AP Syllabus focus:
‘Cytokinesis divides the cytoplasm, forming a cleavage furrow in animal cells or a cell plate in plant cells to produce two daughter cells.’
Cytokinesis is the physical division of one cell into two, completing cell division by partitioning cytoplasm, organelles, and membranes. Its mechanisms differ in animals and plants because plant cell walls prevent membrane “pinching.”
Side-by-side schematic comparing animal and plant cytokinesis. The animal cell panel highlights cleavage furrow ingression driven by a contractile ring, while the plant cell panel shows cell plate formation and outward expansion to partition the cytoplasm into two daughter cells. Source
What cytokinesis accomplishes
Cytokinesis ensures that the products of mitosis become two separate daughter cells with their own plasma membranes and appropriate cellular contents.
Cytokinesis: Division of the cytoplasm (and associated membranes/organelles) that separates one cell into two daughter cells after nuclear division.
It is tightly coordinated with late mitosis so that the division plane aligns with the separated chromosome sets, reducing the risk of trapping DNA in the closing division site.
Core idea: division plane and cytoplasmic partitioning
Positioning the division site
Cells must “choose” where to split so each daughter cell receives a nucleus and sufficient cytoplasm.
In many eukaryotes, the mitotic spindle helps specify the cell’s equator.
Microtubules and associated proteins help localise factors that organise the machinery of cytokinesis at the correct site.
What must be distributed
Cytokinesis partitions:
Cytosol (soluble components and metabolites)
Organelles (e.g., mitochondria, chloroplasts in plants) by cytoplasmic segregation
Plasma membrane (new membrane surface added and reshaped during division)
Cytokinesis in animal cells: cleavage furrow
Cleavage furrow formation (specification focus)
In animal cells, cytokinesis occurs by creating a cleavage furrow—an indentation that deepens until the cell pinches into two.
Contractile ring mechanism
Key steps and structures:
A belt-like contractile ring forms just under the plasma membrane at the cell equator.
The ring is primarily actin filaments plus myosin motor proteins.
Myosin-driven sliding of actin filaments tightens the ring, like a drawstring, pulling the membrane inward.
Important features:
The furrow ingresses from the outside toward the centre, progressively narrowing the cytoplasmic bridge between the forming daughter cells.
Final separation (often called abscission) resolves the thin connection so two distinct cells form.
Why this works in animals
Animal cells lack rigid walls, so the plasma membrane can deform and constrict. Membrane trafficking also helps provide additional membrane surface as the furrow deepens.
Cytokinesis in plant cells: cell plate
Cell plate formation (specification focus)
Textbook diagram of cytokinesis showing (a) animal cleavage furrow formation by an actin-based contractile ring and (b) plant cell plate formation from Golgi-derived vesicles transported to the cell center. The figure emphasizes the key contrast that animal cytokinesis constricts inward, whereas plant cytokinesis builds a new partition that expands outward to the cell periphery. Source
In plant cells, cytokinesis occurs by building a cell plate that becomes the new dividing boundary, because the rigid cell wall prevents a cleavage furrow from pinching inward.
Vesicle-driven construction
Key steps and structures:
Vesicles (commonly derived from the Golgi apparatus) move along microtubules to the cell’s centre.
Vesicles fuse to form a membranous, disc-like structure: the cell plate.
Continued vesicle fusion enlarges the cell plate outward until it reaches and fuses with the existing plasma membrane.
What the cell plate becomes:
The fused vesicle membranes become the new plasma membranes of the two daughter cells.
Vesicle contents contribute to the new cell wall material (especially the middle lamella and primary wall components) laid down between the daughter cells.
Directionality difference
Animal cytokinesis: membrane constriction moves inward (outside → centre).
Plant cytokinesis: cell plate grows outward (centre → outside).
Common outcomes and common errors
Successful outcome: two daughter cells
Cytokinesis completes the formation of two physically separate daughter cells by:
Finalising the boundary between cells
Restoring membrane continuity
Establishing distinct intracellular compartments
If cytokinesis fails
Consequences can include:
Binucleate or multinucleate cells (nuclei divide but the cell does not split)
Abnormal cell size and altered cell function due to improper cytoplasmic partitioning
Disrupted tissue organisation when many cells in a lineage fail to separate properly
FAQ
A rigid cellulose cell wall resists inward pinching.
The cell plate strategy builds a new partition internally, then integrates it with the existing membrane and wall.
Signals from the spindle region help position the furrow at the equator.
Local activation of contractile machinery occurs where these cues concentrate, aligning division with the segregated chromosomes.
Membrane is remodelled and supplemented by vesicle trafficking.
In many cells, targeted vesicle fusion adds surface area where constriction or plate growth is occurring.
The phragmoplast is a microtubule-based structure that guides vesicles to the midline.
It helps organise vesicle delivery and supports outward expansion of the forming cell plate.
Yes; some divisions produce daughters of different sizes.
Asymmetric cytokinesis can differentially allocate cytoplasmic factors, influencing cell fate and specialised function in development.
Practice Questions
State one structural feature used for cytokinesis in animal cells and one used in plant cells. (2 marks)
Animal: cleavage furrow / contractile ring (1)
Plant: cell plate (1)
Describe how cytokinesis produces two daughter cells in (i) animal cells and (ii) plant cells. Include the role of cytoskeletal elements or vesicles in each. (6 marks)
(Animal) Formation of a cleavage furrow at the cell equator (1)
(Animal) Actin–myosin contractile ring constricts to pinch membrane inward (1)
(Animal) Separation completes as the cytoplasmic bridge is resolved (abscission) (1)
(Plant) Golgi-derived vesicles move to the centre and fuse (1)
(Plant) Fused vesicles form and expand the cell plate outward to the plasma membrane (1)
(Plant) Cell plate becomes new plasma membranes and contributes to the new cell wall between daughter cells (1)
