Download Meiosis simulation

Meiosis Simulation
Background:
Name(s)________________________
Recall that chromosomes are composed of DNA and contain the genetic blueprint for an organism. Each species has its
own unique set of chromosomes, and all individuals in a particular species typically have the same number of
chromosomes. Humans have 46 chromosomes. The domestic dog has 78 chromosomes, the domestic cat has 38
chromosomes, and the mouse that it chases has 40 chromosomes!
Within each individual in a species, every somatic (body) cell contains the same number of chromosomes as every
other. Humans (and most other animals) are diploid organisms meaning that each cell contains two complete
chromosome sets. Human gametes (sex cells) are haploid cells, meaning that they have only one complete set of
chromosomes. When fertilization occurs, the haploid sperm cell and haploid egg cell join, producing a fertilized
zygote. This "restores" the diploid chromosome number.
Questions:
1.) Where are the two original chromosome sets initially inherited? ________________
2.) If human somatic cells (body) have 46 chromosomes, how many chromosomes do human gametes have?
_______
3.) How many chromosomes do fertilized human eggs contain? _______ Are fertilized eggs haploid cells or diploid
cells? ______
Activity:
You are going to work through the cellular events involved in meiosis step by step. You will be working with the
chromosomes of a Triffle , a mythical organism. You and your teammates are to complete the following steps, and then
repeat the process until you can go through it without using these instructions. The Triffle has a diploid
chromosome number of four.
1.) What will the haploid chromosome number be?_________________________
Procedure:
Setting up the cell
Lay down the large white paper in the center of your work space. Imagine that this is one sex cell in a Triffle. The
boundary of the paper is the cell membrane.
Setting up the chromosomes and genes:
1. You are going to create a diploid nucleus containing two pairs of chromosomes. First, divide each of your four bags
of play dough in half.
2. Take one of the halves of each color and roll it between your hands to form an elongated, snakelike piece. You will
have four chromosome models. Choose green and yellow play dough to represent the first pair of homologous
chromosomes. Make each chromosome about 2 inches long; do the same with the other blue and red play dough
making them about 4 inches long. We will refer to these pairs as PAIR 1 and PAIR 2. (Refer to the figure below)
Blue
Red
=
***Remember that a chromosome is tightly coiled strand of DNA. Within each
chromosome there are many, many genes. The chromosomes within each pair are said
to be homologous, meaning similar but not necessarily identical. Homologous
chromosomes contain the same genes but not necessarily the same alleles. For
instance, two homologous chromosomes might contain the gene which codes for eye
color, but the allele form might be different--like blue eye allele on one chromosome
and brown eye allele on the other. ***
Green
Yellow
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We are going to examine four Mendelian traits in the Triffle, specified by four genes. Remember that genes are
segments of chromosomes which code for proteins that can result in the expression of detectable traits
(phenotypes). Table II below describes some imaginary traits of our mythical creature and their location on each
chromosome.
Table II -- Imaginary Traits of the Triffle
Genes on PAIR 1
Fur Color
Green Chromosome
D (green fur)
a (straight fur)
Yellow Chromosome
d (yellow fur)
A (curly fur)
Fur Type
Genes on PAIR 2
Eye Color
Eyelash length
Blue Chromosome
R (blue eyes)
L (long eyelashes)
Red Chromosome
r (red eyes)
l (short eyelashes)
3. Label the location of each gene with the appropriate beads. (Your chromosomes should now look like the ones in
figure 2 below!)
Green Yellow
d
D
A
a
Blue
Red
R
r
Figure 2. Map of Triffle Genes
**Note that we are looking at only two genes (two pairs of alleles) on each chromosome, while ignoring hundreds of
other genes. Also, note that the Triffle we are studying is completely heterozygous for all four genes examined. This
does not always have to be the case.**
STOP!! HAVE YOUR TEACHER “SIGN OFF” ON WHAT YOU’VE DONE SO FAR:
1.) What is the genotype of YOU, the Triffle? _______________________________
2.) Where did you get each of your alleles? (2 letters) __________________________
3.) What are the 2 pairs called that are the SAME LENGTH? _____________________
4.) What is YOUR phenotype? _______________________
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Replicating the DNA
1.
Simulate replication by creating a matching chromosome (same shape and color) using the remaining bits of play
dough for each of the four chromosomes in your genome. Label each chromatid with genes so they are exact
copies.
2. Connect sister chromatids together at the centromere by pinching them together.
STOP!! HAVE YOUR TEACHER “SIGN OFF” ON WHAT YOU’VE DONE SO FAR:
1.) How many chromatids are present in the nucleus after DNA replication?_________________
How many chromosomes? _________________
Simulating Meiosis (w/crossing over)
1. Pair the homologous chromosomes.
2. *Simulate crossing over for each of the 4 genes found on the 2 chromosome pairs. (See your textbook if you
need to refresh your memory on what “crossing over” is!)
3. Finish the first phase of meiosis:
 Line up the homologous pairs of chromosomes in the middle of your cell (metaphase)
 Separate the homologous pairs of chromosomes (anaphase)
 The homologous pairs of chromosomes are now on opposite sides of your paper (telophase)
 Tear your large sheet of paper in half…each half is a new cell!
1.) How many cells are the result of Meiosis I? _________
2.) How does the chromosome number of these daughter cells compare to the parent cell?
3.) A second division has to happen next…why?
4. Go through the second phase of meiosis (do the following for EACH of the two cells you created in the first phase
of meiosis)
 Line up the replicated chromosomes in the middle of the cell (metaphase)
 Separate the sister chromatids (anaphase)
 The individual chromosomes are now on opposite sides of your paper (telophase)
 Tear the sheets of paper in half again.
4.) How many cells do you now have? __________
5.) How does the chromosome number of these daughter cells compare with the original parent cell (at the start
of Meiosis)
**Note that each cell has a different genotype (combination of alleles). As a result of gene swapping, each daughter
cell contains one or more chromosomes that is different from both those in the parent cell and those in other
daughter cells.**
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6.) Write the alleles for each of the 4 cells below.
Cell one:_________________
Cell three: ________________
Cell two: ________________
Cell four: _________________
Simulating Reproduction
Now simulate the process of reproduction…find another pair of students who have also created gametes. Choose one
of your gametes to combine with one of their gametes. Answer the questions below about your offspring:
1.) a. What is the genotype of the fertilized egg/zygote? _______________
b. What is the phenotype of the new baby Triffle? Draw this in the space below. Since you only examined 4
traits (fur color, fur type, eye color and eyelash length) you will need to be creative to create a whole
picture of your Triffle!
2.) How many chromosomes were in each gamete (sperm & egg)? _______
3.) How many chromosomes are in your new baby Triffle? __________
4.) Is the baby Triffle identical to either of the parents? ________ Why/why not?
Reflection:
1. How can you tell the difference between meiosis and mitosis? How are they alike?
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