MODELING MEIOSIS
Problem
How does meiosis increase genetic variation?
Introduction
Most cells in organisms that reproduce sexually are diploid. They have two sets of chromosomes
and two complete sets of genes. Gametes are the exception. Gametes are the cells that combine
during sexual reproduction. In animals, these cells are called sperm and eggs. Gametes are haploid
cells with only one set of chromosomes. Meiosis is the process in which haploid cells form from
diploid cells.
In this lab, you will model the steps in meiosis. You will make drawings of your models. You will
also identify points in the process that can lead to greater genetic variation.
Materials
40 red pop-it beads
colored pencils
40 white pop-it beads
scissors
12 pink pop-it beads
1 sheet of 8 ½ “ x 11” paper
Procedure
The diploid cell in your model will have two pairs of homologous chromosomes. In order to keep
track of the pairs, you will make one pair longer than the other. The beads will represent genes.
Part A: Construct the chromosomes for a 2N=6 individual
1. Use ten red beads and a pink centromere bead to construct the long
chromosome as shown in figure 1.
2. Use ten white beads and a pink centromere bead to construct a second
chromosome of the long pair.
3. Use six red beads and a pink centromere bead to construct the medium
chromosome as shown in figure 1.
4. Use six white beads and a pink centromere bead to construct a second
chromosome of the medium pair.
5. Use four red beads and a pink centromere bead to construct the short
chromosome as shown in figure 1.
6. Use four white beads and a pink centromere bead to construct a second
chromosome of the short pair.
7. The red beads will represent DNA from a male source. The white beads
will represent DNA from a female source. The paper the chromosomes
sit on will represent the cell.
8. Randomly position the six chromosomes in the middle of your paper cell.
Draw how they look in space A (page 4)of your data sheet. Label the
centromere on one of the chromosome drawings.
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Figure 1
Long Medium Short
MODELING MEIOSIS
Figure 2
Interphase: Just before meiosis begins, the chromosomes are replicated.
Example of a duplicated
medium chromosome
9. Model the replication of all six chromosomes by making an
identical copy of each. Attach the centromeres of matching sister
chromatids to one another as shown in figure 2.
10. Randomly position the replicated chromosomes in the middle of your
paper. Draw how they look in space B of your data sheet. Label the sister
chromatids on one of the chromosome drawings.
Part B: Meiosis I
During meiosis I, the cell divides into two haploid daughter cells.
11. Pair up the chromosomes to form tetrads as shown in figure 3.
12. Use the longer tetrad to model crossing-over of non-sister
chromatids as shown in figure 4.
13. Line up the tetrads in the center of your paper cell to simulate
Metaphase I. Draw how they look in space C. Label a pair of
non-sister chromatids and the area of crossing-over. In addition,
draw in some spindle fibers and label one of them.
14. Model what happens to the chromosomes during anaphase I.
As you separate the long chromosomes from its tetrad, have
the non-sister chromatids exchange DNA at the point where
they cross-over.
15. Divide the paper cell into two daughter cells. Use space D to make a
drawing of the result.
Figure 3
Example of a medium
chromosome tetrad
Part C: Meiosis II
During meiosis II, the daughter cells divide again.
16. Line up the chromosomes at the center of the first cell. Separate
the chromatids in each chromosome and move them to opposite
sides of the cell.
17. Repeat step 17 for the second cell.
18. Divide each cell into two daughter cells. Use space E to make a
drawing of the four haploid cells.
Part D: Probability
19. In the 2N = 6 diploid cell you just modeled, there would only be eight
different gamete genotypes that could form if no crossing-over took
place. In space F, draw and color all eight possible gamete genotypes.
20. Take the chromosomes apart and place them back into the plastic
baggie. Answer the questions on page 3.
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Figure 4
Long Tetrad
Name ____________________________________________
MODELING MEIOSIS
Date ___________ Period _________ Seat _________
Lab Questions
1. Control Variables Why must you use the same number of beads when you construct the
second chromosome in Step 2?
2. Infer Why is the longer chromosome pair used to model crossing-over?
3. Calculate A diploid cell (2N) has two pairs of homologous chromosomes. How many
different combinations of chromosomes could there be in the gametes?
4. Relate Cause and Effect How does crossing-over increase variation in genes?
5. Use Models Suppose no crossing-over takes place. Use step 14 to explain why meiosis will
still increase genetic variation.
6. Compare and Contrast Compare your models of the four haploid cells with those of other
Groups. Describe and explain any variation that you observe.
7. Calculate What would happen to the possible variation in gametes if the number of
chromosome pairs increased from three to four? What if the number increased again from
four to five?
8. Apply Concepts How can independent assortment help explain genetic diversity in humans?
Hint: How many pairs of chromosomes are in a human diploid cell?
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Name ____________________________________________
MODELING MEIOSIS
Date ___________ Period _________ Seat _________
A: Interphase, 2N = 6
B: Interphase, 2N = 6
C: Metaphase I, 2N = 6
D: Post Telophase I, N = 3
F: Gamete probability, 2N = 23 = 8
E: Meiosis II, N = 3
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