Genetics Unit: Meiosis
Chromosome Simulation Activity
Background information:
Sexual reproduction
requires a reduction in the chromosome
number of the parent cell (diploid or 2N) to half
(haploid or N) in the gamete or sex cell. This
type of cell division, resulting in half the
chromosome number, is called meiosis.
When two haploid gametes (egg and sperm)
combine during fertilization, the diploid
chromosome number is restored. Thus sexual
reproduction provides the mechanism to
produce genetic variation, when the genes of two different individuals
combine.
Meiosis consists of two nuclear divisions (meiosis I and II). This results in
the formation of four daughter cells, each of which has only half the
number of chromosomes of the parent.
Objective: This activity will help you understand the process of meiosis.
Meiosis differs from mitosis in the formation of daughter cells with half the
chromosome number of the parent cell. Sex cells – gametes and spores –
are the result of meiotic cell division. Meiosis can be graphically
demonstrated using strands of pop-it beads with magnetic centromeres.
Working in your lab groups, you will explore the stages of meiosis and
carefully record the stages in your lab notebook.
Materials: Each plastic bag should contain:
Activity: For each stage of meiosis, follow the instructions below, and record the
image simulation in your lab notebook. That is, you will be re-creating the
stages of meiosis with the pop-it beads, and recording (drawing) the
simulation on your lab bench in the results section of your lab notebook.
Interphase
WHAT’S HAPPENING? DNA synthesis occurs, resulting in the formation
of paired chromatids. The centrioles also replicate during interphase.
TO SIMULATE: Your pop-it beads represent the DNA in the chromatin
material.
1. Assemble a strand of red beads and a strand of yellow beads to
represent the chromosomes. You now have paired chromatids
(called dyads).
Genetics Unit: Meiosis
Chromosome Simulation Activity
2. Your table will function as the entire cell. On the table, draw a large
chalk circle (about 0.6 meters in diameter). This circle represents
the nucleus of the cell.
3. Place one strand of red and one strand of yellow beads in the
center of the circle – but remember that distinct chromosomes
aren’t yet visible at this stage.
4. Put two plastic centrioles at right angles to each other, near the
chromosomes. Replicate the centriole by placing another pair near
the first.
Prophase I
WHAT’S HAPPENING? A process called synapsing occurs –
homologous chromosomes move close together and pair up along their
entire length. A tetrad, consisting of four chromatids, is formed.
Centrioles migrate to the opposite poles, and the nuclear membrane
breaks down.
TO SIMULATE:
1. Erase parts of your chalk-drawn nuclear envelope.
2. Align your homologous chromosomes at the center of what was
once the nucleus of the cell.
3. Entwine them at the center of your cell. Separate the centrioles
and move them to opposite sides of the nucleus.
Metaphase I
WHAT’S HAPPENING? Chromosomes disentangle and become aligned
in the center of the cell in homologous pairs.
TO SIMULATE:
1. Position your paired strands in the center of the cell, at right angles
to the centrioles. We won’t use threads time to simulate spindle
fibers – imaginary lines will do.
Anaphase I
WHAT’S HAPPENING? The homologous chromosomes separate and
are drawn to opposite sides of the cell.
TO SIMULATE:
1. Move your strands toward their respective centrioles. They’re
being drawn by the spindle fibers.
Telophase I
Genetics Unit: Meiosis
Chromosome Simulation Activity
WHAT’S HAPPENING? Cell division may occur at this time resulting in
two daughter cells still containing paired chromatids. Centrioles duplicate
at this time.
TO SIMULATE: Move each paired strand to its centriole. Duplicate each
centriole. Draw an imaginary line around each daughter cell.
MEIOSIS II
A second division must now occur to separate the chromatids in the daughter
cells formed by this first division. This will reduce the amound of DNA in each
resulting daughter cell to one strand per chromosome – one-half the original.
Only one homologue from each chromosome pair will be present in each
daughter cell following meiosis II.
Interphase II
WHAT’S HAPPENING? DNA replication does not occur during the
interphase between stages of meiosis. This stage is often called
interkinesis.
TO SIMULATE: Your daughter cells remain as you left them following
telophase I.
Prophase II
WHAT’S HAPPENING? The centrioles move to opposite poles of the two
daughter cells. The chromosomes appear to shorten and thicken.
TO SIMULATE: Move the duplicated centrioles to opposite sides of each
daughter cell and tape them down to your desk. Place your strands
between the centrioles.
Metaphase II
WHAT’S HAPPENING? All of the chromosomes line up, single file, in the
center of the cell.
TO SIMULATE: Line up the strands so they are centered between the
centrioles.
Anaphase II
Genetics Unit: Meiosis
Chromosome Simulation Activity
WHAT’S HAPPENING? The chromatids of each chromosome separate
and are drawn to the opposite poles of each cell. Each chromatid, with a
well-defined centromere, is now a chromosome.
TO SIMULATE: Separate the chromatids at their centromeres and pull
them toward their respective centrioles.
Telophase II
WHAT’S HAPPENING? Cell division is completed and four daughter
cells are formed. Each has half the chromosome number of the parent
cell. A nuclear membrane forms, and one pair of centrioles remain
outside the nuclear membrane.
TO SIMULATE: Place each chromosome strand near its respective
centriole. Draw a chalk-line around each new cell.
Questions
1. Did Prophase I occur during Mitosis?
2. How does Metaphase I in Meiosis differ from Metaphase in Mitosis?
3. What are the differences between Prophase I and II? How do they differ
from prophase in mitosis?
4. How is Metaphase I different from Metaphase II in Meiosis?
5. How is Anaphase II different from Anaphase in Mitosis?
6. How many cells have been formed in Meiosis? In Mitosis? Compare the
resultant chromosome number in the daughter cells formed by each type
of cell division.