Discuss The Significance Of Normal And Abnormal Mitosis And Meiosis In The Lifecycle Of Humans
Discuss The Significance Of Normal And Abnormal Mitosis And Meiosis In The Lifecycle Of Humans.
Introduction:
Mitosis is a type of cell division. The nucleus divides once and produces two identical nuclei. The new daughter cells are genetically identical both to each other and the parent cells. The only source of genetic variation in the cells is via mutations. Mitosis is used for growth and repair. Some tissues must be repaired often for example, the lining of the gut, white blood cells, skin. The skin cell lifespan is only a few days. The same chromosome number is retained from generation to generation.
The duplication of a chromosome is followed by a nuclear and cell division. Nuclear division its self is a continuous process, but for ease of description, four
main stages are recognised. The four stages are known as Prophase, Metaphase, Anaphase and Telophase.
When a cell is preparing to divide, it is said to be in Interphase. At this stage the cell forms new cell organelles to supply the daughter cells. The DNA replicates so the there is sufficient DNA for the two daughter cells. The cell then builds up its store of energy in preparation for the cell division.
Mitosis starts with Prophase, where the chromosomes condense becoming more tightly coiled and folded and so appear more shorter and fatter as Prophase progresses. As soon as the chromosomes begin to condense, the DNA becomes inactive. The condensation of the chromosomes into separate structures enables them to be moved easily. During Interphase they are diffuse and would become entangled if they were moved about the nucleus. In the later stages of Prophase, pairs of sister chromatids can be seen. These chromatids are attached at a point called the centromere. The nucleoli disappear, the nuclear membrane breaks down and a spindle apparatus is formed. The spindle apparatus is made up of microtubules which control the movements of the chromosomes.
The second stage of Mitosis is called Metaphase. During this stage the centromere of all the chromosomes are lined up on the spindles equator and begin to divide. One sister chromatid from each chromosome is attached by microtubules to one pole of the spindle apparatus and the other sister is attached at the opposite pole.
Anaphase, the third stage of Mitosis, begins with the separation of the centromeres. The sister chromatids are drawn apart to the opposite poles of the cells. Once the sister chromatids are separated they are referred to as daughter chromosomes.
The final stage of Mitosis, Telophase, begins when the two sets of daughter chromosomes have reached the two poles of the spindle apparatus. The nuclear membrane and the nucleoli reform, and the chromosomes become less visible under the microscope. At the end of Telophase the spindle apparatus disappears. Mitosis finishes when two identical daughter nuclei are formed.
...
This is a preview of the whole essay
Anaphase, the third stage of Mitosis, begins with the separation of the centromeres. The sister chromatids are drawn apart to the opposite poles of the cells. Once the sister chromatids are separated they are referred to as daughter chromosomes.
The final stage of Mitosis, Telophase, begins when the two sets of daughter chromosomes have reached the two poles of the spindle apparatus. The nuclear membrane and the nucleoli reform, and the chromosomes become less visible under the microscope. At the end of Telophase the spindle apparatus disappears. Mitosis finishes when two identical daughter nuclei are formed.
Mitosis is a form of asexual reproduction. This involves a single parent producing one or more cells by Mitosis. The offspring are therefore genetically identical to each other and to their "parent"- in other words they are clones. Why is this process important to us? Well, it is still going on in our bodies. As our hair grows, the cells are dividing asexually. When our fingernails grow, there is asexual division again. When we cut our fingers and the skin heals, the cells go through this same process. When our bodies grow, this is how our bones get longer and tissues grow: the cells just divide!
Chromosome mutations can happen during Mitosis. A mutation is a change in the amount or chemical structure of DNA. If the information contained within the mutated DNA is expressed (that is, transcribed into mRNA and translated to a specific polypeptide chain) it can cause a change in the characteristics of an individual cell or an organism. In the living cell, DNA undergoes frequent chemical change, especially when it is being replicated. Most of these changes are quickly repaired. Those that are not result in a mutation. Thus, mutation is a failure of DNA repair. Mutations can happen spontaneously as a result of errors in DNA replication, errors during cell division or can also happen during Interphase when the DNA replicates.
Mutations can be very small changes. The information in DNA is carried in the sequence of bases. Changes in just one or two bases can scramble the information and mean that the gene does not work properly. If this mutation is in a gene that controls cell division it can trigger the uncontrolled growth of that cell. This faulty gene is copied to all the new cells and so the uncontrolled growth continues and the new cells form a tumour.
Mitosis is controlled by special proteins produced by oncogenes. Mitosis must be controlled, otherwise growth will occur without limit (cancer). What is cancer? Cancer is essentially a disease of mitosis - the normal 'checkpoints' regulating mitosis are ignored or over ridden by the cancer cell. Cancer begins when a single cell is transformed, or converted from a normal cell to a cancer cell.
Meiosis is a specialized type of cell division that occurs in the formation of gametes such as egg and sperm. Although meiosis appears much more complicated than mitosis, it is really just two divisions in sequence, each one of which has strong similarities to mitosis. Meiosis I, the first of the two divisions, is often called reduction division, since it is here that the chromosome compliment is reduced from 2N (diploid) to 1N (haploid).
Interphase in meiosis is identical to interphase in mitosis and there is no way, by simply observing the cell, to determine what type of division the cell will undergo when it does divide. Meiotic division will only occur in cells associated with male or female sex organs.
Prophase I is virtually identical to prophase in mitosis, involving the appearance of the chromosomes, the development of the spindle apparatus, and the breakdown of the nuclear membrane. Metaphase I in meiosis and metaphase in mitosis has a critical difference. In Metaphase Mitosis, all the chromosomes line up on the metaphase plate in no particular order. In Metaphase I, the chromosome pairs are aligned on either side of the metaphase plate. It is during this alignment that chromatid arms may overlap and temporarily fuse (chiasmata), resulting in crossovers. During Anaphase I the kinetochore spindle fibers contract, pulling the homologous pairs away from each other and toward each pole of the cell.
A cleavage furrow typically forms during Prophase 1 followed by cytokinesis, but the nuclear membrane usually is not reformed and the chromosomes do not disappear. At the end of Telophase I, each daughter cell has a single set of chromosomes, half the total number in the original cell where the chromosomes were present in pairs. While the original cell was diploid, the daughter cells are now haploid. This is why Meiosis I is often called reduction division.
Meiosis II is quite simple in that it is simply a mitotic division of each of the haploid cells produced in Meiosis I. There is no Interphase between Meiosis I and Meiosis II and the latter begins with Prophase II. A new set of spindle fibers forms and the chromosomes begin to move toward the equator of the cell. All the chromosomes in the two cells align with the metaphase plate. The centromeres split and the kinetochore spindle fibers shorten, drawing the chromosomes toward each pole of the cell. A cleavage furrow develops, followed by cytokinesis and the formation of the nuclear membrane (envelope). The chromosomes begin to fade, replaced by the granular chromatin characteristic of interphase. When Meiosis II is complete, there will be a total of four daughter cells, each with half the total number of chromosomes as the original cell.
Meiosis facilitates stable sexual reproduction. Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes than the zygotes from the previous generation. Successive generations would have an exponential increase in chromosome count, resulting in an unwieldy genome that would cripple the reproductive fitness of the species. Polyploidy, the state of having three or more sets of chromosomes, also results in developmental abnormalities or lethality.
Most importantly, however, meiosis produces genetic variety in gametes that propagate to offspring. By crossing over and independent assortment, the gene pool of the species is dynamic and easily adaptable to changing environments and situations. Without genetic variation, progeny would be identical in traits to their parents, a dangerous weakness in a world where survival of the fittest is very much in effect.
Nondisjunction is the failure of a chromosome to split correctly during meiosis. This results in the production of gametes which have either more or less of the usual amount of genetic material, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II phases of cellular reproduction.
This is a cause of several medical conditions in humans, including:
* Down Syndrome -trisomy of chromosome 21
* Patau Syndrome - trisomy of chromosome 13
* Edward Syndrome - trisomy of chromosome 18
* Klinefelter Syndrome - an extra X chromosome in males
* Turner Syndrome - only one X chromosome present
* Jacob syndrome- an extra Y chromosome in males
In females, meiosis occurs in precursor cells known as oogonia that divide twice into oocytes. These stem cells stop at the diplotene stage of meiosis I and lay dormant within a protective shell of somatic cells called the follicle. Follicles begin growth at a steady pace in a process known as folliculogenesis, and a small number enter the menstrual cycle. Menstruated oocytes continue meiosis I and arrest at meiosis II until fertilization. The process of meiosis in females is called oogenesis.
In males, meiosis occurs in precursor cells known as spermatogonia that divide twice to become sperm. These cells continuously divide without arrest in the seminiferous tubules of the testicles. Sperm is produced at a steady pace. The process of meiosis in males is called spermatogenesis.
Both Mitosis and Meiosis are essential to all living organisms. Life its self would not and could not exist without them. Mitosis is essential for growth and repair. Meiosis is responsible for genetic repair.
