In my AP Biology class, I observed dividing onion root tip cells through a light microscope to see the phases of mitosis that the cells were in and calculate the length of time the cells spend in each phase of mitosis. Below is the lab format.
1. Explain how mitosis leads to two daughter cells, each of which is diploid and genetically identical to the original cell. What activities are going on in the cell in interphase?
Mitosis consists of five stages. The first stage is known as interphase. In this phase, a cell prepares to divide. The cell duplicates its DNA, centrosomes, and, in the case of animal cells, centrioles. The replicated DNA is enclosed in the nucleus in the form of chromatin and the nucleus also has one or more nuclei. Also, each centrosome contains two centrioles. In the second phase known as prophase, the chromatin in the nucleus condenses into chromosomes, the nucleoli disappear, and the mitotic spindle develops. Each chromosome is made up of sister chromatids that carry the same alleles. The nuclear envelope begins to break down as well. In the third phase known as metaphase, the nuclear envelope has completely broken down and the chromosomes line up in the center of the cell with the centrosomes at opposite poles. The mitotic spindle attaches to the centromeres, or the connecting point of the sister chromatids, of the chromosomes. In the fourth phase known as anaphase, sister chromatids are pulled apart by the mitotic spindle. The last phase is known as telophase, in which the cell begins to split into two daughter cells that have the same number of chromosomes as the original cell and are each complete with a nucleus. Cytokinesis occurs the moment that the daughter cells each become an individual cell.
Plant cells do not have centrioles like animal cells, just centrosomes. Centrioles are structures made of microtubules that help organize the mitotic spindle, and centrosomes are areas where the mitotic spindle meets. In plant cells, when cytokinesis, or the moment two daughter cells form from a single cell, occurs, a cell wall forms between the two cells. This wall separates the cells. This occurs because the cell wall is rigid and inflexible. In comparison, animal cells do not have cell walls so the daughter cells can completely separate due to the flexibility of the plasma membrane.
3. What is the role of the centrosome (the area surrounding the centrioles)? Is it necessary for mitosis? Defend your answer.
The centrosome is the area where the mitotic spindle meets. The centrosome acts as an organizing center as the mitotic spindle grows out from this area. The mitotic spindle is important in that the spindle separates the chromosomes of a cell during anaphase in mitosis. The mitotic spindle pulls sister chromatids apart at the centromere of chromosomes and the daughter chromosomes move towards opposite poles of the cell. Without centrosomes, mitotic spindles are not organized and chromosomes cannot be separated correctly to make sure that each daughter cell has the necessary chromosomes it needs. For example, if the mitotic spindle was not organized then one daughter cell could have multiple copies of the same allele carried by sister chromatids and not have the other alleles carried by other sister chromatids. This could lead to mutation, which could cause the organism as a whole to not function properly.
1. If your observations had not been restricted to the area of the root tip that is actively dividing, how would your results have been different?
If the area of observation in the experiment had not been restricted to the area of the root tip that is actively dividing, then the results of the experiment would have been different because the cells would supposedly be spending their entire existence in interphase. Since the cells would not be dividing, the cells would all seem to be in interphase. No other phases of mitosis would be observed so the time cells spend in the phases of prophase, metaphase, anaphase, and telophase would be 0 minutes, while the time spent in interphase would equal the total number of cells present multiplied by 1440 minutes, which would equal the maximum range of time the experiment was preformed for (1 day).
2. Based on the data, what can you infer about the relative length of time an onion root tip cell spends in each stage of cell division?
The percentage of total onion root tip cells in each phase observed in this experiment was multiplied by the number of minutes in one day to determine the time spent in each phase. 76.9 % of the cells observed were in interphase. Due to the fact that most of the cells were in interphase, the time calculated for the time cells spend in the phase, which was 1107.4 minutes, was greater than all the other phases. It can therefore be inferred that cells spend the most time in interphase during mitosis. The amount of time spent in each phase decreases in the order that the phases occur. 16.6% of the cells were in prophase, 2.8% in metaphase, 2.2 % in anaphase, and 1.5% in telophase. The amount of cells in each phase decreased, so the time spent in each phase decreased proportionally. The time calculated for prophase was 239.0 minutes, for metaphase was 40.3 minutes, for anaphase was 31.7 minutes, and for telophase was 21.6 minutes. Therefore, cells must spend more time in prophase than metaphase, more time in metaphase than anaphase, and more time in anaphase than telophase.
The University of Arizona. “Online Onion Root Tips.” The Biology Project. The University of Arizona, 20 Aug. 1998. Web. 07 Nov. 2013. <http://www.biology.arizona.edu/cell_bio/activities/cell_cycle/activity_description.html>.