The division of the cell in two daughter cells happens during M phase. It consists on different processes running at the same time that end up with the division of the cellular components to form the two new cells. The components are synthesized in previous cell cycle phases share almost equally by the two daughter cells. The salient components are DNA, synthesized during the S phase, and cytoplasmic organelles and molecules, produced during G1, S, and G2 phases. Mitosis is the chromatin condensation into chromosomes and their separation to be included in the two new cells. Mitosis is a process of M phase that includes several stages: prophase, metaphase, anaphase and telophase. Cytokinesis is another process that runs during telophase and it leads to the splitting of the cytoplasm.
1. Mitosis
Mitosis is a drastic change in the cell, which has to form a molecular machinery for separating the chromosomes: the mitotic spindle. The mitotic spindle is formed by microtubules bearing the chromosomes attached to one of their ends. The animal cell mitotic spindle has two poles where centrosomes are located. There are two types of mitosis: open and closed. Open mitosis involves the nuclear envelope disorganization and the spindle formation in the cytoplasma-nucleoplasm space. Closed mitosis keeps the integrity of the nuclear envelope and the mitotic spindle is intranuclear. In closed mitosis, cell division means the strangling of the nucleus, as well as the cytoplasm, so the cytoplasm and nucleoplasm are always separated by the nuclear envelope. Some species show an intermediate type of mitosis with the nuclear envelope having breakages. Other species may build cytoplasmic mitotic spindle keeping the nuclear intact.
Prophase
Prophase starts with DNA condensation so much that chromatids can be observed at light microscopy. Nucleolus disappears at the beginning of the prophase. Phosphorylation of histones, proteins of the chromatin, triggers DNA condensation. There are also changes going on in the cytoplasm. For example, cytoskeleton filaments undergo organizational changes, cell adhesion is almost lost so that mitotic cells become rounded. This cellular shape is a feature of cells entering in mitosis. In animal cells, the centrosome is duplicated at the end of the S phase. Initially, the two centrosomes remain together, but they are headed toward opposite sites in the cytoplasm at the beginning of prophase, moved by motor proteins associated to microtubules. After that, centrosomes start polymerizing microtubules with high dynamic instability (alternating between growing and shortening). Later, these microtubules form the mitotic spindle (Figure 1). In animal cells, organulles like endoplasmic reticulum and Golgi complex are broken in small pieces, and the vesicular traffic is much less intense. The nuclear envelope is still present.
Some authors propose a further phase, after prophase, named as prometaphase. The nuclear envelope disorganizes during prometamaphase and it is broken in small pieces resembling vesicles. This process is initiated by phosphorylation of the lamina proteins that form the nuclear lamina. In this way, microtubules can access chromatin, which continue the compaction process to become chromosomes. The microtubule plus ends make contact with kinetochores, which are structures located at the chromosome centromeres. Microtubules contacting the kinetochores are known as kinetochore microtubules. Each chromosome has two kinetochores at opposite locations, so that one kinetochore is contacted by microtubules polymerized in one centrosome, and the other kinetochore by microtubules coming from the other centrosome. Thus, every chromosome is linked by microtubules coming from both centrosomes. The number of microtubules converging in one kinetochore is variable, about 20 to 40 in humans, whereas in yeast it is only one. Other microtubules coming from the two centrosomes do not contact chromosomes but interact between each other by their plus ends. These interactions make them more stable by decreasing the dynamic instability at the plus end. These microtubules are known as interpolar microtubules.
Metaphase
At the end of prophase (or prometaphase), sister chromatids join together to form chromosomes, and kinetochore microtubules are contacting kinetochores. Chromosomes are moved to the center of the mitotic spindle by kinetochore microtubules, at a place equidistant from the two spindle poles (centrosomes in animal cells), forming the so-called metaphase plate. Chromosomes are moved by increase and decrease the length of the kinetocore microtubules, as well as by the traction forces of microtubule associated motor proteins. When all chromosomes are positioned and lined up at the metaphase plate, we have the typical metaphase figure. When chromosomes are lining up at the metaphase plate, they can temporarily quit the metaphase plate and enter again. This is indicative of the ongoing pull and push forces produced by the kinetore microtubules, since every chromosome is contacted by microtubules coming from both spindle poles.
The mitotic spindle is a scaffold of microtubules, MAPs (microtubule associated proteins), and motor proteins (dynein and kinesin). It is assembled during late prophase and gets its typical organization during metaphase. The minus ends of microtubules are concentrated in the spindle poles. The plus ends of kinetochore microtubules contact kinetochores of chromosomes lined up at the mitotic plate. Other microtubules, known as astral microtubules, are oriented toward the cytoplasm periphery near to the spindle pole and their plus ends are close to the plasma membrane. There are another microtubules that do not contact the chromosomes but the plus ends of other microtubules coming from the opposite spindle pole. They are known as interpolar microtubules. In large mitotic spindle, where the number of microtubules may be thousands, as in amphibian cells and in the endosperm of angiosperms, there are microbutules non connected with mitotic poles but associated to chromosomes.
Anaphase
Anaphase starts with the split of each chromosome into two sister chromatids. The breakage of the link happens at the centromere and allows sister chromatids to be dragged apart by microtubules toward the opposite spindle poles. The dragging speed is about 1 µm/min. There are two stages: anaphase A, when kinotochore microtubules depolymerize at both minus and plus ends, so they shorten and chromatids are dragged; anaphase B, when interpolar microtubules polymerize and increase in length so that they push the spindle poles in opposite directions, and therefore the kinetochore microtubules and chromatids are also separated. Motor proteins associated to the plus ends of the interpolar microtubules provide the dragging force to move away the spindle poles by sliding one interpolar microtubule coming from one spindle pole over another microtubule coming from the other spindle pole. Other motor proteins associated with the astral microtubules drag centrosomes toward the surface of the cell.
Telophase
During telophase, nuclear envelopes are assembled around the two groups of chromatids (Figure 2) which have been dragged toward the two spindle poles. Then, two nuclei are formed. Phosphorylation of lamins, proteins of the nuclear lamina, initiates this process. Nuclear pores are also assembled and uncoiling of chromatids begins. Previously, microtubules were released from kinetochores.
2. Cytokinesis
Cytokinesis is the last stage of the cell cycle. During cytokinesis, the cytoplasm is divided and gives two new independent cells. This division takes place after the chromatids have been pulled apart, otherwise ploidies (unequal distribution of chromatids between the two daughter cells) may happen. Cytokynesis is different in animal, plant and fungi cells. However, they all follow some similar steps: choosing the orientation of the division plane, assembling the division molecular machinery, and division of the cytoplasm.
In animal cells, the division plane is set by the orientation of the mitotic spindle, and the first evidence of the beginning of cytokinesis is a furrow, known as division furrow, on the plasma membrane (Figure 3). The division furrow is perpendicular to the mitotic spindle and is located in an equatorial position (at the same distance from the two centrosomes). The interactions between actin filaments and myosin motor proteins produces the division furrow during the final part of anaphase. Actin filaments slide over each others pulled by myosin molecules assembling the division ring, which decreases the diameter and strangles the cytoplasm until the ring gets completely closed. The contractile ring is transient and disappears after the division is completed. Before that, the spindle microtubules trapped by the ring must be removed. Furthermore, the opening and sealing of cell membranes that gives two independent cells is a complex process. In both animal and plant cells, it has been observed that organelles involved in the vesicular traffic are needed during the final part of cytokinesis. They provide more membrane for the new plasma membranes and the protein machinery to deal with breakage and fusion of the membranes, processes that are similar to those happening in the vesicular trafficking.
In plant and fungi cells, cytokinesis is different because of the cell wall. The two new cells do not get separated by a ring of actin filaments. Instead, a new cell wall is formed in the interior of the progenitor cell that divides the cytoplasm in two parts (Figure 4). In plants, the first evidence of this new cell wall formation is the phragmoplast. Phragmoplast is a complex structure composed of some microtubules from the mitotic spindle and vesicles delibered by the Golgi apparatus. Vesicles coming from the Golgi apparatus are moved to this central area by microtubules and motor associated proteins. Vesicles fuse between each other and start to form the cell membranes of the new cells, whereas their content forms the medial lamina of the new cell wall. In plants, the cell wall grows centrifugally, i.e. fom the inner part of the cell toward the periphery. In fungi cells, which have no phragmoplat, it is in the other way around, the more external components are the more recently synthesized. A division furrow is not observed in plant cells, but it is present in fungi cells. In plants, the orientation and position of the division plane is established under the influence of the nucleus during the late G2 phase. Microtubule bundles are oriented around the cell nucleus before the M phase starts, and this scaffold, known as preprophase band (PPB), leaves a trace in the cortical cytoplasm. The bundle of microtubules around the nucleus disappears at the beginning of mitosis, but the modifications they made in the cortical cytoplasm affect the formation of the pragmoplast.
Bibliography
Pardo M. 2005. Citoquinesis en células eucariotas. Investigación y ciencia 346:40-49.
Wanke C, Kutay Uz. 2013. Enclosing chromatin: reassembly of the nucleus after open mitosis. Cell 152: 1222-1225.
G2 phase