Lab 7 - Cell Division: Mitosis and Meiosis

Chanel DeJesus


How do eukaryotic cells divide to produce genetically identical cells or to produce gametes with half the normal DNA?

Prelab Questions

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?

In order to become an organism with trillions of cells the first single-celled zygote would have to go through mitosis.  Mitosis had to occur many times, over and over, in order to have trillions of cells.

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

Cell division is important to single-celled organisms because it allows them to asexually reproduce.

3. What must happen to ensure successful cell division?    

To have successful cell division a cell must have sets of chromosomes with DNA and pass all of the three stages of the cell cycle.

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

The genetic information in one body cell is identical to the information in other body cells, unless it is a gamete, because it only has half of the genetic information.

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

One advantage of asexual reproduction is that it consumes less energy.  It also allows all of the genetic information to be passed down.

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

It is important for DNA to be replicated prior to cell division so that in mitosis each daughter cell can get complete set.

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

The chromosomes are replicated and condensed during interphase.  Next, during metaphase, the chromosomes line up at the metaphase plate.  During anaphase, the chromosomes pull apart and go towards the poles.  They reach the poles of the cell during anaphase.

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

Regulatory proteins control the cell cycle.  If the control was defective, the cells would continuously divide and could possibly cause cancer.

Controlling Cell Division

Cell Division is controlled by complexes made up of several specific proteins. The complexes contain enzymes called cyclin-dependent kinases (CDKs). The CDKs turn the various processes in cell division on and off. CDKS partners with a family of proteins called cyclins. One complex, called mitosis-promoting factor (MPF), contains cyclin A or B and CDK. When CDK is bound to cyclin, it is activated. It begins to interact with other proteins that allow the cell to proceed from G2 into mitosis. The levels of cyclin tend to change during the cell cycle and different CDKs are produced during the phases. The cyclins determine which processes in cell division are turned on or off and in what order by CDKs. As cyclin is turned on or off, the cell moves though the stages in the cell cycle because of the CDKs.

Cells must pass through checkpoints to see if the cell has completed all of the tasks necessary so that it can continue on to the next step in the cycle. The first checkpoint, G1 checkpoint, the cells are stimulated by external growth factors. The G2 checkpoint checks for damage after the DNA is replicated. If there is damage, the cell is prevented from continuing on to mitosis. The M-spindle checkpoint makes sure that the mitotic spindles are properly attached to the kinetochores. If the spindles are not anchored properly, the cell will not continue on through mitosis.  

Cell Cycle

The cell cycle is divided into three parts called interphase, mitosis, and cytokinesis. Interphase is separated into three stages. The first growth stage is called G1. In G1 the cell grows and prepares to duplicate its DNA. In synthesis (S), the chromosomes are replicated. The third stage of interphase is the second growth phase (G2). In G2, the cell prepares to divide. In the second stage of the cell cycle, mitosis, the duplicated chromosomes are separated into two nuclei. Followed by mitosis is cytokinesis, which is hen the auto plasm divides and organelles seperate into daughter cells.

Part 1: Modeling Mitosis

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, because the second cell is a copy of the first cell, so the genetic information is the same.

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

They condense so that they can move more easily. It is easier for things to move when they are condensed than if they are in strands.

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

They are separated from each other during cytokinesis  where the cytoplasm divides and organelles separate into daughter cells.

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

If sister chromatids fail to separate it could result in mutations often found in cancer.

Part 2: Effects of Environment on Mitosis

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

My experimental hypothesis was that the lectin will affect the number of cells undergoing mitosis. The null hypothesis is that the number of cells undergoing mitosis will not be dependent on the lectin. The two hypothesis are different because one states that the variable will have an affect, while the other says the variable will not have an affect.

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

I would look at root tips before they are exposed to lectin and record how many cells are undergoing mitosis and how many cells aren't. I would expose the cells to lectin then observe the number of cells undergoing mitosis once again.

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

I would measure the cells that are going through mitosis and the cells that aren't by using a microscope to see all of the cells and then counting which ones are and aren't undergoing mitosis.

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

An appropriate control would be cells of root tips that haven't been exposed to lectin.

Overview of Experiment

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.

To test the effect of lectin on mitosis in root tips, the experiment would include placing root tips in an environment that exposes the root tips to lectin. To see the effect, the experiment would also have to include a trial where the root tips aren't exposed to lectin to act as the control. The control allows us to see if the lectin has an affect on mitosis because there is something we can compare it to.

Analyzing Data

The degree of freedom for this experiment is 1 because the number of treatment groups minus one multiplied by the number of phase groups minus one is 1 ((2-1)(2-1)=1). The p value is .5, so we know the critical value is 3.84. Since the calculated chi-square value is 0.973, which is less than the critical value, the null hypothesis is not rejected. Since the null hypothesis is not rejected, we know the lectin has no effect on the number of cells undergoing mitosis

Postlab Review

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

Collecting the class data allowed us to get more accurate results in determining the affect of lectin on cells undergoing mitosis.

2. Was there a significant difference between the groups?

No, there was not a significant difference between the groups.

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

The lectin didn't have an affect on the number of root tips undergoing mitosis.

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

We could put the root tips in different amounts of lectin to see if the cells react different to different amount of the protein.

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, it does.

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

You could keep track of the time and see how many cells are going through mitosis during a set time interval.

Case 2

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

The cell will kill itself by apoptosis so that it won't turn into a cancer cell.

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

If cells with mutated DNA replicate, the mutation would have a bigger affect than there would have been if the mutation was would have been in one cell.

3. How do cells monitor DNA integrity?

The cells monitor DNA integrity by going through the checkpoints to make sure every task is completed to move to the next step in the cell cycle.

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

Cancer cells aren't able to separate so there are often many more chromosomes than there would normally be in normal cells.

5. How could these differences lead to cancer?

The differences could lead to cancer because the cells continue to replicate even though they are damaged, so they become copies of the mutated cells.

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division.



Modeling Meiosis

1. When is the DNA replicated during meiosis?

DNA is replicated during interphase right before meiosis 1 begins.

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

No, they carry the same genes, but not the same alleles.

3. What is crossing over?

Crossing over is when a segment of sister chromatids break off and reassemble on a different homologous chromosome.

4. What physical constraints control crossover frequencies?

The further away genes are from each other on a chromosome, the further it is for them to cross over together.

5. What is meant by independent assortment?

In independent assortment, it doesn't matter where genes line up during metaphase, so genes that are inherited are inherited independently of each other.

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

The possible number of different kinds of gametes that can be produced could be calculated by multiplying all of the genes of the mother and father and then seeing how many different combinations they could make.

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?

There would be more than the normal number of chromosomes in one of the daughter cells, and less than normal chromosomes in another one of the daughter cells. Down syndrome happens when there is an extra chromosome in chromosome 21. Cri du chat syndrome is a small deletion of genetic material from chromosome 5.

8. How are mitosis and meiosis fundamentally different?

Mitosis produces two identical daughter cells, with the same DNA found in the mother cell, through asexual reproduction, but meiosis produces four gametes with half the genetic DNA as the mother cell, for sexual reproduction. Mitosis has only one cell division, which is why the daughter cells end up being diploid cells, while the daughter cells produced in meiosis are haploid cells because there are two cell divisions. Mitosis is important for cell growth and repair, but meiosis produces gametes for sexual reproduction.

Lab Bench Activity

Evaluating Results

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

Each crossover produces two spores like the parents and two spores that are a result of the crossover. So, to determine the number of crossovers, you must divide the number of asci counted by two since only half the spores in each ascus 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?

It was 31.8 units between the spore color gene and the centromere, which is 5.8 units away from the published distance.

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

Any disparities could have been because we only looked at a small section of the spore instead of the whole thing.  It is more likely that a mutation might have affected the small spore that we looked at more than it would have affected a larger quantity of spores.  the disparities could have also been because of the lack of clarity of the picture making it hard to distinguish between spores.

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

The difference in appearance of spores was caused by crossing over.

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 the Philadelphia chromosome is a result of some other chromosomal abnormality because crossing over would have resulted in equally sized chromosome, it would not have resulted in one chromosome being shorter than the other.  One possible chromosomal abnormality would be any deletion mutations that happened in the gene.  A deletion would have caused a phase shift which would end up making the chromosome shorter.

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

The cell cycle that mitosis goes through would not be good for the cells going through meiosis because mitosis produces two diploid cells that are exactly the same.  This helps the cell grow and repair, but growth and repair aren't needed for meiosis. Meiosis needs daughter cells with half the genetic information as the mother cell so that those sex cells can be used for sexual reproduction.  If mitosis were applied to meiosis, the daughter cells in meiosis would have the same genetic DNA, not half like it needs.

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