Kayla Romeo
October 14, 2011
Lab 3
Lab 3: Mitosis and Meiosis
Objectives



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Compare and contrast mitosis/meiosis in animal and plant cells
Calculate how long it takes for each of the cell cycle stages to take place
Identify the stages of mitosis in a plant cell
Observe crossing over and independent assortment in meiosis
Background
There are two types of cellular division, mitosis and meiosis. Mitosis is used growth and
repair in somatic cells (body cells) and meiosis is used to produce gametes or spores.
We will be observing mitosis in cells in the root tip of an onion. This is the most active
place for cell division. In mitosis, interphase is the first step. DNA is duplicated here and you can
see a distinct nucleus. The next step is prophase, which is when the nuclear envelope breaks and
releases the chromatin that was condensed to create chromosomes and the spindle starts to form.
In metaphase, the spindle attaches to the centromere of each chromosome and they line up across
the middle of the cell. During anaphase, the chromatids are separated and pulled to opposite ends
of the cell. The last stage is telophase which is when the nuclear envelope is reformed and
chromosomes gradually uncoil. Cytokinesis is when a cleavage furrow will form and the two
daughter cells will separate.
In Meiosis, there are two nuclear divisions which result in the production of four haploid
gametes. Crossing-over in meiosis allows for genetic variation. Interphase replicates the DNA,
just like mitosis. Prophase I is when homologous chromosomes join to form a tetrad and synapsis
begins. Crossing-over occurs in prophase. In Metaphase I, the tetrads are lined up in the middle
of the cell while Anaphase I separates the tetrads and pulls them to opposite ends of the cell.
Telophase I prepares the cell for its second division. During meiosis II, interphase does not
replicate the DNA but everything else occurs as usual. The only change is the number of
chromosomes.
We will be studying crossing over in Sordaria fimicola. Sordaria form a set of eight
ascospores, which is called an ascus. We observe crossing over in the arrangement and color of
the asci. If there are 4 tan ascospores and 4 black ascospores in a row, then there has been no
crossing over. If the asci have black and tan ascospores in sets of two or two black ascospores
and four tan in the middle, then crossing-over has occurred.
Procedure
Exercise 3A.1
Kayla Romeo
October 14, 2011
Lab 3
1. Examine pictures of onion root tips and study each individual cell. Sketch and
label the cell in the boxes provided.
Exercise 3A.2
1. Examine the pictures of onion root tips and determine which phase of the cell
cycle each cell is in. Make sure to count at least 200 cells.
2. Record your data in Table 3.1.
3. Calculate the percentage of cells in each phase and record in Table 3.1.
Exercise 3B.1
1. Read the provided information.
Exercise 3B.2
1. Observe the card you have been given and use it to complete Table 3.3. Make
sure to count at least 50 hybrid asci.
Questions
Exercise 3A.1
1. Mitosis leads to two daughter cells through a series of phases. There is
interphase, prophase, metaphase, anaphase, and telophase/cytokinesis. All of
these processes lead up to the division of a cell. In interphase, DNA replication is
occurring and there are two checkpoints.
2. Mitosis differs in plant and animals cells because of their process of cytokinesis.
In animals, microfilaments pinch the cell in a cleavage furrow during cytokinesis.
In plants, cytokinesis involves the formation of a cell plate, which is a fusion of
vesicles that forms new plasma membranes and new cell walls between the cells.
3. The centrosome gives rise to microtubules (spindle). It is important in mitosis
because the centrosomes contain pairs of centrioles.
Exercise 3A.2
1. There would be a very little amount of cells dividing because many of the cells
would still be in interphase.
2. Interphase seems to be the longest phase, but that takes place prior to mitosis.
Therefore, prophase would be the longest stage.
3. See Graph in Data
Exercise 3B.1
Kayla Romeo
October 14, 2011
Lab 3
1. The nucleus is divided once in mitosis and twice in meiosis. Two identical
daughter cells are produced in mitosis and four genetically different cells are
produced in meiosis. The third thing is that crossing over only occurs in meiosis.
2. See Table 3.2.
3. Meiosis I starts with a tetrad and separates the homologous pairs while meiosis II
separates two sister chromatids into haploids.
4. Oogenesis produces egg cells and spermatogenesis produces sperm cells.
5. In meiosis, the chromosome number is reduced to n so that it can be fertilized.
Crossing-over occurs in meiosis also which allows for genetic variation.
Exercise 3B.2
1. See Table 3.3.
2. See Figure 3.1.
Data
Exercise 3A.1
Interphase
Prophase
Metaphase
Anaphase
Telophase
Exercise 3A.2
Table 3.1
Field 1
Interphase
69
Prophase
11
Metaphase 0
Anaphase
6
Telophase
11
Total Cells Counted=256
Number of Cells
Field 2
Field 3
Total
57
15
7
5
5
172
36
12
13
23
46
10
5
2
7
Pie Chart: Time a cell spends in each phase
% of Total
Cells
Counted
67%
14%
4.5%
5%
9.5%
Time in
Each Stage
964.8
201.6
64.8
72.0
136.8
Kayla Romeo
October 14, 2011
Lab 3
Interphase
Prophase
Metaphase
Anaphase
Telophase
Exercise 3B.1
Table 3.2
Chromosome Number of
Parent Cells
Number of DNA Replications
Number of Divisions
Number of Daughter Cells
Produced
Chromosome Number of
Daughter Cells
Purpose/Function
Mitosis
Diploid (2n)
Meiosis
Diploid (2n)
Once
1
2
Once
2
4
Diploid (2n)
Haploid (n)
Growth and repair
Production of gametes
Exercise 3B.2
Table 3.3
Number of 4:4
Number of Asci
Showing
Crossover
Total Asci
% Asci Showing
Crossover
Divided by 2
13
42
55
38%
Figure 3.1
Gene to
Centromere
Distance (map
units)
38
Kayla Romeo
October 14, 2011
Lab 3
Conclusion
We observed and timed mitosis in exercise 3A. We found out that the order in terms of
time for the cell stages went from prophase being the longest, then telophase, anaphase, and
metaphase being the shortest. We simulated meiosis in exercise 3B and then observed crossing
over in Sordaria. The gene to centromere distance was 38 map units in the Sordaria.