Mitosis Essay, Research Paper
In order for a cell?s surface area to provide its volume with sufficient nutrients, a cell must grow and divide. This division, along with the four other phases of mitosis, make up the whole cell cycle. In this graph, ?Cell Division & Its Divisions,? we will explore the many different elements, relationships, and points of importance involved in the cell cycle.
The first phase of the cell cycle is known as interphase, which is the longest phase of the five. Interphase contains three subphases: G1, S, and G2. G1, also known as the growth mode, lasts approximately eight hours. It is the period of major cytoplasmic activity and growth within the cell, which reduces the surface area : volume ratio. The G represents Gap, or a break between cell divisions. However, this is not a period of rest. It is rather a period of rapid growth, especially in younger human cells. During G1, the amount of DNA per cell remains the same at y as the cell grows continually. The cell?s radius increases steadily because of the its cytoplasmic growth. The cell?s MTE, or Membrane Transport Efficiency, is a measure of how well a cell is able to transport nutrients and wastes through its membrane. If the MTE drops below its ?comfort level,? or the level in which the cell can transport these things, it is in danger unless it divides soon afterwards. If the MTE drops below its ?death level,? which is very low, the cell will die from lack of nutrients and excessive wastes. During G1, the MTE decreases and reaches the ?comfort level? by the end of the phase. The end of G1 depends on the rate at which the cell?s MTE drops and passes its ?comfort level.?
The S phase begins the division mode of interphase. This is where the DNA within cells are replicated to provide both daughter genes with their needed genes. This subphase lasts approximately six hours, bringing us to the 14 hour mark of the cell cycle. During the S subphase, the amount of DNA per cell constantly increases until it has doubled itself, reaching 2y. The cell?s radius still increases throughout the S subphase, but it does so at a slower rate than during the G1 subphase. The cell?s MTE decreases at a slower rate than it had been during the latter subphase, but this decrease brings the MTE below the ?comfort level.? This means that the cell should divide soon or else major problems will occur. The DNA replication process has been executed and achieved!
The final division of interphase is the G2 subphase, which prepares the cell for its division. G2 lasts approximately four hours, which makes it the shortest subphase of interphase. During this subphase the cell continues to grow and it makes proteins needed for the cell division process. The G2 subphase continues the division mode of interphase. During G2, the amount of DNA per cell ceases to replicate itself and remains the same at 2y as the cell grows continually. The cell?s radius is still growing larger, but at an even slower rate than that of the S subphase. The cell?s MTE decreases constantly at even a slower rate than that of its latter subphase. Oh no! This is not good news for the cell?s transport efficiency, but at least the MTE level is a safe distance away from its ?death level!? Interphase has ended and the cell is now fully prepared for its division. I?m so excited!
After 18 long hours of interphase, the cell is ready to divide by the process of mitosis. Mitosis includes four phases: prophase, metaphase, anaphase, and telophase. This process lasts only approximately 45 minutes, but a lot can happen in that short amount of time. The amount of DNA per cell begins its sudden and rapid decline down to y, its starting point. This splits the amount of DNA in half to provide both daughter cells with the sufficient number of genes. The cell?s radius continues to grow until mitosis is almost complete. Then the radius divides itself among the two daughter cells and finishes where it began to start growing again. The cell?s MTE continues to decrease, getting closer and closer to its ?death level.? However, at the very last stages of mitosis, the MTE increases rapidly and ends up at its starting point to begin decreasing once again. Mitosis has ended successfully.
Due to the cell cycle, one cell has brilliantly divided itself, including its radius, number of organelles, chromosomes number and amount of DNA per cell. As a result, the cell?s surface area : volume ratio and MTE are increased, which also improves the quality of the cell?s life. Now both daughter cells are immediately ready to begin the cell cycle again.
A student knows that cells from a particular species have 18 chromosomes as their diploid number. This student observed two daughter cells from a recent mitosis, and noted that one had 19 single chromosomes and the other had 17. Apparently some failure in the mitotic process occurred. The most likely and most logical explanation for this abnormality was that the in one double chromosome, centromere separation failed to occur at the end of metaphase, but did occur at some later time in the cell division process.
The reason for this mitotic abnormality was not that cytokinesis failed to occur at the end of telophase. Cytokinesis is known to be somewhat imprecise, which would explain the misplacement of a cell part. However, cytokinesis only distributes the divided cytoplasm and its organelles to the two daughter cells (pg.17). It has no connection with the delivery of the chromosomes to the daughter cell. This delivery is executed through the process of mitosis, which occurs concurrently with cytokinesis. Therefore, if cytokinesis failed to occur, the chromosomes would still be distributed through mitosis. The reason for the incorrect number of chromosomes was not due to the failure of cytokinesis at the end of telophase.
This irregularity did not occur because two of the 18 chromosomes failed to be lined up during metaphase. I don?t know if it is possible, but if two chromosomes did not line up at the equator of the cell, they would be floating around in the cell?s cytoplasm. Those double chromosomes would never divide, which would cause a tremendous anomaly in the individual?s body. 16 chromosomes would line up and divide normally to produce two daughter cells with 16 single chromosomes. However, the two double chromosomes would now be in either or both cells and would cause one of the following effects: both daughter cells would have 18 chromosomes, but the double chromosomes would cause problems; or one daughter cells would have 16 single chromosomes and the other would have 16 single and two double chromosomes. Somehow these results do not seem possible. Also, neither result described the student?s, so this abnormality was not caused because two of the chromosomes failed to be lined up during metaphase.
The chromosomal error of these two cells was not caused by the anaphase stage of mitosis preceding the metaphase stage in this particular cell division. Mitosis is a continuous process split up into four stages: prophase, metaphase, anaphase, and telophase. These stages are used merely for discussion and easy interpretation of this cell process, and cannot be rearranged or changed in any way. There are no breaks or pauses between these stages, thus making this situation highly improbable and, to my knowledge, impossible to occur. I don?t believe that the splitting of centromeres is physically able to occur if the are not first lined up on the cell?s equator. Therefore, anaphase could not have preceded metaphase to bring about this cell abnormality.
A solid false hypothesis for this cell abnormality was that the spindle apparatus on one side of the mother cell was not functional. If the spindle apparatus was disfunctional, then its spindle proteins would fail to produce any structural or contractile fibers in the cell. Unfortunately, the 18 double chromosomes would be held only by the functional ?traction? fibers from the healthy spindle on one side of the cell. Hence, those 18 chromosomes would only be pulled towards one centriole at one pole, ending up in only one daughter cell. The result of this malfunction is one daughter cell with 18 double chromosomes, and one daughter cell with no chromosomes at all and a disfunctional spindle. Although this is a great abnormality, its results do not resemble the student?s results, thus making this a poor hypothesis for this particular situation.
The hypothesis that centromere separation failed to occur at the end of metaphase, but instead at some later time in the cell division process is the most likely explanation for this cellular abnormality. Due to a malfunction of something in a cell during some stage, centromere separation could have occurred at a later time during mitosis. For example, if one or both centrioles failed to pull the chromosomes? centromeres evenly, the chromosomes would fail to align on the metaphase plate. Therefore, centromere separation could have occurred later in the cell cycle, perhaps during cytokinesis at the end of telophase. The formation of the cleavage furrow could have forced one chromosomes to the opposite daughter cell. Consequently, the result just might have been one daughter cell holding 19 single chromosomes and the other containing 17. Therefore, this hypothesis of the false chronology of centromere separation is a solid explanation for this cellular abnormality.
Wow, this student must have been a genius if he or she concluded that the centromere separation of the cell cycle occurred at a later stage than its correct stage, metaphase. Many cell abnormalities can be explained by simple cell malfunctions such as this situation. Knowing about the cell cycle and mitosis in detail can prove to be quite beneficial to genetics students and researchers world wide.