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MITOSIS

Mitosis is the process that facilitates the equal partitioning of replicated chromosomes into two identical groups. Before partitioning can occur, the chromosomes must become aligned so that the separation process can occur in an orderly fashion. The alignment of replicated chromosomes and their separation into two groups is a process that can be observed in virtually all eukaryotic cells.

Both the alignment and separation processes are the consequence of the chromosomes interacting with filamentous proteinaceous structures, known as microtubules. The microtubules become organized into a biconical array known as a spindle, which forms early in mitosis, and then disassembles as mitosis nears completion. Mitotic spindles are visible in living cells with the polarizing light microscope. Some of the spindle microtubules become attached to the chromosomes at sites known as kinetochores. The kinetochores cannot be seen with the light microscope, but they reside near the place on the chromosome known as its centromere, which can be observed with the light microscope. There are two kinetochores on each replicated chromosome (one on each chromatid), and when the replicated chromosome splits apart at its centromere at the onset of anaphase, each daughter chromosome possesses one centromere and one kinetochore. The linkages between kinetochores and microtubules are thought to be central in controlling both the positioning of the replicated chromosome at the central portion of the spindle during the alignment phase, and in moving the daughter chromosomes apart after they split at their centromeres. The separation of daughter cells from each other is a process known as cytokinesis, and is separate from mitosis. In cytokinesis, animal and plant cells differ considerably from each other. These differences are the consequence of having or not having a cell wall. Cytokinesis in fungi reveals some similarities with plant cells, and exhibits other features unique to the group.

THE STAGES OF MITOSIS IN A SINGLE CELL

Prophase -During prophase, the replicated chromosomes undergo extensive condensation (i.e., coiling). The chromosomes are greatly thickened and shortened but are still contained within the nuclear envelope. Late in prophase, within about 6 min of nuclear envelope breakdown, the mitotic spindle begins to grow, and two triangular ‘clear zones’ become visible, with one on each side of the nucleus. In three dimensions, the clear zones are actually conical and the nucleus is spherical. With continued spindle expansion, the nucleus rapidly becomes compressed and appears eliptical in the optical section provided by differential interference contrast optics. Prophase ends with the sudden dispersion of the nuclear envelope (nuclear envelope breakdown), and the chromosomal mass is no longer occupying a discrete, spherically-shaped zone in the cell. Prophase in stamen hair cells can last for as long as several hours.

Prometaphase – Once the nuclear envelope has broken down, the spindle microtubules and the chromosomes are no longer separated by a (double) membrane boundary. The microtubules begin to interact with the chromosomes, and the chromosomes undergo what is known as congressional movement, where they ultimately end up with their centromeres all situated in middle of the spindle, at a site known as the metaphase plate. Each kinetochore of the replicated chromosome is pointed toward one side of the spindle; later, in anaphase, each kinetochore moves to one of the two spindle pole regions as the daughter chromosome The congression of chromosomes and the alignment of centromeres on the metaphase plate represent essential prerequisites for the orderly separation of the replicated genome into two equal parts. The mechanisms underlying congression are under intense scrutiny.

Metaphase – The replicated chromosomes converge toward the center of the spindle, and once they get there, significant movements cease. On either side of each centromere are sites for microtubule attachment to the chromosome; electron microscopists called these plate-like structures kinetochores. The kinetochores are not visible with the light microscope. At several points during metaphase, the chromatid arms may unwind from each other. This unwinding is especially apparent late in metaphase, just 1 or 2 minutes before the chromatids will split apart at their centromeres, with each replicated chromosome giving rise to two daughter chromosomes. The interval between nuclear envelope breakdown and anaphase onset includes prometaphase and metaphase, and requires approximately 33 minutes.

Anaphase – Anaphase commences with the initial splitting of sister chromatids at their centromeres. These daughter chromosomes then begin to separate from each other, each moving away from the metaphase plate and toward one of the two spindle pole regions. The rate of chromosome separation in stamen hair cells is approximately 1.4 micrometers/min. The mechanisms that control chromosome separation clearly involve the interactions between microtubules and components in or near the kinetochore. Anaphase chromosomal movement is a topic of intense interest in the Cell Biology research community.

Telophase/Cytokinesis

1.Cell Plate Vesicle Aggregation

2.Plate Coalescence – About 19 minutes after anaphase onset, the chromosomes have moved close to the spindle pole regions, and the spindle midzone begins to clear. In this middle region of the spindle, a thin line of vesicles begins to accumulate. The vesicle aggregation event is a harbinger to the assembly of a new cell wall that will be positioned midway along the length of the original cell. It will form the boundary between the newly separating daughter cells. Vesicles movement and aggregation in the spindle midzone is facilitated by a microtubule network known as a phragmoplast. This basket shaped structure forms in late anaphase or early telophase and disassembles about the time that the vesicles begin to coalesce.

Nuclear division, or mitosis, typically occupies 5-10%of the total cycle. The cycle may take as little as 20 minutes in a bacterial cell, although it typically takes 8-24 hours.