Cytokinesis is the final event of the cell cycle and is the process that divides one cell into two daughter cells. Cytokinesis was perhaps the first major cell cycle event observed directly. The use of ingenious micromanipulation experiments in echinoderm embryos led to many early insights into the mechanisms of cytokinesis (211
). Unlike other cell cycle events, cytokinesis has been particularly resistant to in vitro biochemical approaches, making progress difficult. However, in recent years, the use of genetically tractable model organisms has been enormously powerful in dissecting out the different steps and molecules involved in cytokinesis.
In general, the goal of cytokinesis is common in all organisms: to physically separate a mother cell into two daughter cells. How different organisms conduct cytokinesis varies, but the major events are universal (Fig. ). In animal cells, the division site is first chosen, generally at the cell equator, and subsequently the cleavage furrow is assembled at the division site (Fig. ). The furrow contains actin, myosin, and other proteins that are organized into a contractile ring called the actomyosin ring. The ring then ingresses or contracts, generating a membrane barrier between the cytoplasmic contents of each daughter cell. The ingressing furrow constricts components of the spindle midzone into a focused structure called the midbody. In the final cytokinetic event, called abscission, the furrow “seals,” generating two completely separate cells. The overall approach to division is well conserved between the fission yeast Schizosaccharomyces pombe and animals. Fission yeast also positions an actomyosin ring at the cell equator (Fig. ). However, unlike animal cells, S. pombe cells synthesize a division septum behind the ring as it constricts, generating new cell wall material between daughter cells. The septum is ultimately degraded by digestive enzymes, physically separating the daughter cells. Unlike animal cells and fission yeast, the yeast Saccharomyces cerevisiae divides by budding (Fig. ). First the site to bud is marked, and then through polarized cell growth the bud grows outward from the mother cell cortex, gradually increasing in size. The contractile ring is assembled at the bud neck, and similar to fission yeast, a division septum is synthesized behind the constricting ring. Plants significantly differ from yeast and animals in that they do not use a contractile ring to divide (Fig. ). In contrast, plants interdigitate microtubules and actin to build a dense structure called the phragmoplast between divided nuclei. Microtubule-directed vesicles containing cell wall material are targeted to the phragmoplast, and fusion of these vesicles causes the phragmoplast to grow outward toward the cell cortex, forming a cell plate. The cell plate fuses to the parent cell wall, dividing the parent cell into two daughters.
FIG. 1. General mechanisms of cytokinesis in eukaryotes. While the process of cytokinesis results in the physical partition of a mother cell into two daughter cells, the approach to cell division differs between several model organisms. (A) Higher plants, after (more ...)
A recent area of intense research has focused on the coordination of cytokinesis and the nuclear cycle. It is crucial that cytokinesis only occurs after mitosis to ensure that equal complements of genetic material are transmitted to each future daughter cell. Important signaling mechanisms and checkpoints have recently been uncovered in many model organisms that spatially and temporally coordinate cytokinesis with mitosis, ensuring the integrity of genetic transfer.
In the following sections, we review recent studies using several eukaryotic model organisms to study cytokinesis, focusing on mammalian cells, invertebrates, yeast, and higher plants. The major events contributing to cytokinesis—including determining the division site, building the division apparatus and mechanically dividing, and coordinating cytokinesis with the nuclear division cycle—will be discussed. We conclude with a brief description of variations on the common theme of cytokinesis important for development.