Cytokinesis

Cytokinesis is the division of the cytoplasm that normally follows karyokinesis and completes the formation of two separate daughter cells; in many cells cytokinesis actually begins during telophase, overlapping with the final stages of nuclear reorganisation rather than waiting for it to finish completely. Because animal cells and plant cells differ fundamentally in whether they have a rigid cell wall, they achieve cytokinesis by two quite different mechanisms, described in the two sub-sections that follow: a contractile ring that pinches the cell in two in animals, and a cell plate that is built outward from the centre of the cell in plants. A useful check on how tightly mitosis is coordinated with the cell's other activities is that transcription (the reading out of genes into RNA) is switched off during mitosis itself, only resuming once the daughter cells have re-formed their nu …

In animal cells, cytokinesis is a contractile process driven by a structure called the contractile ring, positioned just inside the plasma membrane at the cell's equator. The ring is built from a bundle of microfilaments assembled from the proteins actin and myosin - the same pair of proteins responsible for contraction in muscle - and their sliding interaction generates a real contractile force. As the ring contracts, it draws the plasma membrane inward at the equator, producing a visible indentation called the cleavage furrow on the cell's surface; the furrow deepens progressively around the entire circumference of the cell until it pinches the cell fully in two, separating the two daughter cells. This mechanism works well for animal cells, which lack a rigid cell wall and can t …

Plant cells cannot use a contractile ring because a rigid cell wall surrounds the plasma membrane, so cytokinesis instead proceeds by growing a new dividing structure from the centre of the cell outward to the existing lateral walls - described as a centrifugal manner of cell plate formation, and it typically begins during telophase, overlapping with nuclear reorganisation. The structure responsible is the phragmoplast, an assembly of microtubules, actin filaments, and vesicles budded off from the Golgi apparatus and endoplasmic reticulum. These Golgi-derived vesicles are loaded with wall-building carbohydrates such as pectin and hemicellulose; they travel outward along the phragmoplast's microtubules to the cell's equator, where they fuse with one another to build a new stretch of plasma membrane, and the wall materials they carried are deposited to form new cell wall. The very first visible stage of this new wall is a thin dividing line called the cell plate, which progressively stretches all the way across the cell until it forms a continuous layer, the middle …