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?


1. People go from zygotes into multicellular organisms with trillions of cells by continuously undergoing the cell cycle.  The DNA of the zygote will have to be replicated and the zygote must grow, undergo mitosis, and then divide into two daughter cells.  Since every time a cell divides, two new cells are created, this can be modeled by 2^n where n is the number of divisions.  Solving for n, the zygote will have to divide about 47 times in order to make a 100 trillion cell organism.

2. Cell division is important in a single celled organism because cell division is the organism's method of reproduction.  By undergoing the cell cycle, a single celled organism creates 2 daughter cells with the same DNA as the parent.

3. To ensure successful cell division, the cell has to pass three checkpoints during the cell cycle before passing onto the next stage.  The three checkpoints are the G1 checkpoint, which is passed when it is stimulated by external growth factors, the G2 checkpoint, which is passed if the DNA is not damaged during replication, and the M-spindle checkpoint, which checks to make sure that the spindle fibers are properly attached to chromosomes.

4. The genetic information in all body cells is the same because every cell came from the zygote, and the DNA of the zygote was copied so that daughter cells would have the same DNA.  The G2 checkpoint also ensures that there were no problems in replicating DNA that way body cells will have the same DNA.

5. Asexual replication is faster because it doesn't require gamete formation, so it can allow the plant to pass on its genetic information in a shorter amount of time.  Additionally, if the plants are in a secluded area so pollination can't occur that often, it is more advantageous to undergo asexual reproduction.  Also, if the parent already had advantageous traits, then it would be best to just pass down genetic information as is.

6. It's important for the DNA to be replicated before cell division because in the nucleus of each daughter cell, there needs to be a copy of the DNA so the DNA must be copied before the cell divides.

7. First chromosomes are located in the nucleus and replicated during the S phase of interphase.  Then the nucleus dissolves and the chromosomes are lined up at the center of the cell in metaphase and attached to spindle fibers.  The chromosomes are then pulled apart by the spindle fibers in anaphase so the copies of the chromosomes (the sister chromatids) are located at the ends of the cell.  The sister chromatids are then located in two nuclei in telophase which become the nuclei of the daughter cells when the cell splits.

8. The cell cycle is controlled by complexes of CDKs and cyclins.  These complexes cause the cell to move through stages in the cell cycle and don't allow the cell to progress through the cycle automatically, so that at checkpoints, the cell can make sure that nothing went wrong.  If the control was defective, then errors in cell replication can occur and the cell would continue to divide. This can lead to cancerous cells.

How is Cell Division Controlled?

Cell division is controlled by proteins called cyclins and enzymes called CDKs.  When CDKs bind to cyclins, they signal for the cell to move to the next stage in the cell cycle.  This allows for the cell cycle to be checked at 3 different checkpoints, the G1 checkpoint, G2 checkpoint, and the M-spindle checkpoint.  Passing the G1 checkpoint occurs when the cell is stimulated by external growth factors.  Passing the G2 checkpoint occurs if the DNA was correctly replicated.  Passing the M-spindle checkpoint occurs when spindle fibers are correctly attached to the centromeres of the chromosomes.

How Do Eukaryotic Cells Divide to Produce Genetically Identical Cells?

First, during interphase of the cell cycle, the cell grows and replicates its DNA so that there is a duplicate copy of the chromosomes.  The chromosomes are then lined up at the center of the cell during mitosis and spindle fibers attach onto the centromeres of each chromosome.  Each chromosome has 2 sister chromatids which contain a copy of the DNA.  The sister chromatids are pulled apart from each other to opposite ends of the cell.  Nuclear membranes form around the 2 groups of sister chromatids, creating the nuclei of the 2 daughter cells.  Then the cytoplasm spits and 2 daughter cells are formed, each with a copy of the parent cell's DNA.

Pg 87 - Part 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 it does not contain any more genetic information than the cell before the chromosomes ere duplicated because the genetic information in the duplicated chromosome is exactly the same as the original.

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

They need to condense before they are moved so that they can be separated into equal halves during anaphase of mitosis.

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

The sister chromatids are separated during anaphase by the pulling of the spindle fibers, which pull the sister chromatids to opposite ends of the cell.

What would happen if the sister chromatids failed to separate?

If the sister chromatids failed to separate, then 1 daughter cell would have more chromosomes than the other. This can cause the cells to not function properly.  If this occurs in the gametes, it can lead to offspring with trisomy or polyploidy.

Effect of the 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.

To test how lectin affects mitosis in root tips, I will soak the root of one onion in water and another in lectin.  I would first observe the onion bulb root tip under a microscope that is not infected by lectin and count the number of cells undergoing mitosis and the number of cells in interphase.  Then I would look at the lectin infected onion root tip under a microscope and count the number of cells undergoing mitosis and the number of cells in interphase.  Then I will perform a Chi-Square test in order to test my null hypothesis, which is "lectin will not affect mitosis in root tips", and see if the differences between the variables were due to chance or due to lectin causing mitosis to occur more frequently.

Class Data:

Chi-Square Calculations

In order to use the chi-square value to prove or disprove the null hypothesis, we have to first calculate the degree of freedom of the experiment.  The degree of freedom is calculated by multiplying the number of treatment groups minus one by the number of phase groups minus one.  We had 2 treatment groups and 2 phase groups so the df = (2-1)(2-1)= 1.    Then we look at the table below in the row for 1 degree of freedom and compare our chi-square value to the one in the table where the probability is 0.05.  This is where there is a 95% probability that the difference between the variables was not caused by chance.  The value that the chi-square value has to be greater than or equal to in order to have a 95% probability that the difference was not caused by chance is 3.84.  Since our chi-square value is less an that, the null hypothesis is accepted which means that our hypothesis was disproven.


What was the important of collecting the class data?

It was important to collect class data because more data helps to make the experiment more accurate and makes it possible to tell if correlation was because of causality rather than chance.

Was there a significant difference between the groups?

No there was not because after taking the chi square value, the value that we obtained was a lot smaller than the value for the probability to be 0.05.

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

No it did not.

What other experiments should you perform to verify your findings?

We could do the same test with other plant roots in order to see if roots of other plants are not affected by the lectin.

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 doesn't always because even though there is more mitosis, it might be caused by chance rather than a faster rate.

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

You can also determine the rate of mitosis in root tips by taking a section of onion root treated by lectin and not treated by lectin and comparing the number of cells after a week.  If there is a higher change in the number of cells, then more mitosis occured.

Loss of Control of the Cell Cycle

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

If there is a minor change in DNA, the cell can still perform its functions, but if is a large mutation, the shape of proteins will change which will affect its function.

What would happen if the cells with mutated DNA replicated?

More cells with that mutated DNA will be created.

How do cells monitor DNA integrity?

CDKs and cyclins pause the cell cycle so that DNA replication can be checked to prevent mutations in DNA.

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

The chromosomes in cancer cells contain may more mutations because the tumor suppressor genes are mutated so DNA is not checked during replication.

How could these differences lead to cancer?

Because mitosis is not checked, the cells will continue to grow and divide without undergoing apoptosis, passing on the mutations in the tumor suppressor genes.

Nondisjunction is when the chromosomes fail to separate during meiosis or mitosis.  It can lead to offspring with an extra chromosome, such as people with Down Syndrome which is trisomy of the 21st chromosome.  Nondisjunction in mitosis can lead to cancer because it leads to chromosomal mutations so some chromosomes won't have certain genes like the tumor suppressor genes.

Modeling Meiosis

Modeling Mitosis

Pg 93 Qs

When is the DNA replicated during meiosis?

DNA is replicated during interphase I of meiosis.

Are homologous pairs of chromosomes exact copies of each other?

Homologous pairs of chromosomes are not exact copies of each other.  They have the same genes but have different alleles because one came from the mother and the other from the father.

What is crossing over?

Crossing over is when homologous chromosomes which are close together swap genes.

What physical constraints control crossover frequencies?

The position of the gene on the chromosomes control crossover frequencies.  When the genes are closer together, there is a bigger chance that those genes will cross over together.  Also, the farther the gene is from the centromere, the more likely it is to cross over.

What is meant by independent assortment?

Independent assortment describes the random ordering of homologous chromosomes during metaphase I of meiosis.  The homologous chromosome from the father and the chromosome from the mother can pair up in any order.

How can you calculate the possible number of kinds of gametes?

You can calculate the possible number of kinds of gametes by multiplying the number of chromosomes by 4 assuming that crossing over takes place.

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 chromosome fails to separate, 2 daughter cells of meiosis will have 2 copies of that chromosome and one will not have any.  Having extra copies of chromosomes can lead to trisomy or polyploidy when the sperm and the egg form the zygote. Down syndrome is an example of trisomy.  Also pieces of the homologous chromosomes may be lost when they don't separate correctly that way when the sperm and egg combine, one piece of a set of chromosomes in the offspring will be missing.  Cri du chat syndrome is an example and is caused when a piece of chromosome 5 is missing.

How are mitosis and meiosis fundamentally different?

Mitosis creates 4 identical daughter cells that are the same as the parent cell and is used for growth, repair, and asexual reproduction.  Meiosis creates gametes that are haploid and contain varied genetic information due to crossing over and independent assortment.

Lab Bench

Pg 96 Evaluating Results

1. You divide by 2 because crossing over produces only half of the spores result from crossing over.

2. The class data was 5.8 units off.

3. We looked at only 11 asci so the value was not that accurate.  If we looked at more, we would have gotten a more accurate number.

4. During meiosis, crossing over occurred meaning that some homologous chromosomes, since they were paired close to each other managed to swap sections of DNA.  Since the gene for spore color gene was far from the chromosome, there was a higher chance for the spore color gene to swap.

5. I think that Philadelphia chromosome is a result of a different chromosomal abnormality because 2 homologous pairs of chromosomes contain chromosomes of different sizes which means that one section of a chromosome must have been translocated onto a nonhomologous chromosome. Crossing over would lead to homologous chromosomes that still have the same number of genes on each chromosome.

6. I don't think that the cell cycle for mitosis can be applied to meiosis because in meiosis separation occurs twice.  Also in meiosis homologous chromosomes line up at the center of the cell and crossing over may take place.  This does not happen in mitosis.


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