M-phase (Mitosis and Cytokinesis)

Mitosis

Mitosis is one of the most dramatic and beautiful biological processes that we can easily observe. To better illustrate what happens, biologists have traditionally divided mitosis into five arbitrary phases ,namely Prophase, prometaphase, metphase, anaphase, and telophase.  However, the actual process is dynamic and continuous, and not broken into discrete steps. 

Prophase

The first stage of mitosis, prophase, is said to begin when the  chromosome condensation initiated in G2 phase reaches the  point at which individual condensed chromosomes first become visible with the light microscope. Because the condensation process begun in G2 continues throughout prophase, chromosomes that start prophase as minute threads appear quite bulky before its conclusion. Ribosomal RNA synthesis ceases when the portion of the chromosome bearing the rRNA genes becomes condensed.

Spindle Apparatus and Centrioles 

The assembly of the spindle apparatus that will later separate the  sister chromatids occurs during prophase, replacing the normal microtubule structure of the cell that was disassembled in the  G2 phase. In animal cells, the two centriole pairs formed during G2 phase begin to move apart early in prophase, forming between them an axis of microtubules referred to as spindle fibers. By the time the centrioles reach the opposite poles of the cell, they have established a bridge of microtubules, called the spindle apparatus, between them.

In plant cells, a similar bridge of microtubular fibers forms between opposite poles of the cell, although centrioles are absent in plant cells.

In animal cell mitosis, the centrioles extend a radial array of microtubules toward the nearby plasma membrane when they reach the poles of the cell. This arrangement of microtubules is called an aster. Although the aster’s function is not fully understood, it probably braces the centrioles against the membrane and stiffens the point of microtubular attachment during the retraction of the spindle. Plant cells, which have rigid cell walls, do not form asters.

Nuclear Envelope Breakdown 

During the formation of the spindle apparatus, the nuclear envelope breaks down, and the endoplasmic reticulum reabsorbs its components. At this point, the microtubular spindle fibers extend completely across the cell, from one pole to the other. 

Their orientation determines the plane in which the cell will subsequently divide, through the center of the cell at right angles to the spindle apparatus.

Prometaphase

The transition from prophase to prometaphase occurs following the disassembly of the nuclear envelope. During prometaphase the condensed chromosomes become attached to the spindle by their kinetochores. Each chromosome possesses two kinetochores, one attached to the centromere region of each sister chromatid see figure above. 

Attachment of Microtubules

As prometaphase continues, a second group of microtubules grows from the poles of the cell toward the centromeres. These microtubules are captured by the kinetochores on each pair of sister chromatids. This results in the kinetochores of each sister chromatid being connected to opposite poles of the spindle. This allows the proper separation, or disjunction, of the sister chromatids.

This bipolar attachment is critical to the process of mitosis; any mistakes in microtubule positioning can be disastrous. For example, the attachment of the kinetochores of both sister chromatids to the same pole leads to nondisjunction (a failure of the sister chromatids to separate): The two sisters will be pulled to the same pole and end up in the same daughter cell, with the other daughter cell missing that chromosome.

Movement of Chromosomes to the Cell Center

Each chromosome is attached to the spindle by microtubules running from opposite poles to the kinetochores of sister chromatids. The chromosomes are being pulled simultaneously toward each pole, leading to a jerky motion that eventually pulls all of the chromosomes to the equator of the cell. At this point, the chromosomes are arranged at the equator with sister chromatids under tension and oriented toward opposite poles by their kinetochore microtubules.

The force that moves chromosomes has been of great interest since the process of mitosis was first observed. Two basic mechanisms have been proposed to explain this: (1) Assembly and disassembly of microtubules provides the force to move chromosomes, and (2) motor proteins located at the kinetochore and poles of the cell pull on microtubules to provide force. Data have been obtained that support both mechanisms.

In support of the microtubule-shortening proposal, isolated chromosomes can be pulled by microtubule disassembly. The spindle is a very dynamic structure, with microtubules being added to at the kinetochore and shortened at the poles, even during metaphase. In support of the motor protein proposal, multiple motor proteins have been identified as kinetochore proteins, and inhibition of the motor protein dynein slows chromosome separation at anaphase. Like many phenomena that we analyze in living systems, the answer is not a simple either/or choice. Both mechanisms are probably at work.

Metaphase

The alignment of the chromosomes in the center of the cell signals the beginning of the third stage of mitosis, metaphase. When viewed with a light microscope, the chromosomes appear to array themselves in a circle along the inner circumference of the cell, just as the equator girdles the Earth (figure ). An imaginary plane perpendicular to the axis of the spindle that passes through this circle is called the metaphase plate. The metaphase plate is not an actual structure but rather an indication of the future axis of cell division.

Figure:Metaphase. In metaphase, the chromosomes are arrayed at the midpoint of the cell. The imaginary plane through the equator of the cell is called the metaphase plate. As the spindle itself is a three-dimensional structure, the chromosomes are arrayed in a rough circle on the metaphase plate.

Positioned by the microtubules attached to the kinetochores of their centromeres, all of the chromosomes line up on the metaphase plate. At this point, their centromeres are neatly arrayed in a circle, equidistant from the two poles of the cell, with microtubules extending back toward the opposite poles of the cell. The cell is prepared to properly separate sister chromatids, such that each daughter cell will receive a complete set of chromosomes. Thus, metaphase is really a transitional phase in which all the preparations are checked before the action continues.

Anaphase

Of all the stages of mitosis anaphase (figure above) is the shortest and the most amazing to watch. It begins when the proteins holding the sister chromatids together at the centromere are removed. Up to this point in mitosis, sister chromatids have been held together by cohesin proteins concentrated at the centromere.

The key event in anaphase, then, is the simultaneous removal of these proteins from all of the chromosomes.
Freed from each other, the sister chromatids are pulled rapidly toward the poles to which their kinetochores are attached. In the process, two forms of movement take place simultaneously, each driven by microtubules. These movements are often called anaphase A and anaphase B to distinguish them.
First, during anaphase A, the kinetochores are pulled toward the poles as the microtubules that connect them to the poles shorten. This shortening process is not a contraction; the microtubules do not get any thicker. Instead, tubulin subunits are removed from the kinetochore ends of the microtubules. As more subunits are removed, the chromatid-bearing microtubules are progressively disassembled, and the chromatids are pulled ever closer to the poles of the cell.

Second, during anaphase B, the poles move apart as microtubular spindle fibers physically anchored to opposite poles slide past each other, away from the center of the cell.
Because another group of microtubules attaches the chromosomes to the poles, the chromosomes move apart, too. If a flexible membrane surrounds the cell, it becomes visibly elongated.
When the sister chromatids separate in anaphase, the accurate partitioning of the replicated genome—the essential element
of mitosis—is complete.

Telophase 

In telophase, the final phase of mitosis, the spindle apparatus disassembles as the microtubules are broken down into tubulin monomers that can be used to construct the cytoskeletons of the daughter cells. A nuclear envelope forms around each set of sister chromatids, which can now be called chromosomes because they are no longer attached at the centromere. The chromosomes soon begin to uncoil into the more extended form that permits gene expression. One of the early groups of genes expressed after the completion of mitosis is the rRNA genes, resulting in the reappearance of the nucleolus.

Telophase can be viewed as a reversal of the processes of prophase, bringing the cell back to the state of interphase. Mitosis is complete at the end of telophase. The eukaryotic cell has partitioned its replicated genome into two new nuclei positioned at opposite ends of the cell. Other cytoplasmic organelles, including mitochondria and chloroplasts (if present), were reassorted to areas that will later separate and become the cytoplasm of the daughter cells.

Cytokinesis

Cell division is still not complete at the end of mitosis, because the division of the cell body proper has not yet begun. The final phase of the cell cycle, in which the cell actually divides, is called cytokinesis. It generally involves the cleavage of the cell body and cytoplasm into roughly equal halves.

Cytokinesis in Animal Cells

In animal cells and the cells of all other eukaryotes that lack cell walls, cytokinesis is achieved by means of a constricting belt of actin filaments. As these filaments slide past one another, the diameter of the belt decreases, pinching the cell and creating a cleavage furrow around the cell’s circumference (figure a).
As constriction proceeds, the furrow deepens until it eventually slices all the way into the center of the cell. At this point, the cell is divided in two (figure b).

Figure:Cytokinesis in animal cells. a. A cleavage furrow forms around a dividing frog egg. b. The completion of cytokinesis in an animal cell. The two daughter cells are still joined by a thin band of cytoplasm occupied largely by microtubules.

Cytokinesis in Plants

Plant cell walls are far too rigid to be squeezed in two by actin filaments. Instead, these cells assemble membrane components in their interior, at right angles to the spindle apparatus. This expanding membrane partition, called a cell plate, continues to grow outward until it reaches the interior surface of the plasma membrane and fuses with it, effectively dividing the cell in two (figure below). 

Cellulose is then laid down on the new membranes, creating two new cell walls. The space between the daughter cells becomes impregnated with pectins and is called a middle lamella.

Figure:Cytokinesis in plant cells. In this photomicrograph and companion drawing, a cell plate is forming between daughter nuclei. The cell plate forms from the fusion of Golgi-derived vesicles. Once the plate is complete, there will be two cells.