Lab 7: Cell Divison: Mitosis and Meiosis


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

2. Pre Lab Questions

1. How did you develop from a single-called 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?

The first days and weeks after conception: mitotic cell divisions begin, converting the zygote to a multicellular organism,

Day 1: first cleavage - 1 cell becomes 2,

Day 2: second cleavage - 4-cell stage,

Day 3: 6-12 cell stage - can test at this stage for genetic diseases if done by IVF,

Day 4: 16-32 cell stage - solid ball of cells - morula,

Day 5- Solid morula develops into hollow, fluid-filled blastula. The embryo will develop from the inner cell mass, or embryonic disc

Day 6-7 : The embryo will implant onto the uterine lining of the mother.

Days7-10: Days 7-10: Gastrulation: major cellular reorganization into 2 or 3 tissue (germ) layers. This occurs when cells start to differentiate. They will form the layers of the Ectoderm: skin, nervous system, Endoderm: lining of gut and internal organ, and the Mesoderm: muscles, bones, heart . This Gastrula: Early embryo with 3 tissue layers will have  different cells which will  begin to "turn on" (or "express") different genes to become different organs.

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

It aids in reproduction with the formation of gametes, allows growth to occur, and repairs damaged cells

3. What must happen to ensure successful cell division?

The most important condition for a successful division is replication of the DNA.   The DNA needs to be replicated correctly so it goes through a variety of checkpoints throughout the cell cycle.

G1 checkpoint:Checks for the right cell conditions, growth factors present,

G2 checkpoint: Checks for damage after the DNA is replicated and if there is damage, it prevents the cells from going into mitosis

M-spindle (metaphase) checkpoint :Make sure that the spindles properly attach to the anchor sites on the chromosome, kinetochores. If they aren't anchored properly then the cell doesn't continue on through mitosis

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

All cells have the same DNA.

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

Plants can reproduce asexually, without the fertilization of gametes, by either vegetative reproduction or apomixis. Vegetative reproduction is results in new plant individuals without the production of seeds or spores while apomixis occurs in plants when plants produce seeds without fertilization. This is advantageous because the plants will have an increased rate of maturity and a sturdier adult plant. Asexual reproduction is usually faster than sexual and so a new plant can colonize an area more quickly. Examples include moss, ferns, and spores.

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

It needs make gametes that each have 23 chromosomes.Each daughter cell must have the identical DNA and related structures as the parent cell. Without this genetic information, the cell cannot understand how to continue its biological processes and so it will die.

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

During the process of cell division, DNA is packed into chromosomes that move inside the cell in a highly organized manner. First, chromosomes migrate to the center of the cell during metaphase. Then the sister chromatids of each chromosome move to opposite poles in the cell during anaphase.  The cell begins to pinch inward and Nuclear membranes begin to form around the separated chromatids.

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

Cell cycle is controlled by regulatory proteins such as the PTEN and BAD proteins which inhibit cell division. The cell checkpoints (G1, S, and G2)  also help control the cell cycle because they help identify the conditions ( size, DNA amount, etc) of a healthy good-to-go cell. If the control were defective then even faulty cells to pass through the  checkpoints and mutated  regulatory proteins would allow the faulty cells to continue to grow and divide,  promoting the abnormal rate of the faulty cells that could potentially lead to cancer.

3. Cell is controlled by the different checkpoints ( G1, S, M )in the cell cycle which are  controlled by complexes of several proteins containing enzymes called CDKs, cyclin-dependent kinases. These complexes turn on or off various processes that take place in cell divisions. They allow the cells to go through the different phases.

They make sure the cell meets all the conditions required for successful cell division. If cells fail to meet to these conditions and are faulty , apoptosis, cell suicide,  occurs. If these checkpoints weren't  working cells healthy or not would be dividing. This could lead tumors which could cause serious illnesses such as cancer.


How do eukaryotic cells divide to produce genetically  Identical cells?

4. During the M phase of the cell cycle, mitosis occurs. First DNA condenses into chromosomes and duplication occurs. Centrioles move to the opposite sides of the cell. Then spindle fibers from then centrioles align the chromosomes in the middle. Then the chromosomes are pulled apart at the centromere and move to the centrioles placed at opposite poles of the cell. At this point, the half chromosome is called a  sister chromatid. The cell begins to pinch inward and Nuclear membranes begin to form around the separated chromatids. At the end of mitosis, the cell is pinched into two cells with identical cells with the same number of chromosomes

5. Modeling Mitosis

Q1; If a cell contains a set of duplicated chromosomes , does it contain any more genetic information than the cell before the chromosomes were duplicated ?

A1: No because duplicated chromosomes are just identical copies of the original chromosome. They are duplicated so they can be separated to make two identical daughter cells in the end.

Q2: What's the significance of the fact chromosomes condense before they are moved?

A2: They condense so they can move easily and quicker when undergoing the stages of mitosis.

Q3: How are the chromosomes copies , called sister chromatids, separated from each other?

A3: They are separated after they line up on the metaphase plate. Spindle fibers from the centrioles placed at opposite poles of the cell attach to the centromeres of the chromosome and pull half of the chromatids to one side of the cell and the other half to the opposite cell pole.

Q4: What would happen if the sister chromatids failed to separate ?

A4: If they failed to separate then the daughter cells would have an abnormal number of chromosomes, too little or too much. This could lead to diseases such as Down Syndrome or Autism.


Investigate Whether Pathogen Lectin Affects the Number of Cells Undergoing Mitosis

EXPERIMENTAL HYPOTHESIS: Fungal Lectin will increase the number of cells undergoing mitosis in a plant's root tips

NULL HYPOTHESIS: Fungal Lectin will have no effect in the number of cells undergoing mitosis .

The null and experimental hypotheses are not the same. The null states the independent variable (fungal Lectin)  has no effect on the dependent variable ( cells going through mitosis) while the experimental states the effect the independent variable ( fungal Lectin) will have on the dependent variable ( increase the number of cells going through mitosis) .


I would have two groups to test the hypothesis . One group of onion bulbs that are treated with Lectin and one group of onion bulbs that are not treated with Lectin ( these bulbs would be just in water).  After slicing them open and staining them, I will put both of them under a microscope and count how many cells are undergoing mitosis.

I would measure the number of cells by placing the onion bulbs in a solution that freezes mitosis in the cells where it's happening and then stain the bulbs with a dye so I could visually see the number of cells that are undergoing mitosis whether they are  in anaphase, metaphase, propase, telophase, etc. easier. I will put each  bulb in a slide and play that under a microscope. Then I will individually count the onion bulb cells undergoing interphase or any stage of mitosis.

The control in this experiment would be the onion bulbs that aren't  treated with Lectin.

Null Hypothesis: hypothesis  that implies no effect or no relationship between two variables. It can be tested and found to be false, which then implies there is a relationship between the two pieces of observed data.

Chi-Squared Test: a statistical test used to compare expected data with what we collected. What a chi-square will tell us is if there is a large difference between collected numbers and expected numbers. If the difference is large, it tells us that there may be something causing a significant change. A significantly large difference will allow us to reject the null hypothesis,  Then we can conclude the  something significant happened with our results.

7. Class Data of  Lectin and Non-treated Onion Bulb Cells

8. Chi-Squared Calculations

9. Critical Values-  How to accept or reject the null hypothesis

The p value is 0.05 , and the critical value is 3.84. If the calculated chi-square value is greater than or equal to this critical value, then the null hypothesis is rejected. if the calculated chi-square value is less than this chi-square value , the null hypothesis is NOT rejected.

10. Should the null hypothesis be accepted or rejected ? What does this mean in terms of your hypothesis?

The null  hypothesis should be accepted in our case because our  chi-square value of .97 was less than 3.84. Therefore in terms of our hypothesis this means that the fungal pathogen lectin had no effect on the number of cells undergoing mitosis.

11. Post Lab Questions

1. What was the importance of the class data?

The more trails we have , the more accurate our result will be.

2. Was there a significant difference between the groups?

Statistically no because the calculated chi-square was .97 and that was less than critical value of 3.84.

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

No because because the chi-square value of .97 was less than 3.84. Therefore the null hypothesis wasn't rejected. Since the null hypothesis says there is no effect between the variables. The fungal pathogen lectin had no effect on the number of cells undergoing mitosis.

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

We could duplicate these procedures in different experiments with different types of roots not just onion bulbs such as carrots, parsnips, and sweet potatoes.

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 a increased number of cells in mitosis means that cells are dividing faster than those found on the roots with the lower number of cells in mitosis.

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

Another way we could determine the the rate of mitosis is having different time interval where we check the number of cells undergoing mitosis or interphase.


12. Case 2 activity questions

Q1: What happens in a normal cell if the DNA has mutations?

A1:  By changing a gene’s instructions for making a protein, it can change the  fucntions of proteins and therefore its shape. This would cause the protein to malfunction or to be missing entirely which then could cause genetic disorders  such as Sickle cell Anemia and Cystic Fibrosis.

Q2: What would happen if cells with mutated DNA replicated ?

A2; Serious diseases such as cancer can develop because faulty cells keep growing an dividing. This abnormal division can cause tumors to form.

Q3: How do cells monitor DNA integrity?

A3: Cells monitor their DNA integrity with various checkpoints throughout their cell cycle. They include the  G1, S, M  checkpoints. The G1 makes sure the cell is big enough and has made the proper proteins for the synthesis phases.The S phase makes sure if the DNA was replicated correctly. The M phase makes sure two identical daughter cells are created. If the cell doesn't meet or complete any of conditions mentioned above, it doesn't continue on to the next phase and undergoes apoptosis with regulatory protein such as BAD and PTEN .

Q4: How are chromosomes different in the cancer cells compared to normal cells?

A4: Cancer cells sometime have duplicated chromosomes or a few missing chromosomes (mutations).

Q5: How could these differences lead to cancer?

A5: Some genes on chromosomes code for tumor-repressor genes that inhibit cell growth with proteins such as PTEN and BAD. Without these proteins, faulty cells will proliferate at an alarming rate causing tumor formations and eventually cancer.  DNA is basically instructions telling proteins how to function. If the instructions are altered so is the protein function. When proteins don't function properly cell checkpoints can also malfunction causing  faulty cells that don't meet the required conditions to clear the checkpoint to pass through and replicate.

13. Nondisjunction-  occurs when homologous chromosomes or sister chromatids fail to separate properly during cell division. It could cause gametes to have more than the   normal number of 23 chromosomes. This could lead to serious condition such as Down Syndrome or Turner Syndrome,


14. Mitosis and Meiosis



15. Questions

Q1: When is the DNA replicated during meiosis?

A1: DNA is replicated right before meiosis I during interphase.

Q2: Are homologous pairs of chromosomes exact copies of each other?

A2: No, homologous pairs of chromosomes are not exact copies of each other because one chromatid comes from the father , and one comes from the mother. Therefore they have different alleles.

Q3: What's crossing over?

A3: Crossing over is when a segment DNA break off and reattaches to its homologous chromosome.

Q4: What physical constraints control crossover frequencies?

A4: The further the allele are from each other on a gene makes it harder it to cross over together to another homologous chromosome and vice versa.

Q5: What's meant by independent assortment?

A5: Independent assortment means that genes are inherited independently of each other because it doesn't matter how they line up during metaphase, meaning the chromosomes lined up randomly on the metaphase plate. Therefore when the daughter cells are made they could be genetically varied.

Q6: How can you calculate the possible number of different kinds of gametes?

A6: You calculate them by multiplying all the genes of the mother and father and see how different combinations they make.

Q7: 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?

A7: If homologous pair of chromosomes fail to separate there would be more than the normal number of 23 chromosomes in one of the daughter cells and less than 23 chromosomes in another one of the daughter cells which cause genetic disorders such as Down Syndrome and cri du chat. Down Sydrome occurs when there is an extra cope of chromosome 21 and Cri du Chat occur when there's a deletion of genetic material from chromosome 5.

Q8: How are mitosis and meiosis fundamentally different?.

A8: Mitosis creates 2 genetically identical diploid daughter cells with original number of 46 chromosomes through asexual reproduction. Meiosis produces four haploid daughter cells with only 23 chromosomes. In mitosis there is one cell division while in meiosis there two. Mitosis are creates cells growth and repair while meiosis creates gametes for sexual reproduction.


16. Lab Bench Activity

17. Evaluating Questions

Q1: Why did you divide the percentage of as I showing crossover (recombinant) by 2?

A1:It's divided by 2 because only half of the spores in each ascus are the result of a crossing-over event.

Q2: The published map distance between the spore color gene and the centromere is 26. How did the class data compare with this distance?

A2:It was 31.8 units , 5.8 units from the published distance.

Q3: How can you account for any disparities between the class data and the published data?

A3:It could attributed to people not being able to distinguish what spores resulted from crossing over due to lack of clarity on the picture.

Q4: Illustrate what happened during meiosis to produce the results you found

A4: Crossing over occured during meiosis because the spores had varied appearances (anything other than The arrangement of  4 black sores and 4 tan spores in a row)

Q5: 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.

A5: I think it's a result of some other chromosomal abnormality because crossing over would result in equally sized chromosomes not having one chromosome shorter than the other.

Q6: Do you think the cell cycle described for mitosis could be applied to meiosis as well? Explain your answer.

A6: No because both processes create different results . In mitosis two identical daughter cells are created for growth and repair. They don't need to go through another cell cycle like meiosis. Meiosis creates 2 daughter cells with the haploid number of chromosomes so those cells can be used for sexual reproduction. This occurs with 2 cell divisions . It wouldn't

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