Mitosis Overview.

Mitosis Overview

In this essay I will outline the sequence of events in the cell cycle, explaining the difference between Interphase and the individual stages of mitosis. Mitosis is a form of cell division which gives rise to new cells called daughter cells, these are genetically identical to parent cells. Mitosis is the most common form of nuclear division and is important for the growth, repair, and replacement of cells with a limited lifespan; it is also a means of asexual reproduction present in plants and lower animals. The production of genetically identical cells allows certain cells to retain the ability to develop into any other type if needed. Mitosis consists of four main stages: Prophase, Metaphase, Anaphase, and Telophase; before these can occur, the cell must undergo interphase which is the preparation for mitosis.

This is a representation of the ratio of time the cell takes to complete the various stages of Interphase and mitosis:

G1, S, and G2 and all stages of interphase, in a 24hr timeframe G1, S, and G2 would take 23hrs and mitosis itself would take about 1hr. However, timeframes can vary with different animals and cells.

Interphase

As this is the cell's time to prepare for splitting into two completely separate cells, this is quite an intensive period of work. The interphase stage consists of three parts: G1, S, and G2:

G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. Note that the G in G1 represents gap and the 1 represents first, so the G1 phase is the first gap phase. The G1 phase can be slowed by reducing the nutrients available in a system - thus the cell will take longer to build up the resources necessary for cell division. If there is a severe depletion in nutrients the cells can virtually stop growing. It is interesting to note that cells that aren't growing are always stopped in the G1 phase. This suggests that once the cell enters the S phase, it is committed to cell division, regardless of the external cell conditions.

S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized; even though DNA synthesis is confined to a narrow window, the synthesis of organelles occurs throughout interphase

G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins such as kinase, which is necessary for cell division at this time and continues to increase in size. The chromosome begins to condense and the proteins necessary for construction of the mitotic spindle also are synthesized. When the chromosomes become visible ...

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In the last part of interphase, the cell still has nucleoli present. The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin. In animal cells, two pair of centrioles formed from the replication of one pair are located outside of the nucleus.

Prophase

All the DNA that was synthesised how condenses into chromosomes, these are identical chromatids connected at the centre by a centromere, forming a X-shaped object. The chromosomes are greatly thickened and shortened but are still contained within the nuclear envelope. The distinguishing feature of prophase is the set-up of the mitotic spindle, which is used to manoeuvre the chromosomes about the cell. Excess parts from the dismantled cytoskeleton form the spindle. The spindle is initially set-up outside the nucleus.

The cell's centrioles are duplicated to form two pairs of centrioles. Each pair becomes the part of the Microtubule Organising Centres (MTOC's) which forms the focus for an array of microtubules, called the aster. The two asters lie side-by-side close the nuclear envelope. Near the end of prophase the asters pull apart and the spindle is formed. The nuclear membrane shows signs of fragmentation and the nucleoli disappear.

In late prophase:

- The nuclear envelope breaks up.

- Polar fibres, which are microtubules that make up the spindle fibres, reach from each cell pole to the cell's equator.

- Kinetochores that are specialized regions in the centromeres of chromosomes, attach to a type of microtubule called kinetochore fibres.

- The kinetochore fibres "interact" with the spindle polar fibres connecting the kinetochores to the polar fibres.

- The chromosomes begin to migrate toward the cell centre.

Metaphase

(Note the metaphase follows the prometaphase, the nuclear membrane dissolves, marking the beginning of prometaphase. Proteins attach to the centromeres creating the kinetochores. Microtubules attach at the kinetochores and the chromosomes begin moving. This stage is generally classified as ‘early metaphase’.)

Here, the chromosomes become attached to the spindle fibres by the centromere; these are sometimes known as kinetochores which consist of specialised proteins which pull on the spindle fibres using ATP as an energy source. The nuclear membrane fragmentation is complete. During this phase, the chromosomes are lined up on the equator of the cell/metaphase plate due to the effect of both set of kinetochores pulling in opposite directions, is the lining.

Anaphase

In anaphase, the sister chromatids separate and begin moving to opposite ends of the cell. Spindle fibres not connected to chromatids lengthen and elongate the cell. At the end of anaphase, each pole contains a complete compilation of chromosomes

During anaphase the state of the cell is as follows:

The paired centromeres in each distinct chromosome begin to move apart.
Once the paired sister chromatids separate from one another, each is considered a "full" chromosome. They are referred to as daughter chromosomes.
Through the spindles, the daughter chromosomes move to the poles at opposite ends of the cell.
The daughter chromosomes migrate centromere first and the kinetochore fibres become shorter as the chromosomes near a pole.
In preparation for telophase, the two cell poles also move further apart during the course of anaphase. At the end of anaphase, each pole contains a complete compilation of chromosomes.

This stage is usually characterised by the trailing arms of the chromatids being pulled away from the metaphase plate.

Telophase and Cytokinesis

In this phase, the chromosomes unwind and become chromatin once again. The spindle disappears, and a nuclear membrane forms around the bundle of chromatin. The cytoplasm divides in the middle of the cell by means of two contractile proteins, which form a cleavage furrow; in plant cells a cell plate is laid down and covered with cellulose to form a separating cell wall.

To conclude; mitosis, a means of asexual reproduction of cells consists of four parts: Prophase, Metaphase, Anaphase, and Telophase; the interphase, which is not part of mitosis, is the time it takes to grow and synthesis the DNA necessary for mitosis. The prophase is characterised by the condensation/coiling of the chromatin in the nucleus; the metaphase is characterised by the break down of the nuclear wall and the alignment of the chromosomes on the equator of the cell; the anaphase is characterised by the trailing arms of the chromatids attached to each spindle; and the telophase is characterised pinching of the cell membrane and cytoplasm into two separate cells.

www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html

http://biology.about.com/library/weekly/aa071300a.htm

http://library.thinkquest.org/C004535/text/prophase.html

http://www.accessexcellence.org/AB/GG/mitosis.html

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http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/cells3.html

AS level notes

Enclyclopedia Brittanica

Biology Text book

Mitosis and growth