Cell Division: Mitosis and Meiosis

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

Prelab Questions

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?

Through the process of cell division, I developed from a zygote into an organism. The three parts of cell division occurred 47 times to produce an organism with 100 trillion cells. The first growth phase in interphase allows for the cells to grow and prepare to duplicate its DNA. In synthesis the chromosomes are replicated and in the second growth phase the cells prepare to divide. Then mitosis occurs when the chromosomes are separated into two nuclei. Then cytokinesis allows the cytoplasm to divide and the organelles separate into two daughter cells.  

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

Cell division allows the organism to replicate and pass on their genetic information to the next generation so for the species to live on, cell division is very important.

3.    What must happen to ensure successful cell division?

There must be checkpoints and controlling factors to make sure that nothing has gone wrong during any of the processes of cell division. The cyclin dependent kinases turn on or off processes that take place in cell division. The cyclins determine which processes need to be turned on or off. There are also multiple check points that the cells must pass through. The G1 checkpoint check to see that the cells have been stimulated by proper growth factors. The G2 checkpoint checks for damage after the DNA is replicated. The metaphase checkpoint assures that the spindles are anchored to the chromosomes.

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

The body cells differentiate from one another in order to preform their certain function. The genetic information is the same in all of the body cells but certain genes are expressed and others are turned off in order for the cell to preform its particular function.

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

Asexual reproduction takes up less energy that can be used during photosynthesis or cellular respiration instead.

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

DNA needs to be replicated before cell division because their needs to be double the amount of chromosomes so that one set can go into each of the two cells that are created during cell division.

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

The chromosomes move inside a cell during mitosis. During anaphase, one set of chromosomes is pulled in one direction by the spindles and the other set is pulled the other way to begin the formation of two separate cells. During cytokinesis the cytoplasm divides which creates two distinct separate cells.

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

The cell cycle is controlled by cyclin dependent kinases that either turn on or turn off certain processes that the cyclins determine which should be on or off. It is also controlled by checkpoints. If the controls were defective the cells would not divide properly which would lead to a disorder in the organism.


How Cell Division is Controlled

Cell division is controlled through checkpoints between stages of cell division as well with cyclin dependent kinases and cyclins. The cyclins are proteins that determine which processes in cell division are turned on or off and the cyclin dependent kinases turn on or turn off these certain processes depending on the cyclins orders. The different checkpoints make sure that the cells have gone through the processes they need to before moving on. The first growth checkpoint before cell cycle initiation ensures that the cells are the correct size before moving on. The second growth checkpoint before metaphase makes sure that the DNA has been replicated inside the cell and there isno damage to it. The last checkpoint before metaphase and anaphase checks to ensure the the spindles are connected to the chromosomes.


Part 6

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?

Through the process of cell division, I developed from a zygote into an organism. The three parts of cell division occurred 47 times to produce an organism with 100 trillion cells. The first growth phase in interphase allows for the cells to grow and prepare to duplicate its DNA. In synthesis the chromosomes are replicated and in the second growth phase the cells prepare to divide. Then mitosis occurs when the chromosomes are separated into two nuclei. Then cytokinesis allows the cytoplasm to divide and the organelles separate into two daughter cells.  

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

Cell division allows the organism to replicate and pass on their genetic information to the next generation so for the species to live on, cell division is very important.

3.    What must happen to ensure successful cell division?

There must be checkpoints and controlling factors to make sure that nothing has gone wrong during any of the processes of cell division. The cyclin dependent kinases turn on or off processes that take place in cell division. The cyclins determine which processes need to be turned on or off. There are also multiple check points that the cells must pass through. The G1 checkpoint check to see that the cells have been stimulated by proper growth factors. The G2 checkpoint checks for damage after the DNA is replicated. The metaphase checkpoint assures that the spindles are anchored to the chromosomes.

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

The body cells differentiate from one another in order to preform their certain function. The genetic information is the same in all of the body cells but certain genes are expressed and others are turned off in order for the cell to preform its particular function.

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

Asexual reproduction takes up less energy that can be used during photosynthesis or cellular respiration instead.

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

DNA needs to be replicated before cell division because their needs to be double the amount of chromosomes so that one set can go into each of the two cells that are created during cell division.

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

The chromosomes move inside a cell during mitosis. During anaphase, one set of chromosomes is pulled in one direction by the spindles and the other set is pulled the other way to begin the formation of two separate cells. During cytokinesis the cytoplasm divides which creates two distinct separate cells.

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

The cell cycle is controlled by cyclin dependent kinases that either turn on or turn off certain processes that the cyclins determine which should be on or off. It is also controlled by checkpoints. If the controls were defective the cells would not divide properly which would lead to a disorder in the organism.


Eukaryotic cells divide when there are signals that a cell must grow and divide due to environmental factors. During G1 the cell begins to grow and divide which occurs due to the cyclin dependent kinases. Organelles, chloroplasts, and other important structures are copied. Before moving onto synthesis the cell must pass the G1 checkpoint. The s phase CDKs signal that the cell is moving from G1 to the S phase. Here the chromosomes are replicated so that there is a pair of each chromosome to be split into two different cells. These duplications are called sister chromatids and they are attached at their center called the centromere. After passing the S phase checkpoint the cell moves onto the G2 phase. Here the cell should be determined to have correct DNA and be large enough to divide. Then, mitosis begins. The chromatids all bunch up in the middle of the nucleus. Spindle fibers then attach to the outside of the cell and the kinetochores or anchoring sites on the chromatids. At the mitotic checkpoint, the spindles are checked to see if they are correctly anchored to the chromatids. Then anaphase begins where the sister chromatids are pulled apart and two separate cells are created. Cytokinesis separates the cytoplasm of the cell to create two identical sister cells.


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1. Cells that contain duplicates of chromosomes have more genetic information but there is no new genetic information. There are only two copies of the same genetic information inside the cell. Therefore there is more genetic information but nothing new that wouldn't be there if there was only one copy.

2. Chromosomes are condensed to enable them to smoothly move without any damage to the DNA. If the DNA was not condensed there would be more chances for it to be damaged. Condensed chromosomes allows a better chance for the DNA to be in good condition before the start of anaphase and the making of two separate cells.

3. The sister chromatids align at the center of the nucleus. The spindle fibers then attach to either chromatid and the kinetochores at the top of the nucleus. The mitotic checkpoint ensures that the spindle fibers are connected correctly. Then the spindle fibers pull the sister chromatids away from each other at the centromere to start the formation of two separate cells.

4. If the sister chromatids failed to separate, then one of the two cells created would have a duplicate of genetic information while the other cell would be lacking some genetic information. In both cases, this failure could lead to deformities in both cells and could end up harming the overall organism.

Effects of the Environment on Mitosis: Lectin Experiment

Experimental Hypothesis: The lectins will promote mitosis in the soybean roots and therefore weaken the plant tissues.

Null Hypothesis: The lectins will have no effect on the rate of mitosis in the soybean plant cells.

No, the hypotheses are not the same because one believes that the lectins will have an effect on the rate of mitosis while the other does not.

Experiment: Place some onion bulbs in a solution with lectins. Put other onion bulbs in a solution with water as a control group. Then after soaking them for a day take the onion bulbs out from the solutions and cut a tiny piece off of each to observe under a microscope. Count the amount of cells that are going through mitosis and the number of cells that are in interphase. Then calculate the chi squared number based on the observed and expected values for both the mitosis and interphase cells. Compare the chi squared value to the .05 probability under the degree of freedom of 1. If the chi squared value is less than the critical value of 3.84 then the null hypothesis is not rejected. If it is equal to or greater than the critical value, than the null hypothesis is accepted. If the null hypothesis is rejected, then the lectins will have had an effect on the rate of mitosis in the onion bulbs. If the null hyptothesis is not rejected, then the lectins have not had a significant effect on the onion bulbs. Therefore, the lectins do not promote mitosis in the onion bulbs. The two variables make no difference.

Measurements: In this experiment, you would find the observed values based on the amount of cells are seen in mitosis and the amount that are seen in interphase. From there the expected values of the cells in mitosis and interphase in both solutions can be determined by using the formulas shown below. Using these two tables and the chi squared formula, the chi squared value can be found. This value will then be compared to the degree of freedom of 1 and probability of .05 to determine whether the null hypothesis will be accepted or rejected.


Control: The control group would be the onion bulbs that are sitting in water instead of the lectin solution. Water does not have any mitosis promoting properties that would influence the rate of the cells cell division rate.

The null hypothesis is not rejected because the chi squared value is less than the critical value of 3.84 under the probability of .05 in the degree of freedom of 1. Since it is less than 3.84, the null hypothesis is true. Therefore the lectins will have no affect on the rate of mitosis in the onion bulbs.

Postlab Questions:

1. It was important to collect data as a class because the more data, the more accurate our observed and expected values will be which will therefore make the chi squared value more accurate rather than if we used one single set round of data. With an accurate chi squared value then we can determine whether the null hypothesis is correct.

2. No, there was not a significant difference between all the groups data. All the values were relatively the same because we were all using the same slides to determine which cells were in mitosis and which were in interphase.

3. No,the null hypothesis is not disproved meaning that the lectins do not increase the number of root tip cells in mitosis.

4. To verify the findings, we should measure if the presence of lectins makes mitosis occur faster in cells. We should time how long it takes for mitosis to occur in a group of cells in a lectin solution and a group of cells that are not in a lectin solution.

5. No, if an increased number of cells are in mitosis that does not necessarily mean that these cells are dividing faster than the cells in the roots. The time that you look at the cells is a factor when determining how fast the cells are going through mitosis. You could look at them at a time when little mitosis is occuring and visa versa.

6. Another way to test how fast the rate of mitosis is occuring is by preparing one lectin solution and one beaker of water and placing some root tips in both. This time, the root tips should be left in the solutions for a longer time, for a week, to see if the lectins actually have an affect on the rate of mitosis.


Part Three:

1. If a normal cell's DNA has mutations, the process of mitosis is usually blocked and the cells can not grow and divide like they should.

2. If cells with mutated DNA are replicated then the cell's checkpoints, CDK's, and cyclins are not regulating the cell cycle properly. At G2 checkpoint, if there was mutated DNA then the cell would have gone through apoptosis so that it would not replicate its mutated DNA. The cell cycle has lost control if it keeps replicating and producing mutated cells. This will lead to cancer or other diseases.

3. Cells monitor DNA integrity with checkpoints throughout the cell cycle. The most prominent checkpoint when determining DNA integrity is the G2 checkpoint. There are also enzymes and proteins that can repair DNA that has mutated or needs repair. If these enzymes decide that the DNA is irreparable then the cell will go through apoptosis.

4. In normal cells all of the 23 sets of chromosomes should be in a pair. However, cancerous chromosomes  have more than 2 chromosomes per pair or can be missing certain chromosomes all together. This can be seen on a karyotype. The chromosomes can also have mutations.

5.  The extra chromosomes can cause defects in the resulting organism. For example trisomy 21 causes Down's syndrome. If a chromosome is missing this can be detrimental to the rustling organism. For example, if regulatory genes are missing or the gene that codes for the way an organism is structured is not present this could result in a miscarriage or a deformity.

NONDISJUNCTION: The failure of homologous chromosomes in meiosis 1 or sister chromatids in meiosis 2 to separate correctly during cell division. This can also happen in mitosis if the sister chromatids fail to separate. If this occurs then one cell has double the amount of chromosomes that it needs and the other cells has half the amount it needs. This can also result in deformities if not discovered by the checkpoints in the cell cycle.

PART FOUR: MODELING MEIOSIS

In meiosis 1, the homologous chromosomes pair up. The corresponding chromosomes line up across from each other. In mitosis, all of the chromosomes line up in the middle of the nucleus. The homologous chromosomes do not pair up during mitosis.

Questions on Page S93

1. The DNA replicates during the S phase of meiosis 1. The individual chromosomes replicate to make two sister chromatids.

2. Homologous pairs of chromosomes are not exact pairs of each other. One of the chromosomes comes from the mother and the other comes from the father. The genes on the chromosomes code for the same parts of the body and their functions but the alleles could be different which would in turn affect the phenotype of the organism.

3. Crossing over occurs during meiosis 1 between the homologous chromosomes. This occurs when the homologous chromosomes tightly wrap around each other and switch genetic information. Some of the mothers genes might attach to the fathers chromosome and visa versa. This increases genetic variation within future generations.

4. The distance that the certain gene is from the centromere of the chromosome affects the crossover frequencies. The farther the gene is from the center of the chromosome the more likely it is to break off during crossing over and attach itself to the chromosome from the other parent.

5. Independent assortment also increases genetic variation within an organism. This is when the chromosomes randomly line up in the middle of the cell during metaphase. This random assortment of chromosomes ensures that when the chromatids are pulled apart during anaphase to create two new cells, that there will be genetic information from both the mother and the father in both new cells.

6. To calculate the number of different gametes, you use the equation 2n * m. In this case, n is the number of heterozygous pairs, m is the number of homozygous pairs.

7. Down's syndrome is caused by trisomy 21. It occurs when a homologous chromosome does not separate. Therefore, there is a third 21 chromosome within the genome of the organism. The organism now has 24 chromosomes which causes Down syndrome. Cri du chat syndrome occurs when a piece of the chromosome 5 is missing. If a homologous chromosome fails to separate in this instance than there will be no genetic information in the 5 chromosomal spot which results in cri du chat and the homologous chromosome that did not separate will be found somewhere else where there is too much genetic information like in trisomy 21.

8. Mitosis is the process of producing body cells for repair, growth, and asexual reproduction. Meiosis is the process of producing gametes for sexual reproduction.


Part 5: Meiosis and Crossing Over

EVALUATING RESULTS QUESTIONS

1. Since crossing over produces two spores that are like the parent spores and two spores that are different you have to divide the resulting percentage by 2 to only count the number of spores that have gone through crossing over.

2. The class data that was obtained was 31.8 units. This is 5.8 more units than the published map distance.

3. The picture of the asci spores was a little hard to decipher. It was hard to determine whether the spores were orange or black colored. Also, maybe some spores were a combination of two colors so it was hard to determine whether it was mostly black or orange. These could lead to disparities in the results.

4. During meiosis, the black and orange ascospores crossed over, switching some of their genetic information. This resulted in a new pattern of ascospores rather than the 4 black and then the 4 orange. Now the orange and black ascospores were mixed together.

5. The Philadelphia Chromosome is definitely a diagram of crossing over because one of the chromosomes has too little genes and the other has too many compared to its homologous pair. This means that during crossing over, the chromosome broke off farther than the half way point or the centromere.

6. No the cell cycle that describes mitosis would not be able to efficiently describe meiosis. Meiosis includes many different processes like separating twice to make two diploid cells and then four haploid cells. Also during metaphase, in mitosis the chromosomes align all in one row. In meiosis the homologous chromosomes pair up and then line up in the middle of the cells. These differences cause the cell cycle for mitosis and meiosis to have to be explained separately.  

Explain Everything : https://docs.google.com/a/npsdnj.org/file/d/0Bzku19OuzZMCUXYxWTFremtsWDQ/edit?usp=docslist_api


Table for Part 6

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