Lab 7 - Cell Division: Mitosis and Meiosis #NPAPBio

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


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?

The zygote undergoes cell division, or mitosis, in order to develop into an organism of trillions of cells. It also undergoes speciation early on in the development of the organism. In order to create an organism with 100 trillion cells, 50 trillion cells need to divide (one time of mitosis), and 25 trillion need to divide to make 50 trillion (two times of mitosis), and so on. The cell would have to repeat this process 40 times (one cell going under mitosis 40 times is equivalent to 1 times 2 to the 40th power, which equals 1 trillion).

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

Single celled organisms reproduce by dividing, which is very important is the organism is to sustain a population of that species of organism in an environment. Single-celled organisms do not undergo mitosis to get larger and more complex, etc., like multicellular organisms.

3.  What must happen to ensure successful cell division?

Proteins must regulate whether the DNA that is replicated is identical to the original DNA, and the cell must be ready to divide. For example, can't be too small, otherwise it would not have enough organelles to survive.

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

Although different genes are expressed, the genetic information of all my cells is the same (other than small mutations that occur naturally that occurs in DNA replication).

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

If a plant is fit for survival, then all of its offspring will be fit for survival. In sexual reproduction, there is a chance that the offspring is less fit for survival and receive the less favorable trails of the parent organisms. Also, it is a much less complex process, and less things can go wrong compared to sexual reproduction. Plants are also able to mass reproduce, and since their biomass makes up a large part of an ecosystem, this is very important to the entire environment.

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

Daughter cells must have identical DNA strands in order to function as they were originally supposed to. If they did not have copies of chromosomes until after cell division, there is a higher chance that there would be an error.

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

Spindle fibers that are attached to centrioles at the poles of the cell attach to the centromeres of the sister chromatids and pull them apart.

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

The cell is regulated by tumor-suppressor genes that make sure that the cell viable to pass the checkpoints in the cell cycle. If the controls are defective, then the balance of the body could be compromised- the cell can go rapid reproduction and produce tumors, the cell could keep reproducing when it's not needed and waste precious materials, and/or the cells can become cancerous.

How Cell Division Is Controlled

The cell cycle is controlled by regulatory proteins. They make sure that, when the cell hits certain checkpoints during its life cycle, it's healthy. If it's not, then those proteins signal for other proteins to come, or for the cell to undergo apoptosis. These proteins are what keeps our cells from becoming cancer cells, which is why they are products from proto-oncogenes/tumor-suppressor genes.. If they are mutated fail to regulate cell division, then mutated cells will be able to divide, and they may form tumors.  

How Eukaryotic Cells Divide

Eukaryotic cells divide through the process of mitosis. The cell spends most of its life in interphase, when it grows into a mature cell. A segment of interphase is S, or "synthesis", when the DNA of the cell undergoes DNA replication and therefore copies itself. The cell now has two complete copies of DNA, and it begins the process of mitosis. The chromosomes of the cell before mitosis are very condensed and  tangled. However, when prophase begins, the chromosomes untangle into sister chromatids (exact copies of each other), that are connected by a centromere. The chromosomes were duplicated when the DNA was replicated. Then, those sister chromatids are lined up along the center of the cell, and are pulled apart by spindle fibers. Now, the cell has a complete copy of DNA on each pole. The cell then pinches across the middle, and the cleavage furrow separates the two cells into identical daughter cells.

S87 Lab Questions

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 duplicated DNA is identical to the original.

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

When chromosomes are condensed, they are able to be moved much easier than if they were entangled with one another.

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

Spindle fibers that are connected to centrioles pull the chromatids from the centromeres towards the poles.

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

Nondisjunction. Also, the checkpoints of the cell cycle will recognize this mistake, and hopefully regulatory proteins will take care of the faulty cell.

Effects of the Environment on Mitosis

"Various fungi can negatively affect the growth of soybeans by producing lectin-like protein. Lectins can induce mitosis in the root apical meristem tissue which will often weaken the plant tissue."

Testable question: Does lectin induce mitosis in the root apical meristem tissue cells of a plant?

Experimental Hypothesis: Lectin will induce rapid and significant cell division in the root apical tissue cells, which will therefore increase the amount of cells undergoing division at a certain time.

Null Hypothesis: Lectin will not induce rapid and significant cell division in the root apical tissue cells.

Experimental design:

In this experiment, we will have two control groups, in which one will be exposed to lectin for a certain amount of time, which the other will not be exposed to any extraneous substances. We will also try to control as many variables as possible, including the environment of the cells, the amount of nutrients they get, the sunlight exposure of the cells, the number of cells, temperature, etc. After an amount of time in which the cells that are treated with lectin begin to react to the substance, we'll observe cells underneath a microscope of the same magnifying view, and count the number of cells that are undergoing mitosis and the ones that are only in interphase. These values will be used in conducting a Chi-Squared test to see whether the null hypothesis will be rejected or not.

Class Data, Observed Values, and Expected Values

Chi-Squared Calculations

Critical Values

The degrees of freedom of the experiment can be found with (columns-1)(rows-1), and so our df = (2-1)(2-1)= 1. According to the table, our data is not significant, and so the null hypothesis was not rejected. Therefore, our experimental hypothesis was not proven, and lectin will not induce rapid and significant cell division in the root apical tissue cells.

Postlab Review

What was the importance of collecting the class data?

It is important to have a lot of data in this experiment, so that the data is significant. If you only used one trial, then the significance of the data decreases dramatically, because it is more likely that the data was a result of chance.

Was there a significant difference between the groups?

No, because the Chi-Square value was 0.967, which is not significant.

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

Because we failed to disprove our null hypothesis, the fungal pathogen lectin does not increase the number of root tip cells in mitosis, according to our data.

What other experiments should you perform to verify your findings?

We should look at how lectin affects individual cells. We should have very identical sister cells that are isolated from each other. All cells would be in the same environments, but half will be in the presence of lectin. We would time the amount of time it takes for these cells to divide.

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 just means that the amount of mitosis per population is increased, which means that the rate of which mitosis occurs is higher. It does not mean that the rate of mitosis is faster.

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

The previous experiment should determine how fast the rate of mitosis is occurring in root tips. If we timed how fast individual cells go through mitosis (and we would have to experiment with many, many cells to make the data valid)  with and without lectin. There are many other factors that go into whether mitosis occurs, and it's very important that we should control as many as possible.

Case 2 Activity pg S92

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

In a normal cell, regulatory proteins will recognize the damaged or mutated DNA, and either fix the issue, or signal for the cell to undergo apoptosis.

What would happen if the cells with mutated DNA replicated?

If the mutated DNA compromises the integrity of the genes of those regulatory proteins, then mutated cells can pass through the cell cycle checkpoints and produce mutated cells. Those cells are then at a higher risk of further mutating, because there will be no way to make sure that the cell is functioning correctly or whether mitosis occurs without mistakes. Those cells will either not be able to function correctly and die on their own time, or they will become dangerous, reproduce rapidly, and consume the nutrients that are necessary for normal functional cells to live.

How do cells monitor DNA integrity?

Regulatory proteins that stem from proto-oncogenes, like the BAD protein, make sure that the integrity of DNA is maintained.

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

The chromosomes are elongated or shorter, because parts of some chromosomes have attached to other chromosomes.

How could these differences lead to cancer?

The broken chromosomes are now faulty, and do not function as they are supposed to. When the DNA of chromosomes 9 and 22 are swapped, then an oncogene called BCR-ABL is made. This gene makes the BCR-ABL protein, or "tyrosine kinase", which causes these cancerous cells to reproduce and grow rapidly and out of control.

Concept of Nondisjunction

Nondisjunction is the failure of sister chromatids of homologous chromosomes to properly split during mitosis or meiosis. When these chromosomes don't separate correctly, the resulting cells, especially gametes, are genetically compromised.



S93 Bottom Half

When is DNA replicated during meiosis?

DNA is replicated during interphase of meiosis.

Are homologous pairs of chromosomes exact copies of each other?

No- one of the pair is from the mother, while the other is from the father. Unless the mother and father are identical twins, which is impossible unless both are hermaphrodites, the homologous pairs of chromosomes can never be exact copies of each other.

What is crossing over?

Crossing over is the exchange of genes between pairs of homologous chromosomes on contact. It increases genetic variation of the gametes that are produced.

What physical constraints control crossover frequencies?

The size and length of the chromosome controls crossover frequencies. The closer a gene is to the centromere, the less likely it will be able to cross over to the other homologous chromosome.

What is meant by independent assortment?

Independent assortment is when the homologous chromosomes pair up and line across the metaphase plate, and the paternal and maternal chromosomes independently assort themselves along the plate, which results in two cells at the end of meiosis that contains both "mom" and "dad" chromosomes.

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

If no crossing over occurs, then the number of different kinds of gametes is the combination of 23 chromosomes (paternal and maternal) that can be produced from 46 chromosomes, otherwise known as 2^n. Since our n=23, the amount of different gametes that we can produce is 8388608- without crossing over. When crossing over occurs, then the number is essentially infinite.

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 pair of chromosomes fails to separate, two gametes will have extra chromosomes, while the other gametes will be devoid of a part or the entirety of the chromosome that failed to separate. Down syndrome and cri du chat syndrome occur when there is an absence or extraneous presence of a part or a whole of a chromosome. If the gametes with the missing or extra chromosomes are fertilized, then the resulting infant will have a genetic disorder.

How are mitosis and meiosis fundamentally different?

Mitosis is used for asexual reproduction, growth, and repair. The function of meiosis is to produce haploid gametes for sexual reproduction. Not only are the functions different, but the processes are very different- mitosis itself is the creation of identical cells, which is a relatively simple procedure. Meiosis consists of two parts, meiosis I and meiosis II, in which chromosomes are paired up and swap genes to add to genetic variation, to create four haploid cells.

Lab Bench Activity

Evaluating Results S96

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

We divided the percentage of asci showing crossover by 2 to find the distance between the gene for spore color and the centromere in map units.

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

No class data.

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

No class data.

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

See picture underneath.

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 believe that the Philadelphia chromosome is a result of

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

Explain Everything

How does meiosis followed by fertilization ensure genetic variation in sexually reproducing organisms?

Explain Everything link:

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