Bio 160: Activity
Name: ___________________________
Meiosis & Genetic
Variability
OBJECTIVES:
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List and explain the principal events of the stages of meiosis.
Define and explain the following terms: diploid, haploid, homologous chromosomes, alleles,
synapsis, tetrad, crossing over
Explain and understand the difference between the first and second meiotic divisions
List and explain the similarities and differences between meiosis and mitosis
To understand how crossing over contributes to genetic variability
INTRODUCTION
Meiosis consists of two nuclear divisions (meiosis I and meiosis II) and results in the production of
four daughter nuclei, each of which contains only half the number of chromosomes (and half the
amount of DNA) characteristic of the parental cells.
During meiotic reduction of the chromosome number to half, however, chromosomes are not
just divided into two sets at random. In diploid organisms, chromosomes occur in matched pairs
called homologous chromosomes. These are identical in size, shape, location of their centromeres,
and types of genes present. One member of each homologous pair is contributed by the “male”
parent and one is contributed by the “female” parent during the process of sexual reproduction.
Meiosis provides as precise a mechanism as possible for separating these homologous chromosomes
so that daughter cells carry one member, or homologue, of each chromosomal pair.
Meiosis also serves as a mechanism for increasing genetic diversity. One way that genetic
diversity is increased is by crossing over. This is when two homologous chromosomes exchange
corresponding segments.
In this exercise, we will be following the fate of two different chromosomes (#14 & #8) in a
diploid cell as they go through the process of meiosis. These are the same chromosomes that you put
together for the mitosis exercise. Step through the process of meiosis, as described below. As one
team member works their way through the exercise, the other team member should be making a
labeled diagram of each stage. At the end, reverse the roles so each team member has a chance to
walk through the steps.
PROCEDURES
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Begin by drawing a large circle representing a cell on the pad. In the cell draw a nucleus
large enough to contain the chromosomes you have made.
Place the homologues for chromosome #14 (one red and one yellow) separately in the
nucleus.
Place the homologues for chromosome #8 (one red and one yellow) separately in the nucleus.
Interphase 1:
• Chromosomes duplicated - For each homologue, attach its copy (sister chromatid) (e.g., red
with 14 beads to red with 14 beads) at the centromere.
Prophase 1:
• Chromosomes condense and become visible – (yes they are already visible, but note that it
doesn’t occur until this stage).
Biol 160: Activity
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Homologous chromosomes stuck together in pairs (4 sister chromatids - called a tetrad) –
Attach duplicated chromosome from mother (red) to duplicated chromosome from father
(yellow) at centromeres.
Crossing over occurs – For chromosome #14, remove a segment of red beads from one
homologous chromosome and exchange it with the same number of yellow beads from the
other homologous chromosome. Do the same with chromosome #8.
Nuclear envelope breaks down – Erase it from the pad.
Centrosomes start to move towards poles of cells – Move pennies to represent centrosomes
(one heading north, one south).
Mitotic spindle forms & attaches to chromosomes at the centromere - For each tetrad, the
microtubules from the “northern” centrosome will attach to one of the duplicated
chromosomes, and the microtubules from the “southern” centrosome will attach to the other
duplicated chromosome. Draw lines representing this.
The tetrads begin moving towards the metaphase plate.
Metaphase 1:
• Tetrads will be lined up along the metaphase plate – Move the tetrads until they form a line
along the midline of the cell.
• Each duplicated pair of homologous chromosome will be attached to a microtubule – Erase
and redraw the microtubule lines reaching from the centrosomes to the centromere of each
duplicated chromosome. Again, the “northern” centrosome should have a microtubule
attached to the duplicated chromosome closest to it, and the “southern” centrosome should
have a microtubule attached to the duplicated chromosome closest to it.
Anaphase 1:
• Pairs of duplicated chromosomes separate and start to move towards “north” and “south”
poles of cell – Separate the tetrad by pulling the centromeres of the pairs of homologous
chromosomes apart from each other.
• Cell elongates as microtubules push poles of cell further apart.
Telophase 1:
• Duplicated homologous chromosomes now at separate ends of cells – Move them to the poles
of the cell.
• Nuclear envelope reforms - Draw a nuclear envelope around the chromosomes at each pole.
• Spindle goes away – Erase it.
Cytokinesis 1:
• A division occurs along the midline of the cell separating it into two cells – Draw a line
separating the two cells.
Using the drawings your teammate made, compare the cell at the end of Interphase 1 with the cells
you end up with at the end of Cytokinesis 1.
Starting with one of the cells you end up with after the first round of meiosis, continue with the second
round of meiosis.
Prophase 2:
• Centrosomes start to move towards poles of cells – Move pennies to represent centrosomes
(one heading north, one south).
• Nuclear envelope breaks down – Erase it from the pad.
• Mitotic spindle starts to form and to attach to chromosomes at the centromere - For each
chromosome, the microtubules from the “northern” centrosome will attach to one sister
chromatid, and the microtubules from the “southern” centrosome will attach to the other sister
chromatid. Draw lines representing this.
• The chromosomes will begin moving towards the midline (equator) of the cell (called the
metaphase plate).
Biol 160: Activity
Metaphase 2:
• Chromosomes will be lined up individually along the metaphase plate – Move the
chromosomes until they form a single file line along the midline of the cell.
• Each sister chromatid will be attached to a microtubule – Erase and redraw the microtubule
lines reaching from the centrosomes to the centromere of each sister chromatid. Again, the
“northern” centrosome should have a microtubule attached to the sister chromatid closest to
it, and the “southern” centrosome should have a microtubule attached to the sister chromatid
closest to it.
Anaphase 2:
• Sister chromatids of each chromosome separate and start to move towards “north” and
“south” poles of cell – Separate the 2 sister chromosomes by pulling the centromeres apart
from each other.
• Cell elongates as microtubules push poles of cell further apart.
Telophase 2:
• Each sister chromatid (now separate chromosomes) is now at separate ends of cells – Move
them to the poles of the cell.
• Nuclear envelope reforms - Draw a nuclear envelope around the chromosomes at each pole.
• Spindle goes away – Erase it.
Cytokinesis 2:
• A division occurs along the midline of the cell separating it into two cells – Draw a line
separating the two cells.
Using the drawings your teammate made, compare the cell at the end of Interphase 1 with the cells
you end up with at the end of Cytokinesis 2.
Biol 160: Activity
NAME: ______________________________
OTHER QUESTIONS TO ANSWER1. How does the arrangement of chromosomes differ when comparing metaphase of meiosis
I and mitosis? (In other words, what would you see lined up at the metaphase plate in
each of these two phases?)
2. What happens to the sister chromatids during anaphase of meiosis I?
3. In meiosis, how does metaphase II differ from metaphase I? (Again, it may be helpful to
think about what you would expect to see lined up at the metaphase plate in each of
these two phases!)
4. How does metaphase II compare to metaphase of mitosis?
5. How many cells were formed due to the process of meiosis? How many cells were formed
during the process of mitosis?
6. List three major differences between meiosis and mitosis.
7. How does the process of crossing over contribute to genetic variability?
TO TURN IN-
Turn in your answers to the questions for Part I and all of Part II below.