Download BRING YOUR TEXTBOOK TO LAB.

SIMULATION OF CHROMOSOME BEHAVIOR DURING MITOSIS AND MEIOSIS
Prior to performing this lab you should be familiar with the following terms:
chromosomes, sister chromatids, homologous chromosomes, haploid, and diploid.
BRING YOUR TEXTBOOK TO LAB.
Introduction
A eukaryotic cell, whether autonomous or part of a multicellular organism, has
the capacity to divide. In order to do so, three major events must occur. The cell must
first replicate all of its genetic material. The next major event is a “nuclear division” that
involves precisely dividing the replicated genetic material into two equal pools. Lastly,
the cell must physically separate these two pools by dividing the cytoplasm, thereby
creating daughter cells. Today we recognize two very different kinds of nuclear
divisions, namely mitosis and meiosis. While these two nuclear divisions have many
similarities, there are some significant differences. For example, mitosis involves a
single nuclear division while meiosis involves two nuclear divisions. This single
mechanistic difference is tied to all other differences noted between these two
processes.
In mitosis, individual chromosomes behave independently such that two daughter
cells receive a full diploid set of chromosomes. Replication of the genetic material
results in “duplicated” chromosomes, each of which consists of two sister chromatids.
Mitosis involves pulling the “duplicated” chromosomes in half, or separating sister
chromatids. Each of the two genetic pools that are created by this separation consists
of a complete diploid set of the cell’s genetic information. Division of the cytoplasm and,
consequently, the two equal pools of genetic material results in identical daughter cells.
There are similarities and differences between mitotic and meiotic cell divisions.
As for a mitotic cell division, prior to meiosis the meiocyte must first replicate all of its
genetic material. Thus, every chromosome is duplicated and consists of two sister
chromatids. In Meiosis I (the first nuclear division), homologous chromosomes pair to
form a tetrad or bivalent. The tetrads line up at the cell equator and the homologues of
a tetrad are then separated from one another. The resulting daughter cells now contain
the haploid number of chromosomes, but each chromosome is still in the “duplicated”
form. During the second stage of meiosis (referred to as meiosis II), the sister
chromatids are separated. Thus, after the second stage of meiosis, each of four
daughter cells has only a haploid set of chromosomes. It is the behavior of
chromosomes during meiosis that leads to the inheritance patterns observed for various
genetic characteristics.
In this exercise you will study chromosome behavior during mitosis and meiosis.
In addition, to see how the alleles of a gene segregate during meiosis, you will follow
the movement of 2 different genes during meiosis. Strands of pop-it beads will be used
to simulate individual chromosomes (see Figure 1). Each strand contains a thin
magnetic centromere. Strands will be red and yellow in color. Consider a yellow strand
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to be a maternally inherited chromosome, while a red strand being a paternally inherited
chromosome. Individual genes will be located on a chromosome using masking tape.
Figure 1. Strands of pop-it beads simulating individual chromosomes. You will be
given a total of 8 strands – two of each of the 4 strands shown. The additional strands
will be used to change chromosome structure upon replication. Yellow strands
represent maternal chromosomes that are contributed by the egg. Red strands
represent paternal chromosomes that are contributed by a sperm.
1 homologous pair
of chromosomes
consisting of one
maternally inherited
chromosome
(yellow) and one
paternally inherited
chromosome (red)
Another homologous
pair of chromosomes
consisting of one
maternally inherited
chromosome (yellow)
and one paternally
inherited chromosome
(red)
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A.
Mitosis
The time it takes for an individual cell to divide, thus forming two daughter cells is
called the cell cycle. Mitosis is but one stage of the cell cycle and specifically refers to
the nuclear division, or the partitioning of the chromosomes into two identical pools. For
convenience, the major stages of the cell cycle are listed and briefly described below:
Interphase
Gap 1 (G1) Phase
Cell undergoes general growth, increase in numbers of organelles, and an
increase in the volume of cytoplasm.
Synthesis (S) Phase
Chromosomes are duplicated.
Gap 2 (G2) Phase
The cell makes special molecules and structures that will be needed for
division.
M Phase
Mitosis
Prophase
Centrioles move to opposite poles;
Nuclear membrane and nucleolus disappear;
Chromosomes condense and become visible as string-shaped
structures;
Spindle fibers begin to form;
Chromosomes move towards an imaginary equatorial plane at the
center of the cell (pushed & pulled by spindle fibers).
Metaphase
Duplicated chromosomes line up at equatorial plane.
Anaphase
Duplicates uncouple at the centromere and move towards each of the
poles;
Spindle fibers shorten;
Telophase
Unduplicated chromosomes arrive at the poles;
Nuclear membrane reforms;
Nucleolus reforms;
Chromosomes stretch out into invisible threads;
Spindle fibers begin to disassemble.
Cytokinesis
This process involves the division of the cytoplasm and its contents by the
formation of new cytoplasmic membrane and/or cell walls. It usually starts
in late anaphase or in telophase and is completed either in late telophase
or in G1 of the next interphase.
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Procedure
1. Obtain 8 strands of plastic beads as follows:
2 strands that are long and red
2 strands that are long and yellow
2 strands that are short and red
2 strands that are short and yellow
A single strand of beads is meant to simulate one unreplicated chromosome, with
the thin magnate serving as the centromere. Assume that you are beginning with a
somatic cell that has a diploid number of four.
2. Draw the chromosome model that you create with the strands of beads to represent
a somatic cell in G1 that has a diploid number of four. Note in your model
chromosome color and form based on the position of the centromere (e.g.
metacentric, etc.).
How many pairs of homologous chromosomes do you have in this model?
Describe them.
How many sister chromatids are present in the current model?
How many “different”, or nonhomologous, chromosomes are present? Describe
them.
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3. Draw the model that would result after duplication of these chromosomes.
During which stage of the cell cycle does this occur?
How many chromosomes are present in the cell?
How many homologous pairs are present?
How many sister chromatids (total) are present?
4. Draw a model representing metaphase of mitosis for this cell.
Describe the location and arrangement of homologous chromosomes in the cell.
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5. Draw a model representing anaphase of mitosis for this cell.
Describe what has physically happened to the chromosomes during this phase.
How many chromosomes are at each pole of the cell?
Describe the chromosomes in each resulting daughter cell by color and form.
How many chromosomes did your cell begin with?
How many chromosomes are in the resulting daughter cells?
Are these two daughter cells absolutely identical to each other?
Is one, both or neither identical to the original starting cell?
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B. Meiosis
Meiosis is used by organisms to form the sexual gametes (spermatozoa and ova)
needed for reproduction and the distribution of genetic characteristics. Gametes generally
contain the haploid number of chromosomes. These gametes may then participate in
fertilization to produce the diploid zygote. In this exercise you will use strands of beads to
simulate chromosomes and their behavior during meiosis.
Procedure
1. Use the same 8 strands of plastic beads that were used in Part A. Again, assume that
a single strand is an unreplicated chromosome, and that you are beginning with a
meiocyte in G1 that has a diploid number of 4. Draw the chromosome model that you
create with the strands of beads to represent this cell (note chromosome color and
form in model):
How many pairs of homologous chromosomes do you have?
Describe the pairs of homologous chromosomes:
How many sister chromatids are present in this model?
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2. Draw the model that would result after duplication of these chromosomes:
How many chromosomes are now present?
How many pairs of homologous chromosomes do you have?
How many sister chromatids are now present?
3. Draw the model representing metaphase I:
Describe the location and arrangement of homologous chromosomes in the cell:
4. Draw the model representing anaphase I:
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What has physically happened to the chromosomes?
How many chromosomes are at each pole?
How many sister chromatids are at each pole?
What was the effect of Meiosis I on chromosome number?
5. Draw the model representing the above daughter cells in metaphase II:
Are there homologous chromosomes in the secondary meiocytes?
6. Draw the model for anaphase II:
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What has physically happened to the chromosomes in anaphase II?
How many chromosomes are at each pole?
Meiosis II is mechanically similar to what other type of cell division?
Why is it necessary that Meiosis II occur?
C. Behavior of Genes During Meiosis
As you know, genes are located on chromosomes. In this part of the exercise, you
will use masking tape to locate the alleles of two different genes on the set of
chromosomes you have been using and follow their movement through meiosis. One
of the genes is involved in carbohydrate metabolism. In humans, an abnormality
known as galactosemia (a disorder of carbohydrate metabolism) is dependent on gene
represented as “g “. The normal form of this gene (or the normal allele) is represented
as “G ”. The second gene you will consider is associated with brain function. A
neurological disorder called Huntington’s disease is determined by gene “H ”, and the
normal allele of this gene is “h “.
1. Consider a man who has the allele composition Gg and Hh in his (diploid) somatic
cells. Draw the strands of beads/chromosomes as they would appear in G2
assuming that the genes for these two disorders are on nonhomologous
chromosomes (i.e. the two genes are not linked). Use masking tape to “locate” the
alleles of these two genes on your chromosomes and a pen or marker to label each
(G, g, H, or h).
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2. Draw the two possible chromosome alignments that could occur in meiocytes
during metaphase I of meiosis. Note the location and identification of each allele on
your chromosomes.
3. Take each of the above meiocytes through the remaining steps of meiosis. Draw
the chromosomes that each gamete would contain noting the location and
identification of each allele.
How many different types of gametes with respect to allele combinations are
possible?
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4. When genes are linked, one allele of each different gene is located on a given
chromosome. Consider a man who has the allele composition Gg and Hh in his
(diploid) somatic cells. Draw the chromosomes as they would appear in G2 if these
two genes are linked on the large metacentric chromosomes and the man’s father
suffered from galactosemia (e.g. his father was gg) and died of Huntington’s
disease (e.g. his father was Hh), while his mother was normal for both traits.
Remember that the maternal chromosomes are the yellow strands and the paternal
chromosomes are red.
5. Take the above meiocyte through the remaining steps of meiosis (assume no
crossing over occurs). Draw the chromosomes that each gamete would contain
noting the location and identification of each allele.
How many different types of gametes with respect to allele combinations are
possible?
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