Meiosis Lab
Asexual reproduction in multicellular organisms is accomplished through mitosis. Sexual
reproduction requires a special form of cell division, meiosis. The process of meiosis reduces
the number of chromosomes in the cell by half to make haploid (1N) cells. This is called
reduction division. In this way, gametes (egg and sperm) are formed that have half the number
of chromosomes as somatic (body) cells. This is important because during fertilization (the
joining of egg and sperm to make a zygote) the number of chromosomes is doubled, returning
to the cell to its normal diploid form, 2N, (half from the egg and half from the sperm).
Part 1: Simulation of Meiosis
In this lab you will study the process of meiosis using chromosome simulation kits. Your kit
should contain two different color strands of beads. One strand of beads represents the
chromosome given by one parent. The other color strand is the chromosome given by the other
parent. Together they are known as a homologous chromosome pair. The second strand of
each color represents the chromosome’s duplicate following the “S” phase of interphase. The
magnet in the middle of each strand is the chromosome’s centromere.
Use the beads provided to demonstrate your understanding of meiosis.
 Begin with a cell with 4 chromosomes (2 pairs of homologous chromosomes making a
diploid number of 4 (2N).
 Draw a large circle on a piece of notebook paper. (This will represent the cell).
 Move the chromosomes around the cell as you work your way through the phases of
meiosis.
 As you complete each phase, draw the placement of the chromosomes in each of the
circles in Figure 1.
o Use color to represent the different chromosomes.
o Include centrioles, spindle fibers and chromosomes in your drawing.
 Label one of each in your drawings.
o Be sure to include at least two areas of crossing over in your simulation.
 When you have completed the drawings, bring them with you when you demonstrate
your understanding of meiosis by repeating the simulation for me.
o If several people are waiting, go on to Lab 2 and come back later.
Lab 2: Sordaria Tetrad
The following are pictures of the fungus Sordaria. There are two strains, black and tan. The
long strands are called asci. Meiosis occurs during the formation of the asci. Inside the asci are
the spores of the fungus. Look at a single ascus and you will see that it contains both black and
tan spores. The ascus that have a spore pattern of 4 tan to 4 black were produced from cells in
which no crossing over occurred. These asci are called non-recombinants. Other asci contain
black and tan spores that are found in 2:4:2 patterns or 2:2:2:2 patterns. Only these asci result
from cells in which crossing over has occurred. They are called recombinants. Because the
recombinant patterns result only from crossing over, how often they occur is a measure of how
often crossing over occurs.
The formation of non-recombinant asci can be drawn quite easily. Two homologous
chromosomes line up during Metaphase of Meiosis I. The two chromatids of one chromosome
each carry the gene for tan spore color (tn) while the other two chromatids carry the gene for
black core spore color (+).
The first meiotic division (MI) results in two cells, each containing one spore type (either tan or
black). In this division the spores are being segregated (separated).
The spores then undergo Meiosis II. Each cell is haploid at the end of Meiosis I. At the end of
Meiosis II, the cells undergo mitosis, creating four haploid cells.
Figure 2:
Lab 2: Sordaria Tetrad
1. Your task is to look at the Sordaria in the photos provided and count the number of asci
that show recombinant and non-recombinant patterns.
a. Count a total of 50 asci, recording your observations in Table 1.
b. Calculate the percentage of crossing over that occurs.
c. Collect total asci data from the class.
d. Calculate the percentage of crossing using the class data.
Table 1
Number
of 4:4
Number of Asci
Showing
Crossing Over
Total Asci
(Your data)
% Asci
Total
% Asci
Map Units
Showing
Asci
Showing
(Class data)
Crossing
(Class data)
Crossing
Over
Over
(Your data)
(Class data)
The frequency of crossing over appears to be controlled largely by the distance between genes,
or in the case of Sordaria, between the gene for spore color and the centromere. The probability
of crossing over occurring between two genes on the same chromosome (linked genes)
increases as the distance between those genes also increases.
A map unit is the distance between two linked genes. The number of map units between two
genes or between a gene and the centromere is equal to the percentage of recombinants. Due to
the relationship between distance and crossover frequency, we use the map unit.
2. Using the data in Table 1, determine the distance between the gene for spore color and
the centromere.
a. Calculate the percentage of crossovers in the class data. Record this number in
Table 1.
b. To calculate the map distance, divide the percentage of crossovers by 2. (This is
done because only half the spores in each ascus are the result of crossing over).
Record your answer in the final column of Table 1.
3. On a separate sheet of unlined paper, draw a pair of chromosomes that pass through
meiosis I and II and undergo crossing over, ending up with a 2:4:2 spore arrangement.
(Hint: Refer to Figure 2 as a starting point).
Summary Questions
1. Compare Meiosis and Mitosis by completing the chart:
Mitosis
Meiosis
Chromosome number of parent cells
(haploid/diploid)
Number of times DNA replicates
Phase(s) when DNA replication occurs
Number of time the nucleus divides
Number of daughter cells produced
Chromosome number in daughter cells
(haploid/diploid)
Does crossing over occur?
Phase when crossing over occurs
Process purpose/function
2. List 3 major differences between the events of mitosis and meiosis. (These should be
different than what you wrote in the chart above).
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3. How are meiosis I and meiosis II different?
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4. Why is meiosis important for sexual reproduction?
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