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?

Erin Clewell



1. How did you develop from a single-celled zygote to an organism with trillions of cells? How many mitotic cell divisions would it take for one zygote to grow into an organism with 100 trillion cells?

A single cell can divide into two cells and those cells can then divide. There would have to be 47 divisions.

2. How is cell division important to a single-celled organism?

Cell division is the only way that a single celled organism can reproduce.

3. What must happen to ensure successful cell division?

To ensure successful cell division, the cells must have chromosomes that can be divided. These chromosomes need to have DNA.

4. How does the genetic information in one of your body cells compare to that found in other body cells?

The body cells all have the same genetic information but sperm and eggs on have half of that genetic information.

5. What are some advantages of asexual reproduction in plants?

Asexual reproduction is more advantageous in plants because it takes less energy to be carried out. Also, it ensures that all of the genetic information is passed to the offspring.

6. Why is it important for DNA to be replicated prior to cell division?

It is important that the DNA is replicated because once the cell divides, the two new cells need to have all of the DNA and have equal amounts of DNA. In meiosis, this is important so that when the cell is divided twice, each new cell can have half the DNA.

7. How do chromosomes move inside a cell during cell division?

In each phase of mitosis, the chromosomes move differently. First, in interphase, the chromosomes are replicated. Then, in metaphase, they are lined up. In anaphase, they are pulled apart to the two opposing poles of the cell. Finally, in telephase, the cells reach the opposing poles.

8. How is the cell cycle controlled? What would happen if the control were defective?

There are proteins that control the cell cycle and make sure the phases are proceeding properly. If it were defective, the cells might not cease dividing and there would be too many cells. Also, defective cells might divide and then the new cells might divide as well.  

Genes are used to control cell division. With close regulation of the cell division process, a dividing cell can be copied properly. If there are problems with copying the DNA, they can be repaired so that each cell that is produced has the correct DNA and all of its chromosomes. The cell division process is regulated closely at several checkpoints. The genes, at these specific points, can check for problems and possibly stop the process to fix any problems. The three checkpoints are the G1, G2, and m-spindle checkpoints. A cell can pass through the G1 checkpoint when it is stimulated by appropriate external growth factors. At the G2 checkpoint, a cell can only pass through if it has no damage to the DNA after replication. A cell can pass through the m-spindle checkpoint if the mitotic spindles have properly attached to the kinetochores.  If the genes cannot repair the DNA, the cell might go through apoptosis. This process of apoptosis, or programmed cell death, helps get rid of cells that the body doesn't need or cells that could be potentially dangerous.

Part 1: Mitosis

The DNA first needs to replicate so that there are double the amount of chromosomes. Then the cell enters mitosis. In mitosis, the cells line up at the center of the cell and then the spindle fibers attach to the centromeres. Ones they attach, the spindle fibers pull the sister chromatids apart so that they are on opposite poles of the cell. Once the nuclei are created, the cytoplasm splits so that there are two individual cells. Each cell is a daughter cell that has the identical DNA from the parent cell.

Modeling Mitosis 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?

It does not contain any more genetic information because it can only contain the information that was provided by the parent cell.

2. What is the significance of the fact that chromosomes condense before they are moved?

They need to condense so that they can be separated equally during anaphase.

3. How are the chromosome copies, called sister chromatids, separated from each other?

The sister chromatids are separated when the spindle fibers attach to the chromatids and pull them apart and to the opposite poles of the cell.

4. What would happen if the sister chromatids failed to separate?

If the sister chromatids failed to separate, than one cell would end up having more chromosomes compared to the other cell. This could cause many problems and may cause the cell to undergo apoptosis if cannot be fixed. In gametes, the offspring could have a form of polyploidy.

Part 2- Effect of Environment on Mitosis

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.

1. What is your experimental hypothesis? Your null hypothesis? Are these the same?

Experimental hypothesis- Lectin will increase the amount of cells undergoing mitosis.

Null hypothesis- Lectin will decrease the amount of cells undergoing mitosis.

They are not the same.

2. How would you design an experiment with onion bulbs to test whether lectins increase the number of cells in mitosis?

I would take two small pieces of onion bulbs and look at both under a microscope. One would have the lectins and the other would not. I would count the amount of cells undergoing mitosis in each and then compare the amounts to see which one has more mitosis occurring.

3. What would you measure and how would you measure it?

I would measure the amount of cells undergoing mitosis by counting the cells that look like they are splitting or having their DNA split.

4. What would be an appropriate control for your experiment?

A control would be a group that does not have the Lectin.

The experiment would be to submerge some onion root tips in lectin and some in water. Then, a thin slice from each would be examined and the amount of cells undergoing mitosis would be counted in each. If there is a significant difference between the amount of cells, then the hypothesis can be proved or disproved. This would have to be done with many different samples to make sure the conclusions are accurate.

Null Hypothesis

A null hypothesis is a hypothesis that states that there is no relation between variables or that one variable has zero effect on something.

Chi Squared Test

The chi squared test is used to see what happened in the experimental outcome vs. the expected outcome. It is a test to see if this difference occurred from chance or for a different reason (possibly something that occurred in the experiment). Using the chi square distribution chart can be used in conjunction with the formula to find out why there was a difference.

Chi squared calculations

In this experiment, the degree of freedom is 1 and the probability that is being used is 0.05. This means that the critical value is 3.84. If the calculated chi square value that is found is more that 3.84, then the null hypothesis is rejected. This would mean that Lectin does induce meiosis. However, since the calculated chi square value is .969, the null hypothesis cannot be rejected and therefore has the be accepted. This means that lectins do not increase the number of cells in mitosis for the onion root tip.

Post Lab Review

1. What was the importance of collecting the class data?

With more data, more accurate conclusions can be made. It reduces the chance that the data resulted from chance.

2. Was there a significant difference between the groups?

No. The chi square test proved there was no significance.

3. Did the fungal pathogen lectin increase the number of root tip cells in mitosis?

The null hypothesis was not disproved, therefore the Lectin does not increase the mitosis in onion root tip cells.

4. What other experiments should you perform to verify your findings?

We should measure if Lectin makes mitosis occur faster in cells by timing how long it takes for some cells that have lectins and some that don't.

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?

No, it just means that the amount of mitosis has increased; there was no faster division of the cells.

6. What other way could you determine how fast the rate of mitosis is occurring in root tips?

By testing individual cells, infecting some with Lectin and some without, and then timing how long each individual cell takes to go through mitosis, you could determine how fast the rate is mitosis is occurring.

Part 3- Loss of Control of the Cell Cycle

Case 2 Activity

1. What happens in a normal cell if the DNA has mutations?

If a normal cell has mutated DNA, proteins will usually recognize it. The cell will either undergo apoptosis or attempt to fix the problem.

2. What would happen if cells with mutated DNA replicated?

If cells with mutated DNA replicated, then the mutation would spread. Cancer is a possibility as well. Sickle cell anemia is another effect of mutated DNA spreading.

3. How do cells monitor DNA integrity?

Cells monitor DNA integrity during the different checkpoints in the cell cycle. Enzymes check the DNA at each checkpoint. If a problem is recognized, then certain enzymes will go in and try and fix the problem. For example, if nucleotides are missing, the enzyme will try and make sure the cell will be able to function.

4. How are the chromosomes different in the cancer cells compared to normal cells?

In cancer cells, there are commonly deletions, translocations, or inversions of the chromosomes. A gain or loss in the chromosomes could mean that many of the other chromosomes are affected too. A particular example is in Leukemia. In many types of leukemia, there is a loss of a whole chromosome.

5. How could these differences lead to cancer?

These differences could lead to cancer because one damaged and malfunctioning cell could divide and multiply into thousands of cells that are not working properly. With a chromosome missing, the cell may not have the right genetic information and may start making proteins wrong, or going about other cell processes the wrong way.

Nondisjunction- when either the homologous chromosomes or the sister chromatids fail to separate properly. This would mean that both of the chromosomes go to one daughter cell, leaving the other daughter cell with none. It could lead to discrepancies in the chromosome number and therefore be the cause of conditions such as trisomy 21 (Down's Syndrome) or monosomy.

Part 4- Modeling Meiosis

The difference between alignment in Mitosis and Meiosis 1 is that in Mitosis, the chromosome align one on top of the other. The centromere of each chromosome is on the metaphase plate. On the other hand, for Meiosis 1, the homologous chromosomes pair up so that the metaphase plate is right in between the two chromosomes.

S93 Questions

1. When is the DNA replicated during meiosis?

DNA replicates in the S phase of meiosis, before the cell enters Meiosis 1.

2. Are homologous pairs of chromosomes exact copies of each other?

No, homologous chromosomes are not exact copies. One is from the mother and one is from the father. They have the same length and appearance, but they are not genetically identical.

3. What is crossing over?

Crossing over is when the homologous chromosomes entangle and become so close that segments of each are exchanged. This occurs during metaphase 1 in prophase.

4. What physical constraints control crossover frequencies?

The locus of the chromosome controls crossover frequencies. Also, two segments that are close together are very likely to cross over compared to segments that are far away from each other.

5. What is meant by independent assortment?

Independent assortment is when the homologous chromosomes pair up and are beginning to align along the metaphase plate. The father's chromosomes are not aligned all on the same side, and the mother's chromosomes are not necessarily all aligned on the same side.

6. How can you calculate the possible number of different kinds of gametes?

The number of gametes can be calculated using the formula 2^n, n being the number of possible chromosome combinations there are. However, since crossing over usually occurs, it would be a hard to calculate this.

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 homologous chromosomes fail to separate, then one of the daughter cells might has more or less chromosomes then the other. This could lead to down syndrome, or trisomy 21 because it means that a certain cell has more of chromosome 21 then it needs. Crib du chat syndrome occurs when a piece of chromosome 5 is missing. This is because part of the chromosome failed to separate, so part of that chromosome was therefore missing for the person.

8. How are meiosis and mitosis fundamentally different?

Although both process involve replicating only once, in mitosis, the cell only divides one time. This results in two diploid cells. On the other hand, in meiosis, the cell divides twice and produces four diploid cells. Additionally, while meiosis only produces gametes, mitosis produces all other body cells.

Part 5- Meiosis and Crossing Over

Evaluating Results Questions

1. Why did you divide the percentage of asci showing crossover (recombinant) by 2?

Because each crossover produces two spores like the parents and two spores that are a result of the crossover, you divide the percentage by 2 (you only want to count the spores that crossed 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 data was that the distance was 31.8 map units. This is 5.8 map units more than the published distance.

3. How can you account for any disparities between the class data and the published data?

The disparities could have been caused by only looking at a small piece of the spore. Maybe this piece does not account for the entire spore. Also, I found the picture to be a bit unclear and for a few of the asci to be indistinguishable from each other. This could be a reason that the two values are not the same.

4. Illustrate what happened in meiosis to produce the results you found.

In meiosis, the ascospores crossed over. This meant that a new pattern of ascospores was made in each, and that the ascospores were no longer in that pattern of four light and four dark.

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.

The Philadelphia chromosome is a result of crossing over because one chromosome has too many genes and another has two few. This probably means that some of the pieces did not attach to the other homologous chromosome.

6. Do you think that the cell cycle described by mitosis could be applied to meiosis as well? Explain your answer.

I don't think the cell cycle can be applied because there are too many differences. For example, the chromosomes align differently at the metaphase plate. Additionally, the cell divides twice in meiosis and once in mitosis.

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