Cell division

Mitosis cell division

 

 

Cell biology  The cell cycle  

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Mitosis tutorial & animation

The Mammalian Cell Cycle
Using a microscope to watch a mammalian cell reproduce culminates with two visually stunning events: when the nucleus divides (termed mitosis) and when the cell physically splits in two (termed cytokinesis) [2,5].
From start to finish these two events take approximately one hour to complete and together are considered one phase of the mammalian cell cycle called the M phase (M = mitosis). Although M phase is visually dramatic, there is quite a long period between one M phase and another (on average 23 hours), a time that under the microscope appears to be an intermission where the cell merely grows in size.
This period of visual inactivity is called interphase—which on the molecular level is, in fact, quite an active time for the cell. Interphase is divided into the three remaining phases of the cell cycle. During interphase, a cell replicates its nuclear DNA, ensuring that when it does divide at the end of M phase each of the newly created daughter cells will contain a full set of genes. The period in which this is accomplished is called S phase (S = synthesis).
Before DNA synthesis begins and after it is completed, are the two remaining phases of the cell cycle. Since these periods occur between M and S phases they are designated gap phases in which the cell continues to grow. The interval between the completion of M phase and the beginning of S phase is the G1 phase (G = gap), while the G2 phase is the interval between the end of S phase and the beginning of M phase. Gap phases constitute a decision making period for the cell [6] It is within these phases that molecular events within the cell will determine whether to initiate M phase (during G2), S phase (during G1), or to extend the gap for a longer period and allow further cell growth.
Mitosis (Nuclear division)
Separation of the cell’s chromosomes is the key requirement to producing two genetically identical daughter cells, a requirement that is accomplished during M phase of the cell cycle [5].
Chromosomes are normally diffuse strands of DNA that begin to become visible under the light microscope as M phase is about to start. At this stage, the chromosomes, which have been replicated during an earlier S phase, appear as long threads that gradually shrink and thicken so that they are physically easier to separate during mitosis. As the chromosomes condense, a cytoskeletal structure called the mitotic spindle is assembled to carry out their separation (see Figure 2). It is composed of long molecular tubes (called microtubules) that attach to the chromosomes and physically pull them into separate daughter cells.





Figure 2: Chromosomes condense. This occurs in a continuous fashion that has traditionally been divided into five stages (the names of each stage are included below—see Figure 3) [2,5]:
1. The replicated chromosomes condense and the mitotic spindle begins to assemble outside the nucleus (prophase).
2. The membrane surrounding the nucleus (nuclear envelope) breaks down and allows the mitotic spindle to contact the chromosomes (prometaphase).
3. All the chromosomes are gathered at the center of the mitotic spindle (metaphase).
4. The chromosomes are split apart and pulled to opposite sides of the cell (anaphase).
5. The nuclear envelope reassembles around the two new sets of separated chromosomes to form two nuclei (telophase).





Figure 3. Steps in Mitosis. Once the two new nuclei are formed, the chromosomes de-condense and gene transcription can resume (i.e. genes can begin to function). All that remains is for the cell to divide in two.
Cytokinesis (Cell Division)
The second feature of M phase is the time in which the other components of the cell— membranes, cytoskeleton, organelles, and soluble proteins—are distributed to the two daughter cells through a process called cytokinesis. This is the final task that a cell must complete to finish its reproduction (see Figure 4).





Figure 4. Cytokinesis An inward wrinkling of the cell membrane is the first sign of cytokinesis. This is caused by a second cytoskeletal structure called the contractile ring [2], which begins to tighten in that area of the cell. The wrinkle occurs perpendicular to the mitotic spindle that is pulling the chromosomes to opposite sides of the cell and thus ensures cellular division will result in one nucleus in each of the daughter cells.
Once assembled, the contractile ring will tighten further and further until the cell is pinched in two. The force required to do this is provided by the molecular architecture of the contractile ring, which is composed of two types of filaments (called actin and myosin filaments) that slide over each other to create a tightening power [2]. In fact, the contractile ring is capable of exerting a force strong enough to bend a fine glass needle inserted into the cell. This is a transient structure that gradually becomes smaller as cytokinesis proceeds and disassembles completely once the cell is cleaved in two.
The Cell Cycle And Continuation of Life
Once cytokinesis is completed, the cell has successfully gone through one turn of the cell cycle and produced two cells from a single precursor. For bacteria or yeast, which are single-celled organisms, this cell division will produce a new and complete organism. In a multicellular organism (like human beings), a fertilized single-celled egg requires many cell divisions to make a new individual. In either case it is the completion of the cell cycle that produces new organisms, a process that can go on throughout life. In an adult human, for instance, adult nerve and muscle cells do not divide at all. Liver cells divide once a year. More than one division a day occurs in blood cell precursors in the bone marrow and in the cells lining the gut. Our survival requires a production of millions of cells a second. This fact was illustrated by the exposure of organisms to massive doses of x-rays that stop all cell division and cause an individual to die within a few days. In the end, it is the cell cycle that ensures life will always be able to produce more life in an organized fashion.

 Reference:         Cell cycle

Meiosis - reproductive cell division