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AP Lab 7 – Cell Division: Mitosis & Meiosis
How do eukaryotic cells divide to produce genetically identical cells or to produce gametes with half the normal DNA?
Background
One of the characteristics of living things is the ability to replicate and pass on genetic information to the next generation. Cell
division in individual bacteria and archaea usually occurs by binary fission. Mitochondria and chloroplasts also replicate by binary
fission, which is evidence of the evolutionary relationship between these organelles and prokaryotes.
Cell division in eukaryotes is more complex. It requires the cell to manage a complicated process of duplicating the nucleus, other
organelles, and multiple chromosomes. This process, called the cell cycle, is divided into three parts: interphase, mitosis, and
cytokinesis (Figure 1). Interphase is separated into three functionally distinct stages. In the first growth phase (G1), the cell grows
and prepares to duplicate its DNA. In synthesis (S), the chromosomes are replicated; this stage is between G1 and the second growth
phase (G2). In G2, the cell prepares to divide. In mitosis, the duplicated chromosomes are separated into two nuclei. In most cases,
mitosis is followed by 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 of multicellular organisms.
Figure 1. The Cell Cycle Showing G1, S, and G2 Phases, Mitosis, and Cytokinesis
Cell division is tightly controlled by complexes made of several specific proteins. These complexes contain enzymes called cyclindependent kinases (CDKs), which turn on or off the various processes that take place in cell division. CDK partners with a family of
proteins called cyclins. One such complex is mitosis-promoting factor (MPF), sometimes called maturation-promoting factor, which
contains cyclin A or B and cyclin-dependent kinase (CDK). (See Figure 2a.) CDK is activated when it is bound to cyclin, interacting with
various other proteins that, in this case, allow the cell to proceed from G2 into mitosis. The levels of cyclin change during the cell
cycle (Figure 2b). In most cases, cytokinesis follows mitosis.
Figure 2a-b. MPF Production During the Cell Cycle
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As shown in Figure 3, different CDKs are produced during the phases. The cyclins determine which processes in cell division are
turned on or off and in what order by CDK. As each cyclin is turned on or off, CDK causes the cell to move through the stages in the
cell cycle.
Figure 3. Levels of CDKs During the Cell Cycle
Cyclins and CDKs do not allow the cell to progress through its cycle automatically. There are three checkpoints a cell must pass
through: the G1 checkpoint, G2 checkpoint, and the M-spindle checkpoint (Figure 4). At each of the checkpoints, the cell checks that
it has completed all of the tasks needed and is ready to proceed to the next step in its cycle. Cells pass the G1 checkpoint when they
are stimulated by appropriate external growth factors; for example, platelet-derived growth factor (PDGF) stimulates cells near a
wound to divide so that they can repair the injury. The G2 checkpoint checks for damage after DNA is replicated, and if there is
damage, it prevents the cell from going into mitosis. The M-spindle (metaphase) checkpoint assures that the mitotic spindles or
microtubules are properly attached to the kinetochores (anchor sites on the chromosomes). If the spindles are not anchored
properly, the cell does not continue on through mitosis. The cell cycle is regulated very precisely. Mutations in cell cycle genes that
interfere with proper cell cycle control are found very often in cancer cells.
Figure 4. Diagram of the Cell Cycle Indicating the Checkpoints
Learning Objectives
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To describe the events in the cell cycle and how these events are controlled
To explain how DNA is transmitted to the next generation via mitosis
To explain how DNA is transmitted to the next generation via meiosis followed by fertilization
To understand how meiosis and crossing over leads to increased genetic diversity, which is necessary for evolution
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Procedures
Part 1: Modeling Mitosis
Pre-Lab Questions:
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?
2. How is cell division important to a single-celled organism?
3. What must happen to ensure successful cell division?
4. How does the genetic information in one of your body cells compare to that found in other body cells?
5. What are some advantages of asexual reproduction in plants?
6. Why is it important for DNA to be replicated prior to cell division?
7. How do chromosomes move inside a cell during cell division?
8. How is the cell cycle controlled? What would happen if the control were defective?
Materials:
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4 long pipe cleaners: 2 long pipe cleaners of one color and 2 long pipe cleaners of a different color.
4 short pipe cleaners: 2 short pipe cleaners of the one color 2 short pipe cleaners of a different color.
8 beads of the same color.
Procedure:
**As you work with the pipe cleaners, use colored pencils to diagram each stage in the circles of the diagram. You may also
choose to take pictures of each stage as you model through the cell cycle. These drawings or pictures need to be included in
what you turn in for AP Lab 7.
1.
2.
3.
4.
5.
Imagine that a cell with 4 chromosomes (2 long each a different color, and 2 short each a different color) is in Interphase G1.
Set up the pipe cleaners and beads to represent Interphase G1.
Squeeze an identical pipe cleaner through each centromere to represent the result of replication in Interphase S.
Continue through G2
Modify the configuration of pipe cleaners to illustrate:
o prophase
o metaphase
o anaphase (add centromeres as required)
o telophase
o cytokinesis
Some examples of pipe cleaners and beads as chromosomes:
one chromosome,
single stranded in
Interphase G1
one chromosome,
double stranded after
Interphase S
a pair of homologous
chromosomes,
double stranded
Interphase G2
Analysis 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? Explain.
2. What is the significance of the fact that chromosomes condense before they are moved?
3. How are the chromosome copies, called sister chromatids, separated from each other?
4. What would happen if the sister chromatids failed to separate?
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Part 2: Effects of Environment on Mitosis
Scientists reported that a fungal pathogen, may negatively affect the growth of soybeans (Glycine max). Soybean growth decreased
during three years of high rainfall, and the soybean roots were poorly developed. Close relatives of R. anaerobis are plant pathogens
and grow in the soil. A lectin-like protein was found in the soil around the soybean roots. This protein may have been secreted by
the fungus. Lectins induce mitosis in some root apical meristem tissues. In many instances, rapid cell divisions weaken plant tissues.
You will investigate whether the fungal pathogen lectin affects the number of cells undergoing mitosis in a different plant (onion),
using root tips.
You will be provided with untreated and lectin-exposed roots. You should be comfortable identifying cells in mitosis or interphase
before you begin examining chromosome squashes.
Hypothesis:
Experimental hypothesis (make a hypothesis to describe the effect of lectin on the stages of mitotic division):
Null hypothesis (statement of assertion that the things you were testing are not related and your results are the product of random
chance events):
Materials:
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Onion bulbs (A & B –
treated and untreated)
metric ruler
scalpel
1M hydrochloric acid
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Carbol fushsin stain
Depression plate
distilled water
microscope slides (3)
coverslips (2)
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forceps
microscope
clock or watch
paper towels
Procedure:
Part A: Preparing Chromosome Squashes
1. Obtain 1 onion bulb A and 1 onion bulb B.
2. Cut 4-5 roots (approximately 1 cm in length) from the onion bulb A using a scalpel. Caution: The scalpel is extremely sharp.
3. Place the roots in a spot plate with 50°C 1M HCl. Soak for 5 minutes.
4. Using forceps, transfer the roots to a well containing carbol fushsin stain. Stain for 2-3 minutes.
5. Using forceps, transfer the roots to a well containing distilled water. Removed pink-colored water and replace with new
water numerous times until the water is light pink.
6. Place roots on a paper towel and trim to 3-5mm in length – BE SURE TO KEEP THE TIP!
7. Place the 4-5 tips on the center of a glass microscope slide.
8. Place a coverslip over the tips and squash the roots by placing a second slide perpendicular to the first and pressing down
on its center.
Part B: Comparing Phases of Mitosis
9. Using low magnification on the microscope, focus on the root cells. Switch to medium power or high power and adjust the
light as necessary to easily visualize the inside of the onion root cells.
10. Study all of the squashed tissue to locate cells in each stage of the cell cycle. Note: All stages of mitosis may not be present
within a single field of view.
11. Within the field of view, count the cells in interphase and the cells in mitosis. Record your data in Table 1.
12. Repeat the counts for the remaining root tips.
13. Repeat steps 2-12 using the root tips from onion bulb B.
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Table 1: Number of Cells in Interphase and Mitosis (Group Data)
Number of Cells
Mitotic
Interphase
Total
Root 1
Root 2
Root 3
Root 4
Root 5
Total
Root 1
Root 2
Root 3
Root 4
Root 5
Total
Onion
Bulb A
Onion
Bulb B
Table 2: Number of Cells in Interphase and Mitosis (Class Data)
Onion Bulb A
Onion Bulb B
Interphase Cells
Mitotic Cells
Total
Table 3: Percentage of Cells in Each Phase (Class Data)
% of Cells
Group
Interphase
Mitotic
Control
Treated
Part C: Calculation of Chi-Square
1. Enter the class data number of treated onion bulb cells in interphase and mitosis as observed (o) in Table 4.
2. To determine the expected numbers (e), multiply the percentages of untreated cells in each phase by the total number of
cells in the treated group. Record these values in Table 3.
3. Calculate the chi-square (x2) value for the test.
4. Complete Table 4 using the AP Biology Equations and Formulas sheet.
5. Answer the analysis questions.
Table 4: Calculation of Chi-Square X2 =  (o-e)2/e
# observed
# expected
(o-e)
(o-e)2
(o-e)2/e
Interphase
Mitosis
X2 =
Degrees of Freedom =
Accept or Reject Null Hypothesis?
Explanation:
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p=
Critical Value =
Analysis Questions:
1. What was the importance of collecting the class data? How might your chi-square calculation have differed if you had only
used group data?
2. Was there a significant difference between the groups? Which group did you determine to be the lectin exposed? Which
group was not exposed to lectin?
3. Did the lectin increase or decrease the number of cells in mitosis?
4. What other experiments could you perform to verify your findings?
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?
6. What other way could you determine how fast the rate of mitosis is occurring in root tips?
Part 3: Loss of Cell Cycle Control in Cancer
Pre-Lab Questions:
1. How is the cell cycle controlled in normal cells?
2. What are cyclins and cyclin-dependent kinases? What do these proteins do in a cell?
3. How are normal cells and cancer cells different from each other?
4. What are the main causes of cancer?
5. What makes some genes responsible for an increased risk of certain cancers?
6. Do you think that the chromosomes might be different between normal and cancer cells? Explain.
Materials:
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3 karyotype sheets: 1 normal, 1 HeLa, 1 chronic myelogenous leukemia (Philadelphia chromosome)
Scissors
Glue
3 karyotype boards
Procedure:
1. Carefully cut out the individual chromosomes on the karyotype sheet.
2. Arrange the chromosomes in order of decreasing size, from largest to smallest on a karyotype board.
3. Use the size, centromere location, and banding pattern on each chromosome to match homologous pairs of chromosomes.
(Refer to the table below for centromere locations on human chromosomes)
4. Glue the chromosomes to the karyotype board and label it.
Analysis Questions:
1. Compare each unknown karyotype with the normal karyotype. Note the gender of the person and the type of error.
2. Research each of the chromosomal errors present and write a brief summary of the cancer caused by the error present.
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Part 4: Modeling Meiosis
Pre-Lab Questions:
1. How is meiosis important to a sexually reproducing organism?
2. What would happen if eggs and sperm were produced by mitosis instead of meiosis?
3. How can crossing over between homologous chromosomes be detected?
4. How do meiosis and fertilization affect genetic diversity and evolution?
5. How do sexually reproducing organisms produce gametes from diploid cells?
6. How does the process increase gamete diversity?
7. What are the outcomes from independent assortment and crossing over?
8. How does the distance between two genes or a gene and a centromere affect crossing over frequencies?
Materials:



4 long pipe cleaners: 2 long pipe cleaners of one color and 2 long pipe cleaners of a different color.
4 short pipe cleaners: 2 short pipe cleaners of the one color 2 short pipe cleaners of a different color.
8 beads of the same color.
Procedure:
**As you work with the pipe cleaners, use colored pencils to diagram each stage in the circles of the diagram. You may also
choose to take pictures of each stage as you model through the cell cycle. These drawings or pictures need to be included in
what you turn in for AP Lab 7.
1.
2.
3.
4.
5.
6.
Imagine that a cell with 4 chromosomes (2 long each a different color, and 2 short each a different color) is in Interphase G1.
Set up the pipe cleaners and beads to represent Interphase G1.
Squeeze an identical pipe cleaner through each centromere to represent the result of replication in Interphase S.
Continue through G2
Modify the configuration of pipe cleaners to illustrate:
o prophase I
o metaphase I
o anaphase I (add centromeres as required)
o telophase I
o cytokinesis I
Modify the configuration of pipe cleaners to illustrate:
o prophase II
o metaphase II
o anaphase II (add centromeres as required)
o telophase II
o cytokinesis II
Some examples of pipe cleaners and beads as chromosomes:
one chromosome,
single stranded in
Interphase G1
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one chromosome,
double stranded after
Interphase S
a pair of homologous
chromosomes,
double stranded
Interphase G2
Part 5: Meiosis and Crossing Over in Sordaria
The fungus Sordaria fimicola exchanges genetic material when two mycelia meet and fuse. The resulting zygote undergoes meiosis
to produce asci; each ascus contains eight haploid spores. A single gene determines the spore color.
A cross was made between wild type (+; black) and tan (tn) strains. The resulting zygote produces either parental type asci, which
have four black and four tan spores in a row (4:4 pattern) or recombinant asci, which do not have this pattern.
Materials:
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microscope
Sordaria prepared slides and/or pictures
Procedure:
1. Obtain a slide and/or pictures of Sordaria asci.
2. Count at least 50 asci, and score them as either parental or recombinant (crossing over).
3. Enter the data for your group and the class in Table 4 and make the calculations. One map unit equals one recombinant per
100 total events. The percentage of asci showing crossing over divided by 2 equals the map units in this activity. This is
done because each spore produced by meiosis undergoes a mitotic division.
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Table 5: Analysis of Sordaria Results (Group and Class Data)
Number of Asci
Number of Asci
Showing Crossover
Showing 4:4
2:2:2:2 or 2:4:2
(parental) pattern
(recombinant)
pattern
Total # of Asci
% of Asci Showing
Crossover Divided
by 2
Gene to Centromere
Distance (map
units)
Group Data
Class Data
Analysis Questions:
1. Why did you divide the percentage of asci showing crossover (recombinant) by 2?
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?
3. How could one account for any disparities between the class data and the published data?
4. Illustrate what happened during meiosis to produce the results you found.
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.
6. Do you think they cell cycle described for mitosis could be applied to meiosis as well? Explain your answer.
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