# Cell Division: Mitosis and Meiosis

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

## 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