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?

Pre Lab Section

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 goes through cell division or mitosis over and over until a blastocyst is formed.

Blastocyst-composed of many stem cells which then specialize into the many different types of cells found in an organism.


The specialized cells from the Blastocyst then divide and come together to form tissue, the tissue eventually turns into an organism. It would take 10 million mitotic cell divisions to make an organism with 100 trillion cells.

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

Cell division is important to a single celled organisms because it is the only way that they can reproduce. Without cell division there would not be any single celled organisms.

3. What must happen to ensure successful cell division?

There are checkpoints that ensure successful cell division.

G2 phase to M phase- it checks for DNA replication completion, DNA damage/mutations, and cell size

M Spindle- Checks from spindle fiber attachment to chromosomes

G1- Checks for cell size, Growth factors, and environment

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

All the genetic information in all body cells are the same, the one difference is what is being expressed.

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

if plants cant find another plant to reproduce with, plants can reproduce asexually. This keeps the species alive in that area. There are also less chances of mutations or things to go wrong if plants reproduce asexually.

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

If the DNA is not replicated before the cell divides then the daughter cells will only have half the necessary chromosomes and genetic code; it would be n rather than 2n if mitosis is the process that occurred.

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

The chromosomes are inside the nucleus and replicate during the S phase of interphase.  Then in metaphase the chromosomes are attached to spindle fibers. The spindle fibers pull apart the chromosome in anaphase so that the sister chromatids are at the end of the cell. The sister chromatids end up in 2 nuclei in telophase, the sister chromatids end up becoming the daughter cells once the cell splits.

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

It is controlled by cyclins and CDKs. Cyclins attach to the CDKs which then allows the CDKs to do something to make the cell advance to the next part of the cell cycle an example of this would be chromosome replication. If the control was defective the cell cycle could have many problems. Such as the checkpoints not checking for the proper things.

How is cell division is controlled?

Cell division is controlled by cyclins, cdks, apc, and checkpoints. There are three checkpoints that the cell must pass. There is the G1, G2, and the M-spindle. At each of the checkpoints the cell checks if it has completed all the parts, and if it is ready to move on.

Cells pass G1 when they stimulate the appropriate external growth factors

Cells at G2 check for damage after the DNA is replicated, if there is damage it prevents the cell from going under mitosis.

The M-spindle checks for the mitotic spindles to be properly attached to the kinetochores.

The cell cycle is controlled very carefully, mutations in the cell cycle could lead to cancer.

Explain how eukaroytic cells divide to produce genetically identical cells

This process can be put into three phases, inter-phase, mitosis, and cytokinesis

interphase- DNA Replication is checked for integrity. Errors are corrected, and repairs are made, in the synthesis the chromosomes are replicated.

Mitosis- the duplicated chromosomes are separated into two nuclei

Cytokinesis- When the cytoplasm divides and organelles separate into daughter cells.

This type of cell division is asexual and important for growth, renewal, and repair o multicellular organisms.


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 all the genetic information is the same, as before the cell was duplicated.

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

It is easier and safer to move when they are condensed

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

They are separated into two nuclei and the cytoplasm divides.

What would happen if the sister chromatids failed to separate?

One daughter cell with an extra chromosome and one daughter cell with a missing chromosome.

Hypothesis: Lectin will increase the number of cells that are going through mitosis in the roots.

Null Hypothesis: Lectin will not increase the number of cells going through mitosis.


1. Cut up the onion bulb root tip, put the cut onion bulb root tip under a microscope and record how many cells there are.

2. Have many different petri dishes to put onion bulb root tips and different amount of lectin. The control group would be the petri dish with no lectin.

3. Check how may cells there are after every hour.

4. Check the data to see if we prove our hypothesis

Page S90

What was the importance of collecting the class data?

To get more data and more accurate results, because in a larger size there is more accuracy.

Was there a significant difference between the groups?


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

No it did not have an effect.

What other experiments should you perform to verify your findings?

Have the onion root tips in different amounts of lectin.

Does an increased number of cells in mitosis mean that these cells are dividing faster than the cells in the roots with lower number of cells in mitosis?


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

Keep track of how fast each of the onion root tip undergo mitosis.

Null Hypothesis and Chi Squared                                  Test

Null Hypothesis-a hypothesis which the researcher tries to disprove

Chi Squared Test-

Loss of Control of Cell Cycle

Case 2 Page S92-

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

A normal cell that has DNA that has mutations will cause the cell to not perform the tasks it needs to.

2. What would happen if cells with mutated DNA replicated ?

There would be more mutated cells, and there would most likely be problems with the DNA.

3. How do cells monitor DNA integrity?

Cells monitor DNA integrity by using CDK's and cyclins. These proteins are used in the checkpoints and check for DNA that has been damaged or hasn't gone through the right steps.

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

Chromosomes are different in cancer cells than in normal cells because they are most likely damaged or had a mutation so the chromosome might not be able to complete it's task.

5. How could these differences lead to cancer?

The differences lead to cancer because the cells could be missing certain vital genes and could lead to cancer.

Nondisjunction- When homologous chromosomes or sister chromatids do not separate properly during cell division

Modeling Meiosis

1. What is crossing over?

Crossing over is the creation of recombinant chromosomes. It is the swap of genetic material from the Maternal to the Paternal chromosome and the Paternal to the Maternal

2. What physical constraints control crossover frequencies?

The closer the chromosomes are to each other it makes it easier to cross over.

3. What is meant by independent assortment?

This is the way the chromosomes line up in the cell.

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

multiply the total number of pairs of chromosomes by four

5. 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 homologous pairs of chromosomes failed to separate the gametes would have twice as many gametes or maybe even none at all. 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. Some 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.

6. How are mitosis and meiosis fundamentally different?

Mitosis creates somatic cells and only creates two daughter cells from the parent cell, while meiosis creates gametes and produces four daughter cells from the parent cell.

Meiosis and Crossing Over

Evaluating Results

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

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

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

The class data we collected was 5.8 units off

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

We only looked at the 11 asci, so it was not very accurate.

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

Crossing over occurred. This means that homologous chromosomes were very close to each other and exchanged sections of their DNA. The gene for spore color was the farthest down there is a high chance that the spore color gene got exchanged.

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 think that Philadelphia chromosome is a result of crossing over. One chromosome seems to be shorter than the other. So that could mean that one of the sections that were going to get swapped got lost.

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

No I do not think that the cell cycle describe for mitosis could be applied to meiosis as well because the end results for mitosis and meiosis are very different so I don't think that the cell cycles are regulated in the same manner.

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