AP Biology
Name: ___________________________
Meiosis Pre-Lab: Answer the questions below after reading Campbell text sections 13.2 – 13.4,
reading and annotating the attached lab, and before coming to class on lab day!
1. Briefly distinguish between the events of meiosis I and meiosis II?
2. During which phase of meiosis do synapsis and crossing over occur?
3. Define synapsis:
4. Define crossing over:
5. What is a tetrad (using the terms sister chromatids & homologous chromosomes)? When do
tetrads exist?
6. During which phase of meiosis do sister chromatids separate?
7. What is the scientific name of the model organism used to demonstrate crossing over?
8. How many TOTAL ascospores are present when mitosis follows meiosis for the ascomycete
fungus?
9. What pattern(s) of ascospores would you see when there is no crossing over? (draw or use
numbers)
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Laboratory 18: Meiosis
UNIT 9 CONTENT and PROCESS TARGETS:
I: Describe the process of meiosis. Include the words meiosis I, meiosis II, crossing over, tetrad, sister
chromatid, haploid, diploid and homologous pairs.
II: Compare and contrast sexual and asexual reproduction. Describe examples of organisms that perform
each type and the impact on genetic diversity.
III: Describe how segregation, crossing over, and random fertilization produce genetic variation in a
population.
B: Use bead chromosomes to model the process of meiosis including crossing over.
Meiosis reduces the chromosome number by one half during the formation of gametes in animals
and during the formation of spores in plants and fungi. During the meiosis, also called "reduction
division," the chromosomal pairs are partitioned so that each gamete (or spore) contains one of each
chromosomal pair (haploid). When haploid gametes unite during fertilization, the resulting zygote is
diploid, having received one chromosome of each pair from each parent.
Meiosis involves two successive nuclear divisions that produce four haploid cells. The first division
(meiosis I) is the reduction division and produces two haploid daughter cells. The second division
(meiosis II) separates the sister chromatids and produces a total of four haploid daughter cells.
Meiosis produces daughter cells that are genetically different from the original parent cell and
therefore increases genetic variation within a sexually reproducing population. Each diploid cell
undergoing meiosis can produce 2n different chromosomal combinations, where n is the haploid number.
In humans, n = 23. Therefore, humans can produce 223 or over eight million different combinations of
chromosomes for a gamete.
In addition, meiosis increases variation because, during meiosis I, each pair of chromosomes unite in
a process known as synapsis. Chromatids of homologous chromosomes may exchange parts in a process
called crossing over. You can estimate the relative distance between two genes on a given chromosome
by calculating the percentage of crossing over that takes place between them.
Part A: Modeling Meiosis using Bead Chromosomes
Procedure
1.
In this exercise you will study the process of meiosis using chromosome simulation kits. Your kit
contains 4 different colored chromosomes. Two colors represent the chromosome contribution of
the female parent, and the other two colors represent the chromosome contribution of the male
parent. The two different colored chromosomes that are the same length, one from mom and one
from dad, are homologous chromosomes.
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2.
Interphase. Place one chromosome from each parent that are the same size near the center of your
work area. You should have four chromatids, and each is a different color. (Recall that chromosomes
at this stage would exist as diffuse chromatin and not as visible structures.) DNA synthesis occurs
during S phase of interphase and each chromosome, originally composed of one strand, is now made
up of two strands, or sister chromatids, joined together at the centromere region. Simulate DNA
replication by bringing the magnetic centromere region of the second red strand in contact with the
centromere regions of the first red strand. Do the same with its homolog, the yellow strand.
3.
Prophase I. Homologous chromosomes come together and synapse along their entire length. The
pairing of homologous chromosomes represents the first big difference between mitosis and
meiosis. A tetrad consisting of four chromatids is formed. Entwine the two chromosomes to
simulate synapsis and then simulate the process of crossing over. Popping the beads apart on a sister
chromatid and doing the same a non-sister chromatid of the homologous chromosome to simulate
crossing over and the exchange of genetic information. Proceed through prophase I of meiosis and
note how crossing over results in recombination of genetic information.
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4.
Metaphase I. The crossed-over tetrads line up in the center of the cell. Position the chromosomes
near the middle of the cell
5.
Anaphase I. During anaphase, the homologous chromosomes separate and are “pulled” to opposite
sides of the cell. This represents a second significant difference between the events of mitosis and
meiosis.
6.
Telophase I. Place each chromosome at opposite sides of the cell. Centriole duplication takes place
at the end of telophase in preparation for the next division. Formation of a nuclear envelope and
division of the cytoplasm (cytokinesis) often occur at this time to produce two cells, but this is not
always the case. Notice that each chromosome within the two daughter cells still consists of two
chromatids.
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7.
A second meiotic division is necessary to separate the chromatids of the chromosomes in the two
daughter cells formed by this division. This will reduce the amount of DNA to one double-helical
strand per chromosome. This second division is called meiosis II. It resembles mitosis except that
only one homolog from each homologous pair of chromosomes is present in each daughter cell
undergoing meiosis II.
8.
Interphase II (Interkinesis). The amount of time spent “at rest” following telophase I depends on
the type of organism, the rate of formation of new nuclear envelopes, and the degree of
chromosomal uncoiling. Because this “interphase” that occurs between meiosis I and meiosis II
does not necessarily resemble the interphase consisting of G1, S, G2 segments of the cell cycle, it is
often given a different name - interkinesis. DNA replication does not occur during interkinesis.
This represents a third difference between mitosis and meiosis.
9.
Prophase II. Replicated centrioles separate and move to opposite sides of the chromosome groups.
10.
Metaphase II. Orient the chromosomes so they are centered in the middle of each daughter cell.
11.
Anaphase II. The centromere regions of the chromatids now appear to be separate.
Separate the chromatids of the chromosomes and pull the daughter chromosomes toward the
opposite sides of each daughter cell. Now that each chromatid has its own visible separate
centromere region, it can be called a chromosome.
12.
Telophase II. Place the chromosomes at
opposite ends of the dividing cell. At this
time a nuclear envelope forms and the
cytoplasm divides. The picture on the right
indicates one possible outcome of meiosis.
Are any of the gametes the same? Imagine the
diversity one can achieve if 2N = 46!
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Part B: Meiosis of Sordaria
Sordaria fimicola is an ascomycete fungus that can is an excellent model organism used to
demonstrate the results of crossing over during meiosis. Sordaria is a haploid fungus for most of its life
cycle. The fungus becomes diploid only after the fusion of two haploid mycelia (filament-like groups of
cells) of two different fungus strains. The fusion of two different types of haploid nuclei forms a diploid
nucleus. The diploid nucleus then undergoes meiosis to produce haploid spores. The haploid spores grow
by mitosis to create new haploid fungi mycelium.
Meiosis followed by mitosis, in Sordaria results in the formation of eight haploid ascospores
contained within a sac called an ascus (plural, asci). Many asci are contained within a fruiting body
called a perithecium (you think the perithecium is a mushroom). When the ascospores are mature, the
ascus ruptures, releasing the haploid spores. Each spore develops into a new haploid fungus is
environmental conditions are favorable for fungus growth. The life cycle of Sordaria fimicola is shown in
Figure 18.1:
FIGURE 18.1: Life Cycle of Sordaria
To observe crossing over in Sordaria, one must make hybrids between wild-type and mutant strains.
Wild-type Sordaria have black ascospores (+). One mutant strain has tan spores (tn). When mycelia of
these two different strains come together and undergo meiosis, the asci that develop will contain four
black ascospores and four tan ascospores (4:4 pattern). The arrangement of the spores directly reflects
whether or not crossing over has occurred. In figure 18.2, no crossing over has occurred.
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FIGURE 18.2: Sordaria with no crossing over events
If crossing over has occurred, the pattern of the ascospores will be changed. The patterns may be a
2:2:2:2 or 2:4:2. Figure 18.3 shows the result of a crossing over between the centromere of the
chromosome and the gene for ascospore color.
FIGURE 18.3: Patterns of ascospores after crossing over event(s)
Part B: Counting Sordaria Ascospores Procedure
1. Use the prepared slides of Sordaria to look for the ascospores.
2. View several different Sordaria cards with pictures of asci. Locate a group of hybrid asci (those
containing both tan and black ascospores). Count at least 50 hybrid asci and enter your data into
Table 18.2.
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Part B: Results
TABLE 18.2:
4:4 Asci
2:2:2:2 OR 2:4:2 Asci
Total # Asci Counted
Group Data
Class Data
Meiosis Lab Analysis Questions: Answer using complete thoughts and sentences on
separate paper (typed responses preferred!)
1. If a cell of an organism had 46 chromosomes before meiosis, how many chromosomes would exist in
each daughter cell nucleus after meiosis?
2. Suppose one sister chromatid of a chromosome has the gene represented by a capital letter “H”. What
gene letter exists on the other, sister chromatid? (Assume crossing over has not taken place).
3. Draw the chromosomes of a cell, with a diploid number of 6 (2n = 6) and use different colors to
distinguish between each homologous pair:
a. as the chromosomes would appear during G1 of the cell cycle (if you could see individual
chromosomes).
b. as the chromosomes would appear before the M phase of the cell cycle (end of G2).
c. after the cell completes meiosis I (draw the correct number of daughter cells as well!).
d. after the cell completes meiosis II (draw the correct number of daughter cells as well!).
4. Suppose that two alleles on one homologous chromosome are A and B, and the other homologous
chromosome has alleles a and b. How many different genetic types of gametes would be produced
without crossing over? What are those types? If crossing over were to occur, how many different types
of gametes could occur? List them.
5. Name and describe two differences between mitosis and meiosis.
6. Name and describe two similarities between mitosis and meiosis.
7. From an evolutionary viewpoint, explain the significance of random fertilization after meiosis. (DO
NOT ANSWER: survive and reproduce! THINK and reference your natural selection flow chart)
8. Name and discuss two ways that meiosis creates genetic variation in organisms who sexually
reproduce.
9. Using your data in Table 18.2, determine the distance between the gene for spore color and the
centromere. Calculate the percent of crossovers by dividing the number of crossover asci
(2:2:2:2 or 2:4:2) by the total number of asci and multiplying this answer X 100.
10. Determine the genetic map distance between the centromere and the spore color gene by
dividing the percent of crossover asci by 2. The percent of crossover asci is divided by 2
because only half of the spores in each ascus are the result of crossing over. (The other half are
created by mitosis- see figures 18.2 and 18.3).
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