Cytokinesis and Cell Division in Plants and Animals
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You should be familiar with the stages of mitosis, particularly anaphase and telophase, and understand the basic structure of the plasma membrane and the cytoskeleton.
Why this matters
Mitosis separates chromosomes, but cytokinesis physically separates the cell into two distinct entities.
While the nuclear division (mitosis) ensures that each daughter nucleus receives an identical set of genetic material, it does not create two separate cells. Cytokinesis is the final phase of cell division where the cytoplasm divides, resulting in two genetically identical daughter cells. This process differs fundamentally between animal and plant cells due to the presence of a rigid cell wall in plants. Understanding this distinction is crucial for grasping how multicellular organisms grow and repair tissues, as well as how single-celled organisms reproduce asexually.
The Fundamental Difference: Cleavage vs. Cell Plate
In animal cells, which lack a cell wall, cytokinesis occurs through a process called cleavage. The cell membrane pinches inward at the equator, forming a structure known as the cleavage furrow. This constriction eventually separates the cytoplasm into two distinct daughter cells. In contrast, plant cells possess a rigid cell wall made of cellulose that prevents the membrane from pinching. Instead, plant cells build a new cell wall from the inside out. Vesicles derived from the Golgi apparatus accumulate at the centre of the cell, fuse together, and form a disc-like structure called the cell plate. This plate expands outward until it fuses with the existing cell wall, dividing the cytoplasm and establishing the boundary between the two new cells.
Cytokinesis in Animal Cells
The mechanism of cleavage furrow formation is driven by the actin cytoskeleton. A contractile ring composed of actin filaments and myosin II motor proteins assembles just beneath the plasma membrane at the metaphase plate. As myosin II slides along the actin filaments, the ring contracts, pulling the membrane inward. This process is tightly coordinated with the mitotic spindle; the position of the cleavage furrow is determined by the location of the spindle midzone, ensuring that the division plane is equatorial and that both daughter cells receive a fair share of cytoplasm and organelles.
The contraction is an active, energy-dependent process requiring ATP. If the contractile ring is disrupted, for example by drugs that depolymerize actin, cytokinesis fails, resulting in a multinucleated cell (a condition known as binucleation). This highlights that while mitosis is primarily a nuclear event, cytokinesis is a cytoplasmic event that relies heavily on the structural integrity of the cytoskeleton.
Cytokinesis in Plant Cells
Plant cytokinesis is a constructive rather than a destructive process. During late anaphase and telophase, vesicles containing cell wall materials (such as pectins and hemicelluloses) and membrane components are transported along microtubules to the centre of the cell. These vesicles fuse to form the cell plate. The cell plate grows outward toward the periphery, guided by a phragmoplast, which is a structure of microtubules and actin filaments that remains between the separating chromosomes.
As the cell plate expands, it eventually fuses with the existing plasma membrane and cell wall. The vesicles continue to deposit cell wall material, thickening the new wall until it is structurally identical to the original. This process ensures that the new cell wall is properly aligned and integrated with the existing cellular architecture. Unlike animal cells, plant cells do not undergo membrane invagination; instead, they synthesise new membrane and wall material to partition the cytoplasm.
Coordination with the Cell Cycle
Cytokinesis is not an independent event but is tightly coupled to the cell cycle. It typically begins during late anaphase and is completed during telophase. The completion of cytokinesis marks the end of the M phase (mitotic phase) and the entry into G1 of the next cell cycle. Regulatory proteins, such as cyclins and cyclin-dependent kinases (CDKs), control the transition between these phases. For instance, once securin has been destroyed to start anaphase, the destruction of cyclin B (and the fall in CDK1 activity) allows cytokinesis to proceed. If cytokinesis fails, the cell is left with two nuclei and double the normal DNA content.
The fidelity of cytokinesis is essential for maintaining genomic stability. If the division plane is misaligned, one daughter cell may receive more or fewer organelles, or in extreme cases, part of the nucleus, leading to cellular dysfunction or apoptosis. Thus, the precise timing and spatial regulation of cytokinesis are critical for the survival and proper function of the daughter cells.
The concept
The cell cycle and mitosis
How a cell duplicates and divides its contents faithfully.
A common misconception is that cytokinesis is simply the "end" of mitosis. In reality, mitosis and cytokinesis are separable: megakaryocytes (the large cells that produce platelets) repeat mitosis without completing cytokinesis and become polyploid, and the early fruit-fly embryo divides its nuclei many times inside one shared cytoplasm. Furthermore, the mechanism of cytokinesis is not universal; while animal cells use a contractile ring, some protists and fungi use different mechanisms, such as budding or fission, highlighting the evolutionary diversity of cell division strategies. C
Worked example
Problem. A spherical animal cell with a diameter of \( 20\ \mu\mathrm{m} \) undergoes cytokinesis. The cleavage furrow deepens until the cell is divided into two equal halves. Calculate the total surface area of the plasma membrane of the two daughter cells immediately after cytokinesis is complete, assuming the daughter cells are perfect spheres. Compare this to the surface area of the original parent cell.
- Determine the radius of the parent cell: \( r_{\text{parent}} = \frac{20\ \mu\mathrm{m}}{2} = 10\ \mu\mathrm{m} \).
- Calculate the volume of the parent cell: \( V_{\text{parent}} = \frac{4}{3}\pi r_{\text{parent}}^3 = \frac{4}{3}\pi (10)^3 = \frac{4000\pi}{3}\ \mu\mathrm{m}^3 \).
- Since the cell divides into two equal daughter cells, the volume of each daughter cell is \( V_{\text{daughter}} = \frac{V_{\text{parent}}}{2} = \frac{2000\pi}{3}\ \mu\mathrm{m}^3 \).
- Find the radius of a daughter cell using \( V = \frac{4}{3}\pi r^3 \): \[ \frac{4}{3}\pi r_{\text{daughter}}^3 = \frac{2000\pi}{3} \implies r_{\text{daughter}}^3 = 500 \implies r_{\text{daughter}} = \sqrt[3]{500} \approx 7.937\ \mu\mathrm{m} \]
- Calculate the surface area of one daughter cell: \( A_{\text{daughter}} = 4\pi r_{\text{daughter}}^2 = 4\pi (7.937)^2 \approx 4\pi (63.00) \approx 791.7\ \mu\mathrm{m}^2 \).
- Calculate the total surface area of the two daughter cells: \( A_{\text{total}} = 2 \times 791.7 = 1583.4\ \mu\mathrm{m}^2 \).
- Calculate the surface area of the parent cell: \( A_{\text{parent}} = 4\pi (10)^2 = 400\pi \approx 1256.6\ \mu\mathrm{m}^2 \).
- Compare the areas: The total surface area of the daughter cells is greater than that of the parent cell, which is expected because dividing a sphere into two smaller spheres increases the total surface area to volume ratio.
Units check. The calculation involves squaring length units (\(\mu\mathrm{m}\)), resulting in area units (\(\mu\mathrm{m}^2\)), which is consistent with the definition of surface area.
Three quick tests
- What structure is responsible for the physical separation of cytoplasm in animal cells?
Answer
The contractile ring, composed of actin filaments and myosin II, which forms the cleavage furrow. - How does the mechanism of cytokinesis differ in plant cells compared to animal cells?
Answer
Plant cells form a cell plate from vesicles that fuses with the existing cell wall, whereas animal cells pinch the membrane inward via a contractile ring. - Why is the coordination between mitosis and cytokinesis important for genomic stability?
Answer
It ensures that each daughter cell receives a complete and equal set of chromosomes and cytoplasmic components, preventing aneuploidy or cellular dysfunction.
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Lecture notes drafted with a local model and checked line by line by the editor; every number in the worked example was recomputed.