Mitosis Worksheet (30 points)
Name:______Gibson, John____________________________
(2 pt) Describe what occurs during Interphase of the cell cycle (be specific regarding G1, S and G2).
Interphase is when the cell is not dividing, basically all phases except the M phase. So, interphase includes the following 3 phases,
G1: The cell grows, and CDK1 is low, so that DNA chromosomes are licensed for preparation of replication with mcm27 and cdt1 cdc6 bound to ori sites.
S: The DNA molecules are replicating, with elongation factor E2F , but the licensing stops with a high CDK1 level.
G2: The cell grows and transcribes and pre-accumulate protein molecules (tubulins, kinetochore proteins, etc.) needed for mitosis next when transcription will be difficult with condensed chromatin in mitosis. Remember, transcription factors will not be able to bind to target promotors when mitosis comes and chromosomes are bound tightly to histones.
(1 pt) What is the purpose of hydrochloric acid when preparing the onion root tips?
HCl breaks down cell walls so that nucleus can be stained with methylene blue dye.
(1 pt) What is the purpose of methylene blue in preparing the onion root tips?
It dyes the nucleus blue-purple, for visual identification of chromosomes.
(3 pts) Onion cells spend 12 hours (720 minutes) in mitosis. The length of each phase can be calculated using the following formula:
Time in phase = #cells in phase x 720 minutes
Total # cells counted
Fill in the table below with the class data and calculate the time spent in each phase:
(2 pts) Based on the class data from lab and your calculations above, which phase is the longest? Which phase is the shortest?
Prophase is the longest.
Anaphase is the shortest.
(3 pts) Using Microsoft Excel (or similar graphing program), plot the data from above in a pie chart. You may attach the graph to the end of the worksheet. Make sure you have used an appropriate/informative title and label of each pie section.
Table 1. Group Onion Cell Counts For Each Phase
Figure 1. Onion Cell Mitosis Percent Time Spent In Each Phase
Caption: Telophase 112 minutes; Anaphase 53 minutes; Metaphase 68 minutes; Prophase 487 minutes.
(2 pt) Write up a discussion section describing what you did to generate your data (ie-counting cells in specific phases of mitosis from the onion root tip prepared slide), what you see in your figure, and what your conclusions are.
Discussion
After obtaining the prepared microscopy slide of 3 onion roots, the cells were counted for each phase (Prophase, Metaphase, Anaphase, Telophase/Cytokinesis) for 6 trials. The field of view for trial #1 was near the meristem for the left-most root. The field of view was moved up without overlapping and with a small gap for trial #2. Trials #3 and #4 were performed on the middle root with the same procedure as before; trials #5 and #6 were performed again on the right-most root with the same procedure as before. The result cell counts for the phases ranged from 6 to 60 as shown in Table 1. Prophase had the highest cell count, so it is the longest phase the cells spent time on. Telophase had the fewest cell count, so it is the shortest phase the cells spent time on.
When 1 cell goes through the 120 minute cell cycles, and taking snapshots randomly during the time period, only 1 phase exists in each snapshot, with higher counts of snapshots for the longer phases. When many cells go through the cycle and pictures are taken together, the higher counts of snapshots for longer phases for 1 cell become multiple capturing of in-the-act of more cells in the picture. So, proportionally, the longer the phases, the more cell counts.
Conclusion is that prophase is a longest phase because DNA replication takes a long time for organisms with long DNA molecules.
(1 pt) What is the common name for allium cepa?
Onion.
(1 pts) How does cytokinesis differ in plants and animals (describe how it occurs in both)?
In plants, the division of the daughter cells in cytokinesis occurs within the confines of a cell-walled area. Daughter cells do not expand outward.
In animals, the division of the daughter cells in cytokinesis occurs without confines. Daughter cells expand outward.
In both cases, actin contractile ring pinches off the 2 daughter cells.
(1 pts) Based on your observations, how do the size of the chromosomes in the plant cells compare to those in whitefish? Explain.
The anaphase chromosomes of an onion cell occupied about 1/20 of the field view with 400x magnification. The anahase chromosomes of an onion cell, on average, occupied about 1/10 of the field view with 1000x magnification. Because field width is inversely proportional to magnification, the field width of 1000x magnification is 400/1000 = 0.4 fraction of 400x magnification’s field width. Even though fish chromosome appeared 2 times larger in the view, factoring in the field width, the fish chromosomes’ size is 2*0.4 = 0.8 fraction of the size of onion’s chromosomes’ size, on average.
(2 pts) What is the difference between a homologous chromosome and a sister chromatid? Are two homologous chromosomes or two sister chromatids more genetically alike?
Homologous chromosomes are not identical molecules. One homologous chromosome comes from father and one homologous chromosome comes from mother, for dioecious diploid organisms, such as humans.
Sister chromatids, without considering recombinations, are identical molecules. Sister chromatids are DNA replication products preparing the cell for division.
Two sister chromatids are more genetically alike.
(1 pts) What is the difference between a centromere and a centrosome?
The centromere is a DNA section that holds 2 sister chromatids together before anaphase. The centrosome is an organelle that organizes mitotic or meiotic spindles with microtubules that prepare the sister chromatids to split.
(10 pts) Based on the modeling activity in lab, recreate figures 1-5 this time adding in microtubules, centromeres, the nuclear and plasma membranes.
Figure 1.
Figure 2
Figure 3
Figure 4
Figure 5
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