Mitosis and asexual cell division

Asexual cell division in unicellular eukaryotic organisms is synonymous with growth. Division is usually by binary fission after a single nuclear division. The parent cell divides, usually longitudinally, into two even-sized identical progeny cells. Division can be by multiple fission where many nuclear divisions occur, producing either a large multinucleate coenocyte or many uninucleate progeny cells. All cells produced from mitosis are genetically identical to their parent.

In both cases somatic cell division is preceded by the mitotic division of the cell nucleus. In mitosis the replicated DNA from the S phase of the cell cycle is separated equally into two progeny cells.  The events of mitosis can be separated into four stages for convenience, but each flows into the other as a continuous process.

The first phase in mitosis is the prophase. In this phase microtubules form from the microtubule organizing centers (MTOCs).  In fungi   these structures are known as spindle pole bodies (SPBs), located close to the nuclear envelope. In the motile species of protista the MTOC is called a centriole and it becomes surrounded by microtubules in a process termed aster formation. The MTOCs begin to move towards opposite poles of the nucleus, and spindle microtubules appear between them. Single chromosomes, which have been duplicated to form chromatids, are joined together at their centromeres. These centromeres are also attached to spindle microtubules. By late prophase–early metaphase, MTOCs are opposite each other and the spindle is complete, with chromosomes aligned across the center in a metaphase plate.  In many species of fungi, chromosomes remain extremely indistinct during mitosis, and they do not appear to assemble across a metaphase plate.

The nuclear envelope may disappear between the prophase and the beginning of the meta- phase in some eukaryotic microbes, but in some protista the nuclear envelope remains intact throughout the process, the microtubules of the spindle penetrating through it.

In the next phase of mitosis, the anaphase, pairs of chromatids that were held together at the centromere begin to separate simultaneously, and spindle microtubules begin to pull them towards the two poles of the cell. By the end of the anaphase the chromatids have been pulled close to the MTOCs. In many species of the fungi there is an asynchronous chromosome separation during the anaphase.

In the final phase of mitosis, the telophase, the aster microtubules disappear, and the nuclear envelope re-forms if it has disintegrated. In the two progeny nuclei the MTOC duplicates, and cell division commences with the division of the cytoplasm by an invaginating plasma membrane or the formation of a cell plate by Golgi-derived vesicles across the midline between the two nuclei. In some protista separation can be by budding, producing a progeny cell that is much smaller than the parent.