The cell theory was first proposed by Schleiden in 1838 and Schwann in 1839. This theory was then extended upon by Rudolf Virchow in 1855 declaring that new cells only came from pre-existing cells. Shortly after, in 1887, Weismann suggested a specialist form of division occurred in the manufacture of gametes. These two forms of division are called Mitosis and Meiosis respectively. By definition, Mitosis and Meiosis are very similar, both being methods of cell division. However, the way in which the daughter cell is produced in these processes vary. The biological differences in these two processes lie between Mitosis and Meiosis I, as Meiosis II is almost identical to Mitosis.
As a consequence of Mitosis, a parent nucleus divides into two daughter nuclei, each with the same number of chromosomes as the parent nucleus. The division of the whole-cell follows this. In order to accomplish this chromosomes firstly replicate themselves during interphase. The two replicated chromosomes are known as chromatids and separate during mitosis. Cell division is a continuous process with no sharp distinction between the phases. There are 3 main stages:
Interphase: this is the episode of synthesis and growth. The cell produces many materials essential for its own growth and for carrying out all its functions. DNA replication occurs during interphase.
Mitosis: this is the process of nuclear division
Cell division: this is the process of division of the cytoplasm into two daughter cells.
Although these are the main stages, many sub-stages exists allowing further detail into what happens during the cell cycle. Prophase is the longest stage in Mitosis. The chromosomes become visable as long threads, and start to coil up and become shorter and thicker. During Mitosis in animal cells, the centriole divides and moves to opposite poles of the nucleus. Spindle fibres are formed by the protein microtubules that have developed from each centriole, some extending pole to pole. Towards the end of the phase each chromosome can be seen to consist of two chromatids held together by a Centromere. The stage is completed by the nucleolus disappearing and the breakdown of the nuclear envelope. The chromosomes now lie free in the cytoplasm.
Metaphase is a shorter-lived stage, as the chromosomes move toward the equator of the spindle; they attach themselves to a spindle fibre by means of a Centromere. Anaphase is also a brief stage. The Centromere’s holding each pair of chromatids divide, and the free chromatids move to the poles, Centromere first. The contraction this movement induces on the spindle fibres causes the chromatids to disintegrate. Now the chromatids have reached the poles, the stage of telephase begins. This concluding phase of mitosis can almost be referred to as the reversal of prophase. The nucleolus reappears as the nuclear envelope forms around each cluster of chromosomes. The cytoplasm then divides by a process called cytokenisis.
Mitosis produces cells that are an exact copy of the parent cell. These daughter cells have the same amount of chromosomes and are genetically identical to the parent cell. It is significant for many processes. It is essential for growth, as a growing multicellular organism requires and increasing number of cells. Repair of tissues also requires Mitosis, as damaged cells must be replaced with identical ones. Finally it is important for asexual reproduction, as the offspring are identical to the parent.
Meiosis is a form of nuclear division in which the chromosome number is halved from the diploid number, to the haploid number. As a result of Meiosis four daughter cells are formed from one parent cell. Each daughter cell has half the number of chromosomes of the parent cell. Meiosis occurs during the formation of sperm and egg cells, therefore is essential for sexual reproduction. Like Mitosis, Meiosis is a continuous process and is conveniently divided into the prophase, metaphase, anaphase and telephase. However, this cycle is repeated twice to produce the four daughter cells. I will be detailing Meiosis I, as Meiosis II is almost identical to Mitosis.
Prophase I: The chromosomes contract and are seen to have divided into two chromatids. As in Mitosis, the chromosomes coil, causing them to shorten and become thicker. One of the key characteristics of prophase I of Meiosis is that the paternal and maternal chromosomes come together in homologous pairs. This pairing is called synapsis. Each homologous pair is called a bivalent, and so consisting of four strands. The points of exchange of chromatid material are called chiasmata, and the swapping of materials is referred to ‘crossing over.’ This is the longest and most complicated phase in Meiosis.
Metaphase I: At the beginning of this stage, the spindle will have formed, and as in meiosis the chromosomes accumulate on the equator of the spindle. The key distinction in the Meiosis metaphase in relation to the Mitosis metaphase is that the chromosomes are joined in bivalents.
Anaphase I: The chromosomes in each bivalent separate during anaphase I as a result, each pole of the nucleus receives only one of each homologous pair of chromosomes. Each pole receives a haploid number of chromosomes, induced again by the contraction of the spindle fibres.
Telophase I: this occurs when the chromosomes reach opposite ends of the spindle. The nuclear envelope forms around each group of haploid chromosomes. Usually they stay in their condensed form and Meiosis II follows on immediately. Cytokenisis occurs to produce two haploid cells.
Sexual reproduction and genetic variation are two of the processes that necessitate Meiosis. It is essential in sexual reproduction. During fertilization, the nuclei of two gametes combine. Each gamete has one set of chromosomes (haploid.) The product of this fusion is a zygote that has two sets of chromosomes (diploid.) if Meiosis did not transpire; than the fusion of gametes would consequence in the doubling of chromosomes for each consecutive sexually reproduced generation. This condition is prevented in the life cycle of all sexually reproduced organisms by the incidence of cell division involving a decrease in the diploid number of chromosomes to the haploid number.
The importance for gametes to carry genetic information is immense. The genes the chromosomes in the gametes carry determine the genetic makeup of the offspring. Regarding genetic variation, Meiosis provides opportunities for new combinations of genes to occur in the gametes. This concludes in genetic variation in the offspring produced by fusion of the gametes.
So what defines Mitosis and Meiosis?
The most obvious difference is the type of cell division. Where Mitosis produces somatic cells involving one nuclear division, Meiosis produces gametes, involving two nuclear divisions. Somatic cells are referred to as diploid (2n) whereas gametes are haploids (n), thus explaining that Mitosis produces 2 cells where Meiosis produces 4.
During the prophase, differences arise. For example, in mitosis, the homologous chromosomes remain separate whist they pair up in meiosis. In metaphase of mitosis, the chromosomes line up singly, however in metaphase I of Meiosis, bivalents align on the metaphase plate. The 2 double chromosomes are called a tetrad when they are lined up side by side.
Crossing over occurs during the formation of the tetrad, resulting in the formation of chiasmata. Anaphase accommodates the centromeres to divide during Mitosis, whereas they remain whole in Meiosis. The arrangement of chromosomes also differs between the two processes. Daughter cells of Mitosis have 2 sets of chromosomes and are genetically identical to the parent cell. Meiosis daughter cells have only one member of each pair of chromosomes and only half the genes from the parent cell. This stage occurs in the telophase. Crossing over and random separation of homologous pairs of chromosomes allows genetic diversity.
The occurrence of these cells also differs. Where mitosis cells occur in haploid, diploid or polyploid cells, during the formation of somatic cells, Meiosis only occurs in diploid or polyploid cells, during the formation of gametes and spores. Mitosis and Meiosis are both immensely important in life, meiosis for creating it, and mitosis for maintaining it.
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