cortex (the cytoskeleton just beneath the membrane).
Important Linker Proteins:
Formin, septin, and anillin are some of the key linker proteins involved in attaching the actomyosin ring to the cellular cortex.
Cdc15 and Mid1 are crucial proteins that help link the actomyosin ring to the plasma membrane. In mutants lacking both Cdc15 and Mid1, the actomyosin ring constricts in the cytoplasm rather than at the plasma membrane, meaning cytokinesis fails.
Failure to Attach to the Plasma Membrane:
If the actomyosin ring doesn’t attach to the plasma membrane, cytokinesis cannot occur. This means the cell will not divide properly.
rhoa gtpase and its activation
RhoA is a small GTPase in the Ras superfamily that plays a central role in orchestrating cytokinesis. It cycles between an inactive GDP-bound form (RhoA-GDP) and an active GTP-bound form (RhoA-GTP). The activation status of RhoA is regulated by GTPase-activating proteins (RhoGAPs), which promote GTP hydrolysis and inactivate RhoA.
- RhoA Activation and Function During Cytokinesis:
When activated (RhoA-GTP), RhoA coordinates the assembly and constriction of the contractile ring through two major pathways:
Actin Filament Formation via Formin:
Active RhoA binds to the actin-nucleating protein formin. This interaction promotes the polymerization of actin filaments, a critical component of the contractile ring structure that drives cell membrane constriction.
Myosin II Activation via ROCK:
RhoA-GTP also activates Rho-associated coiled-coil kinases (ROCK). ROCK phosphorylates the regulatory light chain of myosin II, activating its motor function and promoting actomyosin contraction. Additionally, ROCK inhibits myosin phosphatase, sustaining myosin II activity during furrow ingression.
- Spatial Regulation of RhoA Activity:
The localization of active RhoA (RhoA-GTP) is highly specific—it accumulates at the equatorial cortex of the cell just prior to cleavage furrow formation. This spatial regulation ensures the precise placement of the contractile ring. In sea urchin embryos, fluorescent biosensors have visualized this RhoA activation pattern, highlighting its role in defining the cytokinesis plane.
- RhoA as a Cytokinetic Landmark:
The focal accumulation of RhoA-GTP determines where the contractile ring will form, effectively marking the division site. This ensures accurate and symmetrical cell division by guiding the machinery responsible for furrow ingression to the correct location.
models
During cytokinesis, the site of contractile ring formation is tightly regulated to ensure accurate cell division. Several models explain how cells determine the division plane, especially how RhoA is activated at the cell equator. These models include:
- Astral Stimulation Model
Mechanism: Astral microtubules emanating from the spindle poles extend toward the cell cortex and continuously grow and shrink (dynamic instability). When microtubules from opposite poles meet at the cell equator, they stabilize.
Effect: Stabilized microtubules facilitate the delivery of RhoA activators to the equatorial cortex. This localized activation defines the site of contractile ring assembly.
Relevance: This model is more applicable in smaller cells, where microtubules can easily reach and interact with the cortex.
- Central Spindle Stimulation Model
Mechanism: RhoA activators are generated or localized at the central spindle (midzone of the mitotic spindle) during anaphase.
Effect: These activators then diffuse toward the cortex, and the region of cortex closest to the central spindle becomes the site of RhoA activation and contractile ring formation.
Relevance: This model is favored in larger cells, where the spindle midzone is more distant from the cortex and diffusion can define a broader region.
- Astral Relaxation Model
Mechanism: Astral microtubules interact with the polar regions of the cell cortex and induce cortical relaxation by removing or inhibiting myosin activity.
Effect: This suppression of cortical tension at the poles leads to a relative enrichment of active myosin in the equatorial region, resulting in contractile ring formation there.
Relevance: Suggests positioning by negative regulation at the poles rather than direct activation at the equator.
During cytokinesis, the spatial activation of RhoA is a critical determinant of where the cleavage furrow will form. One key model explaining this is the astral stimulation model, which suggests that RhoA activators are delivered to the cell cortex by astral microtubules extending from the spindle apparatus. This ensures that RhoA becomes active specifically at the equator of the cell, precisely where constriction is required. At the heart of this process is the centralspindlin complex, composed of CYK-4—a RhoA GTPase-activating protein (GAP)—and MKLP1, a kinesin motor protein. Although CYK-4 is a GAP, in this context it paradoxically facilitates RhoA activation, which in turn triggers actin filament nucleation and myosin II activation, leading to the formation of the contractile ring. RhoA must cycle between its active (GTP-bound) and inactive (GDP-bound) forms to ensure proper timing and spatial control of constriction. Additional studies using optogenetic tools have demonstrated that RhoA can be artificially activated by light in non-contractile anaphase cells, confirming that localized RhoA activation alone is sufficient to initiate furrow formation. Moreover, dynein, a motor protein, plays a supporting role by transporting myosin II away from the poles, contributing to polar relaxation. This process reduces cortical tension at the poles and directs actomyosin activity to the cell center, where constriction occurs. Notably, the exact positioning of the myosin ring may vary depending on the cell type, suggesting that although these mechanisms are conserved, they can be modulated according to cell identity.
the rappaport experiment
Rappaport’s classic experiments provided pivotal insight into how the cleavage furrow is positioned during cytokinesis. Using physical manipulation of dividing cells, he demonstrated that astral microtubules play a more prominent role than previously thought.
Experimental Design
Rappaport manipulated fertilized sea urchin eggs to adopt a doughnut or horseshoe shape using physical constraints.
During the first division, the cleavage furrow formed normally between the chromosomes, directed by the spindle midzone.
After the first division, the now horseshoe-shaped cell underwent a second round of mitosis.
Two spindles formed—one in each arm of the horseshoe.
Unexpected Result
Instead of forming a single cleavage furrow per spindle (which would be expected if only the central spindle dictated cleavage), cleavage occurred between adjacent spindle poles, even in the absence of chromosomes or a midzone.
This resulted in four daughter cells, not two.
Conclusion
This experiment showed that astral microtubules—originating from spindle poles—are sufficient to induce cleavage furrows.
The results challenged the idea that the central spindle is the sole determinant of division plane positioning.
It provided strong evidence that local signals from astral microtubules can direct furrow formation, independent of chromosomes or the central spindle.
the midbody
During the final stages of cytokinesis in mammalian cells, the two daughter cells remain connected by a structure known as the midbody, a remnant of the central spindle that forms after cortical ingression. The midbody is composed of tightly bundled, overlapping antiparallel interpolar microtubules, which interdigitate at the center, creating a scaffold essential for the final separation process known as abscission. A key feature of the midbody is the high level of vesicular trafficking and secretion occurring in this region, which supports membrane remodeling required for plasma membrane fusion. Before abscission can take place, the central spindle must be disassembled, and the cytoplasm must be maintained intact, with careful coordination between cytoskeletal elements and membrane components. The central spindle proteins, which include RhoA regulators, also contribute to organizing the midbody by linking the plasma membrane to the underlying cytoskeleton. This interaction is critical for maintaining mechanical stability in the cleavage furrow, ensuring it remains ingressed and structurally sound during the final stages of division. As division progresses, the midbody itself undergoes progressive constriction, narrowing from one or both sides until the daughter cells are fully separated. This complex interplay between the midbody, cytoskeleton, and membrane components highlights its central role in ensuring successful and accurate cell division.
