Lab 7 - Cell Division: Mitosis and Meiosis
How do eukaryotic cells divide to produce genetically identical cells or to produce gametes with half the normal DNA?
Pre Lab Questions
How is Cell Division Controlled?
1. How many mitotic cell divisions would it take for one zygote to grow into an organism with 100 trillion cells?
Well, every cell divided would make two new ones, which multiply exponentially, so very quickly. In total, the first cell would have to divide approximately 37 times. log(2)100000000000=x; x= 36.54 divisions.
2.How is cell division important to a single-celled organism?
It allows the organism to asexually reproduce. By undergoing the cell cycle, the single celled organism can create two daughter cells.
3. What must happen to ensure successful cell division?
DNA must be replicated correctly and the checkpoints must be passed to make sure that all of the parts have been successfully done.
4. How does the genetic information in one of your body cells compare to that found in other body cells?
The genetic information is the same because the DNA is the same. Although the chromosomes are the same for each cell, what is different is the genes that are being expressed in the cell. Genes can be turned on or off in order to function in their metabolic pathways.
5. What are some advantages of asexual reproduction in plants?
It can replicate with itself so no partner is needed. The genetic code does not change with generations as well.
6. Why is it important for DNA to be replicated prior to cell division?
So the DNA in all generations is the same and all of the DNA in the cells are not messed up. If the DNA is replicated prior to cell division, there will be a functional cell with identical DNA of the parent without having any complications.
7. How do chromosomes move inside a cell during cell division?
Mitosis spindle fibers guide them towards the center of the cell in prophase and metaphase.
8. How is the cell cycle controlled? What would happen if the control were defective?
Certain checkpoints need to be passed. If the checkpoints are not passed, the cell cycle will not let the cell progress. If the control were defective, cell division would be problematic.
How is Cell Division Controlled?
Cell division is controlled by many different processes to make sure that checkpoints are passed and cells are replicated correctly. They are also controlled by proteins called cycling and enzymes called CDKs. When cyclin binds to CDKs, the checkpoint is passed and it lets the cell continue its cycle. There are three checkpoints; the G1 checkpoint, the G2 checkpoint, and the M-Spindle checkpoint.
How do Eukaryotic Cells Divide to Produce Genetically Identical Cells?
During interphase of the cell, the cell grows and replicates its DNA making a duplicate copy of the chromosomes. In mitosis, the chromosomes are lined up at the center of the cell and spindle fibers attach to the on the centromeres of each chromosome. Sister chromatids contain DNA and they are pulled to oppsosite sides of the cell. The nuclear membranes create the nuclei of two daughter cells. After that, the cytoplasm creates two daughter cells, each containing a copy of the parents DNA.
Page s87 Lab Manual Questions
1.If a cell contains a set of duplicated chromosomes, does it contain any more genetic information than the cell before the chromosomes were duplicated?
-No. The chromosomes were duplicated with the same DNA as the original, so there is no way new genetic material could have been introduced.
2. What is the significance of the fact that chromosomes condense before they are moved?
-They need to condense before they are moved so they can be separated into equal halves during anaphase of mitosis. It is also protective of the DNA because it is compact and not twisted.
3.How are the chromosome copies, called sister chromatids, separated from each other?
- The sister chromatids are separated by each other when the spindle fibers pull them to opposite parts of the cell.
4.What would happen if the sister chromatids failed to separate?
- Having this happen would cause it to be detrimental for the cell. In some cases. an extra chromosome in the daughter cell could cause triploidy of polyploidy.
Effects of Environment on Mitosis
Null Hypothesis and Chi-Squared Test
8. Chi Squared Calculations
9. Based on the data below, our value (.9713) was lower than the critical value necessary to disprove our null hypothesis (3.841). In this case, our null hypothesis was proved in that lectin does not have any effect on the rate of mitosis in onion roots. Again, our hypothesis was disproved because it was predicted that lectin would have an increasing effect on the rate of mitosis in onion roots.
6. Experimental Design
“Various fungi can negatively affect the growth of soybeans by producing a lectin-like protein. Lectins can induce mitosis in the root apical meristem tissue which will often weaken the plant tissue.
You have been asked to investigate whether the fungal pathogen lectin affects the number of cells undergoing mitosis in a different plant, using root tips.
Testable Question: Does lectin increase the rate of mitosis in onion root tips?
What is your experimental hypothesis? Your null hypothesis? Are these the same?
- My experimental hypothesis is that having lectin in the roots of plants will increase the amount of cells going through mitosis.
-My null hypothesis would be that lectin has no effect on the rate of mitosis.
These hypothesis aren't the same. The experimental hypothesis states that lectin induced roots will increase the rate of mitosis while the null hypothesis states that lectin will have no effect on the root tips.
How would you design an experiment with onion bulbs to test whether lectins increase the number of cells in mitosis?
To test whether lectins increase the number of cells in mitosis, I would have two experimental groups. One group would have the onion root bulbs soaked in a solution of water. I would then count the amount of cells going through inter-phase and mitosis. I would perform the same procedures with cells soaked in a solution of water and lectin. By comparing these two groups, I would see if the lectin induced root onion bulbs had a big difference in the amount of cells going through mitosis. I would also perform the Chi-Squared test to see if there is a significant difference between both groups. It will then prove or disprove my null hypothesis.
What would you measure, and how would you measure it?
I would measure the amount of cells before and after lectin was introduced and compare the change by counting the amount of cells going through inter-phase and mitosis.
What would be an appropriate control for your experiment?
Controls for the experiment could include the temperature and the amount of time that the roots would be exposed to lectin. These variables should stay the same throughout both trials of the experiment.
A null hypothesis is a statement that is either rejected or accepted that disproves the hypothesis. A chi-squared test is used in experiments such as this in order to determine whether or not there is a difference between two different variables.
A chi-squared test takes two variables and decides whether they occurred just from chance or because something acted upon it. The chi squared test proves or disproves the null hypothesis.
7. Data Tables
Post Lab Questions
1. What was the importance of collecting the class data?
Collecting the class data allows different trials to be tested. The more trials that are tested, the better accuracy of the experiment one has.
2. Was there a significant difference between the groups?
No; statistically speaking, there was no significant difference between the groups.
3. Did the fungal pathogen lectin increase the number of root tip cells in mitosis?
No, the lectin had no effect on the rate of mitosis in the root tip cells.
4. What other experiments should you perform to verify your findings?
More degrees of freedom on the experiment should be performed in order to ensure accurate results and a conclusion.
5. Does an increased number of cells in mitosis mean that these cells are dividing faster than the cells in the roots with a lower number of cells in mitosis?
Yes, the amount of cells present indicate that mitosis is being undergone, and if more cells are present, mitosis must be happening at a faster rate.
6. What other way could you determine how fast the rate of mitosis is occurring in the root tips?
Checking how many cells there are in the root tips at different time intervals.
Part 3: Loss of Control of the Cell Cycle
12. Case 12 Activity Questions
1. What happens in a normal cell if the DNA has mutations?
A mutation in the cell can occur when a base is inserted or deleted from the DNA sequence, thus changing the amino acid sequence. Consequently, the shape and size of the proteins would be different. This can have many negative or positive implications on the cell because the protein is not the correct one for the cell to function properly.
2. What would happen if cells with mutated DNA replicated?
If cells with mutated DNA are replicated, the mutations will simply be passed on to the daughter cell in mitosis.
3. How do cells monitor DNA integrity?
The cell monitors DNA integrity through checkpoints in the cell cycle and cyclins and CDKs. This method of cell control allows the cells to be made and functioning properly. If this doesn't happen, the cells may go through apoptosis.
4. How are the chromosomes different in the cancer cells compared to normal cells?
A difference in chromosomes could be seen in a Karyotype. Usually, the chromosome structure of a cancerous cell will be different than a normal cell. The cancerous cell may have extra or missing chromosomes.
5. How could these differences lead to cancer?
The differences in the chromosomes and DNA sequences might affect the control of the cell cycle. This may not allow the cell to go through the expected checkpoints to make sure that the cell is functioning properly. Instead, the cell will undergo division with these differences leading to cancer.
13. Nondisjunction: The failure of homologous chromosomes or sister chromatids to separate properly during cell division. A result of nondisjunction is genetic diseases and miscarriages.
Part 4 Modeling Mitosis + Meiosis
Questions of S93
1. When is the DNA replicated during meiosis?
The DNA is replicated during inter-phase.
2. Are homologous pairs of chromosomes exact copies of each other?
No. One chromosome comes from the mother and one comes from the father.
3. What is crossing over?
Crossing over occurs in meiosis when homologous chromosomes pair up. The chromosomes wrap around each other and sometimes swap alleles.The swapping of alleles causes genetic information between the chromosomes to be exchanged.
4. What physical constraints control crossover frequencies?
The closer and more wrapped together the chromosomes are, the more likely crossing over is to occur.
5. What is meant by independent assortment?
Independent assortment is how the chromosomes line up on the metaphase plate. Their order is completely random which increases genetic variation. For example, the mother chromosome could be lined up on top of the father cell or it can go the other way around. This happens 23 times so it adds genetic variation.
6. How can you calculate the possible number of different kinds of gametes?
You multiply the number of genes by the number of alleles.
7. What happens if a homologous pair of chromosomes fails to separate, and how might this contribute to genetic disorders such as Down syndrome and cri du chat syndrome?
If a homologous pair of chromosomes fails to separate nondisjunction occurs. This can result in monosomy and trisomy. These cells have too little or too much chromosomes causing them to be defective. Down Syndrome is called trisomy 21. The homologous chromosome pair 21 fails to separate, resulting in trisomy. Cri du chat occurs when part of chromosome 5 is deleted causing monosomy or (one) chromosome. When there isn't a regular amount of chromosomes, these cause defects.
8. How are mitosis and meiosis fundamentally different?
Mitosis has one division and meiosis has two. Mitosis results in two identical daughter somatic cells and meiosis results in four genetically different daughter cells.
Part 5: Meiosis and Crossing Over
1. Why did you divide the percentage of asci showing crossover (recombinant) by 2?
I divided the percentage of asci showing crossover by two since only half the spores in each as us result from crossing over.
2. The published map distance between the spore color gene and the centromere is 26 map units. How did the class data compare with this distance?
The class got 31.8 map units, 5.8 units away from the published map distance.
3. How can you account for any disparities between the class data and the published data?
Any disparities between the class data and the published data make have occurred because of random variation, wrong methods of counting, and possible because the class only looked at a small sample of spores rather than a large group.
4. Illustrate what happened during meiosis to produce the results you found.
Crossing over occurred during meiosis.
5. Do you think the Philadelphia chromosome is a result of crossing over as seen in this part of the investigation or some other chromosomal abnormality? Explain your answer.
I think that this is caused by an error in the copying of the cell. Because one chromosome is larger than another, the replication process was probably obstructed by an error.
6. Do you think the cell cycle described for mitosis could be applied to meiosis as well? Explain your answer.
No. Although they are similar processes, meiosis is a little bit more complex so the cell cycle is different since the end product is 4 cells instead of 2. The cell cycle is still regulated, but differently.