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Cell reproduction began in single celled organisms that reproduced in order to create an identical copy of them selves. These single-celled organisms were the prokaryotes, the ancestor of the eukaryote and the first organisms on earth.
In the later more complex multi-cellular organisms, the eukaryotes, reproduction of cells began to take on a more constructive role, in the growth, development and repair of different parts of the organism.
In all cases of cell reproduction there are four key events that must occur. There must be a reproductive signal to initiate, replication of the genetic material Deoxyribonucleic acid called DNA, segregation where the new DNA is distributed, and finally cytokinesis where the two new cells separate. Despite the fact that all four must occur in both prokaryotes and eukaryotes these events occur very differently.
The mechanism for division differs between the two organism types and depends hugely on the type of cells required and its use in the organism. In eukaryotes, either meiosis or mitosis occurs and in prokaryotes binary fission. Meiosis produces genetic variation and is how we as humans reproduce therefore as we are not all identical this cannot produce identical cells. Mitosis on the other hand produces identical daughter cells with exactly the same genetic information.
In prokaryotes, binary fission is the main method of cell reproduction; essentially the cell grows and divides into two identical cells each with half of the original DNA and half new DNA, due to the DNA replication method. However due to interspecifc competition other methods have evolved.
The reproductive signal for prokaryotic cell division is its environmental conditions and nutrient concentrations. (Sadava D, et al. 2008)
Due to the need to remain competitive, and also viable, the prokaryotic cell must replicate at an appropriate time and in an optimum position within the cell to allow the cells to obtain the full compliment of genetic material and the highest chance of survival. (Angert, 2005)
For the replication of the DNA to occur all of the chromosomes must be duplicated and one of each of the two new sets must be transferred into one of the new cells. Most prokaryotes have only one chromosome in a single long form with proteins bound to it. To be able to fit into the cell the DNA must be compacted via infolding on itself. The positive and negative proteins on the DNA bind to each other. Circular chromosomes are common to almost all prokaryotes and are common in a few of the eukaryotic cells. The chromosome is divided into two different functioning parts during cell reproduction the ori region; the site where chromosome replication starts and the ter region; the site where replication terminates. Replication is carried out by a replication complex of proteins near the center of the cell; this complex contains DNA polymerase, an enzyme vital to many replication types in both Eukaryotes and Prokaryotes. During the replication process the cell expands ready for the segregation process.
The segregation process begins as the now replicated DNAââ‚¬â„¢s ori regions move from the center of the cell to opposite ends. DNA adjacent to the ori region binds proteins that are essential to this segregation. Now that the two new sets of DNA are at opposite ends of the cell the final stage cytokinesis can begin. The first event of this is the pinching in of the cell membrane to form a ring similar to a purse string. Fibers composed of a protein make up this ring. As this happens new cell wall materials are synthesized, which finally separate the two cells. (Sadava D, 2008) Although many of the genes that are involved this cell division have been identified, the actual mechanisms involved to bring this about are still under analysis. (Angert, 2005)
Eukaryotes have evolved two main very different methods of division, meiosis and mitosis. Most complex eukaryotes evolved from a single cell, a fertilized egg. This egg was formed via meiosis and fertilized by the fusion of two gametes. Gametes are the product of meiosis. They each contain one set of chromosomes from the male parent and one from the female parent. This single cell then develops rapidly into a complex multi-celled organism. This development involves both cell reproduction and cell specialization. As with the prokaryotes, eukaryotic cells do not constantly divide whenever the environmental conditions are right.
In actuality cells in such large organisms seldom divide. The reproductive signals for cell division are related not to the environment of the single cell but to the needs of the whole organism. For example if the organism is damaged, repair is carried out via the reproduction of new cells. (Sadava D, 2008)
Before knowing about the cell division, understanding of the cell cycle is needed. When not reproducing cells grow and carry out their functions. The cell cycle shows the sequence of growth, function and replication.
After cytokinesis, the daughter cells are small and have very little energy, in the form of adenosine triphosphate or ATP. They acquire this ATP to grow to the required size during what is called the G1 phase of Interphase. Interphase is the longest part of the cell cycle and hence most cells are observed during interphase. When they have reached sufficient size and ATP levels, the cells then undergo the DNA synthesis previously described, this occurs during the S phase. As this all requires energy the cell must have another growth and energy acquiring stage, the G2 phase. The energy acquired during G2 is used to carry out the cell division, in this instance, mitosis. (Farabee, 2007)
Whilst most prokaryotes have a single chromosome, eukaryotes have many, for example humans which have 46, so the basis for DNA replication whilst essentially the same, is far more intricate. In eukaryotes the chromosomes are very closely associated and known as sister chromosomes and unlike in prokaryotes a different mechanism called mitosis is used for segregation into two new nuclei. Eukaryotic cells have a distinct nucleus, which must be divided into two new nuclei, each containing an identical set of chromosomes. Thus, cytokinesis is distinct from segregation of the genetic material and can happen only after duplication of the entire nucleus.
Whilst the DNA replicates, the centrosome, an organelle in the cytoplasm near the nucleus doubles forming a pair of chromosomes in many organisms each centrosome consists of a pair of centrioles each one a hollow tube lined with nine microtubules. The two tubes are at right angles to each other. At the next phase transition the two centrosomes separate from each other moving to opposite ends of the nuclear envelope. The orientation of the centrosomes correlates with the plane at which the cell will divide, and therefore the special relationship of the two new cells to the parent cell. This relationship may be of little consequence to small cells but it is important to cells that make up part of a body tissue. Surrounding the centrioles is a high concentration of tubulin dimers and these initiate the formation of microtubules, which orchestrate chromosomal movement. (in plant cells which lack chromosomes, a distinct microtubule organizing centre at either end of the cell palsy the same role) the formation of microtubules will lead to the formation of the spindle structure that is required for the orderly segregation of the chromosomes.
Mitosis is divided into many named phases, the first being prophase, during this the cohesion surrounding the chromosomes disappears and the chromatids become visible.
Kinetochores develop in the centromere regions one on each chromatid, as shown in the figure 1 below.
Figure (Purves & Orians, 1998)
Each of the two centrosomes serve as mitotic centers or poles and microtubules form between poles to make the spindle. Spindle divided into two types of microtubules, polar microtubules which overlap in the center and Kinetochore microtubules which attach to Kinetochores on the chromatids. Sister Chromatids attach to opposite halves of the spindle ensuring one chromatid of the pair moves to each pole. http://www.estrellamountain.edu/faculty/farabee/biobk/spindle.gif
The networks of spindles come together in the following way shown in figure 2.
Figure (Purves & Orians, 1998)
During Prometaphase the nuclear envelope surrounding the nucleus and the nucleoli disappear, and the chromosomes are gradually pushed towards the equator of the cell.
Figure (Purves & Orians, 1998)
In Metaphase the chromosome are condensed and all chromatids are connected to one pole or the other by microtubules.
During Anaphase the sister chromatids are separated and begin to move to opposite ends of the spindle, they are now called daughter chromosomes. At this point separase breaks down the cohesion holding the chromatids together and the spindles shorten drawing the chromatids apart as shown in figure 3.
Figure (Purves & Orians, 1998)
In Telophase the spindle breaks down and the chromosomes uncoil. The nuclear envelope and the nucleoli reform. Two daughter nuclei are formed with identical genetic information.
Figure (Purves & Orians, 1998) http://www.estrellamountain.edu/faculty/farabee/biobk/mitosis4.gif
Now that the nucleus has successfully divided cytokinesis can take place. The cytoplasm is divided between the two new cells and the plasma membrane pinches in. After the pinching in the cells divide as in prokaryotes and two new daughter cells are formed.
To form the multi-cellular organisms common to our world the mitotic cycle repeats itself over and over producing identical cells. By this process a single cell gives rise to billions of cells making up one organism. The other form of cell reproduction, meiosis forms only 4 daughter cells, which generally do not undergo further duplication and which are all genetically different. (Sadava D, 2008)
Prokaryotes as the ancestors of Eukaryotes evolved millions of years before and as such have a less evolved way of reproducing. Binary Fission is useful in the single celled prokaryotes but would be impractical for multi-celled Eukaryotes. So as the Prokaryote evolved new ways of surviving in its environment it developed new ways to reproduce itself so that the resultant organisms could then be separated out as the new organism type, the Eukaryotes. The Eukaryotes use mitosis for growth and repair of their large number of cells but use meiosis for the reproduction of the organism as this gives genetic variation.