Bio 122
Spring 2007
Week 3 - Simulations of the Cell Cycle and Inheritance
Introduction
The purpose of this laboratory is to help familiarize you with cell division in eukaryotes so
you can understand its role in processes of genetics. You will be contrasting the timing of DNA
replication and its relationship to chromosome division and critical differences between mitosis and
meiosis. You will distinguish between sister chromatids and homologous chromosomes. (See
Chapters 12, 13 in your text.) Following Activities 1-4 you will simulate Mendelian inheritance.
Part 1: The Cell Cycle, Mitosis and Meiosis
Introduction to chromosome geography
The word "chromosome" can mean two very different things. First, as a concept, a
chromosome is a linked array of genes. Second, as an object, it consists of DNA and protein and is a
microscopic structure that is visible during mitosis and meiosis. We will focus on this second
meaning today. Learn the basic terms used to describe chromosomes.
We describe chromosomes by size, centromere location (Fig 12.4) and, if stained
appropriately, by banding patterns. For any organism, ONE whole set of genes is its genome. A
genome is arrayed into a set of chromosomes characteristic for a species. That set will have N
chromosomes. A karyotype consists of a set of identified chromosomes displayed in an organized
figure (page 240). If an individual has more than one genome per cell then the karyotype will
contain more of each chromosome. For example, a diploid cell has two chromosomes of each type
(Figs. 12.6, 13.4). Homologous chromosomes are two matching chromosomes per diploid organism,
which look similar but come from a different parent. Figure 13.4 shows two PAIRS OF
HOMOLOGOUS chromosomes.
Chromosomes are counted by counting their centromeres. Contrast the largest chromosome
in both parts of Fig. 12.6 with the smaller chromosome on the same figure. The chromosomes have
been replicated and the resulting duplicated chromosomes condense with two identical halves called
sister chromatids. The chromatids are joined at the centromere. Since there is only one centromere
shared between two chromatids, each chromosome appears duplex.
Activity 1- The Mitotic Cell Cycle - Interactive study partner simulations
To begin this exercise, you will need to start your Biology laptop machine and wait until it
finishes booting up. At this point, slide your CD Rom disk that comes with your textbook into the
drive. Click on the 'StartHere.html' icon and when it opens, click on 'Launch Campbell Biology'. A
new screen will appear. Click on 'Select Chapter' and choose 12. You will now see a list of
activities in the middle of the screen.
1. Click on the link 'Mitosis and Cytokinesis Animation' and watch the video and read the text
description of cell division. When the first 'panel' is finished, click onto the next panel. Look for
evidence of sister chromatids and homologous chromosomes. How can you tell the difference
between chromosomes?
2. Click on the bar at the top to scroll up to Mitosis and Cytokinesisis video'. Look for evidence of
sister chromatids in this video. In which panel(s) do you see them?
Bio 122
Cell Cycle and Inheritance
Spring 2007
Activity 2- The Mitotic Cell Cycle - You become a chromosome
A. For this exercise you will be working as pairs of students and cooperating with other pairs of
students to simulate events of the mitotic cell cycle. Each pair of students will get a wad of play
dough (either red or blue) and a file card with a chromosome drawn on it in the same color as the
play dough. Pinch off a piece of play dough about the size of a marble and roll it into a ball. Divide
the remainder of the play dough in half and set one half aside. From the other half, roll out and mold
a chromosome to match the drawing on the card and place the "marble" in position as the
centromere. Wait until all the groups are to this stage. Your lab instructor will have a few things to
say at this point about where you are in the cell and in the cell cycle.
B. Cover the chromosome with a sheet of notebook paper so you can't see it. Get the half of the play
dough you set aside during part A above. Without peeking and keeping it under the notebook paper,
roll it out and mold it into a shape that should match the first one you made. Your lab instructor will
have a few things to say at this point about replication in the cell cycle.
C. OK, cheat a little and uncover your creation. "Improve" the shape of your newly made chromatid
so that it matches the first one and attach it to the centromere of the first one.
•
How many chromosomes do you have?
__________________________
•
How many chromatids do you have?
__________________________
•
What is the comparative term for these chromatids?
__________________________
D. You and your lab partner take your file card with your chromosome on it to the table in the
middle of the room.
•
Are there any chromosomes identical in shape to yours? __________________________
•
If there are chromosomes "identical" to yours, do they
have any special spatial relationship to yours?
__________________________
•
What phase of the cell cycle has begun?
__________________________
E. With all the other groups, line your chromosomes up on the table.
•
What phase of the cell cycle are you in now?
__________________________
F. You and your lab partner should get on opposite sides of the table from each other, next to your
chromosome. Your instructor will now pass among your chromosomes and create two centromeres
from each original centromere. You and your lab partner will grab separate centromeres of your
chromosome and begin to move to opposite sides of the room.
•
How many chromosomes are present in the cell now? __________________________
•
What phase of the cell cycle has begun?
__________________________
Bio 122
Cell Cycles and Inheritance
Spring 2007
G. When all pairs of lab partners approach their respective sides of the room,
•
What phase of the cell cycle is it?
•
What else is happening in the cell as this is occurring? __________________________
•
How many chromatids does each chromosome
have now?
__________________________
Are there identical arrays of chromosomes at
both sides of the room?
__________________________
•
__________________________
•
Is each of those arrays identical to the chromosomes
that were present in the room at the end of 2A above? __________________________
•
When does the cell actually divide?
__________________________
Go back to your lab bench with your partner, taking your card and chromosome with you. Keep
your chromosomes intact
Activity 3- The MEIOTIC Cell Cycle- computer simulation
1. Return to your laptop and click on the top bar and select Chapter 13. Select 'Meiosis Animation'
and watch the video and listen to the text description. Finish with all panels and write down
three processes that occur at meiosis that you did not observe in mitosis.
2. Select the panel 'Origins of Genetic Variation.' After scrolling through all panels, list two
processes that go on that generate genetic variation among daughter cells resulting from a
meiotic division.
Activity 4- The MEIOTIC Cell Cycle - You participate in chromosome tango
A. You and your partner will begin much as you did for mitosis, setting one of your chromosomes
aside and placing the other on your card, covering it with a piece of notebook paper.
B. Without peeking, carry out the process of replication by synthesizing the sister chromatid for your
covered chromosome. (Get the other one you just set aside and fuse its centromere with the
centromere of the one that is covered so that you have a duplex chromosome with two chromatids
and only one centromere).
C. Before you and your lab partner move to the center table, find the other pair of students that have
an identically shaped chromosome of the other color. Do your best to make the two chromosomes
look identical, except for color. Place one of the chromosomes on top of the other, either red or blue
on top and put one card under the other. Cover them up with a piece of notebook paper.
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Bio 122
Cell Cycles and Inheritance
Spring 2007
•
What process have you just simulated?
__________________________
•
Does this process occur in mitosis?
__________________________
•
What phase are we in?
__________________________
•
What term is used to define the two identically
shaped chromosomes of different color?
__________________________
•
How many chromatids are in the structure on the card? __________________________
D. The four of you will move to the table in the center of the room with you and your lab partner on
one side of the table and the other pair of students on the opposite sides of the table next to the
chromosome assembly.
•
What phase are we in now?
__________________________
•
Do the identically shaped chromosomes have any
specific spatial relationship?
__________________________
Is this the same as in mitosis?
__________________________
•
•
How many separate assemblies of chromosomes
are there?
__________________________
E. You and your lab partner will now grab your chromosome's centromere and the other pair of
students will grab their chromosome's centromere. In each group of four students, one pair will take
their card and chromosome and move to one side of the room while the other pair of students moves
to the other side of the room.
•
As you begin to move what phase are we in?
•
Is anything different about what the centromeres do now
compared to mitosis?
__________________________
•
How many chromosomes are in the cell now?
__________________________
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__________________________
Bio 122
Cell Cycles and Inheritance
Spring 2007
F. When everyone is at their respective sides of the room,
•
What phase of the cell cycle is it?
__________________________
•
What else is happening to and around each cluster
of chromosomes?
__________________________
•
If each side of the room is now in a separate cell, how many
chromosomes are in each?
__________________________
•
How many chromatids in each chromosome?
__________________________
•
Are there identical arrays of chromosomes on both sides
of the room?
__________________________
•
Is this the same as in mitosis?
__________________________
•
If we did this again would the same combination of reds
and blues end up on each side?
__________________________
G. Each group of students on each side of the room should move to a counter at that side and place
their chromosomes in a line.
•
What stage of the cell cycle is it as this is happening?
__________________________
Your instructor will have a few things to say before we proceed.
H. Your instructor will move among the chromosomes and create two centromeres from each
original centromere. You and your lab partner will each grab separate centromeres from you
chromosome and begin to move to corners of the room at the same end.
•
How many chromosomes in each cell now?
__________________________
•
What phase of the cell cycle has just begun?
__________________________
I. When all pairs of partners approach their respective corners of the room,
•
What phase of the cell cycle is it?
•
How many chromatids does each chromosome now have? __________________________
•
Are there identical arrays of chromosomes in any of the
corners of the room?
•
__________________________
__________________________
Are any of these arrays identical to the array in the cell that
started this process?
__________________________
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Bio 122
Cell Cycles and Inheritance
Spring 2007
•
Are any of these arrays identical to the array present in the
gametes that formed the cell that started this process?
__________________________
•
If a cell forms around one of these arrays, how many
chromosomes in it.
__________________________
How does this number of chromosomes compare to the
number in the cell at the start?
__________________________
What important process usually found in meiosis have
we left out?
__________________________
•
•
J. We will now explore the significance of this process. Without smooshing your chromosomes, get
back together with your lab partner and the other pair of students that have the chromosome
homologous to yours. Each pair of students needs to restore their chromosome to the state it was at
the end of step C, above. Allow the chromosomes to synapse as before and cover them with a piece
of notebook paper. While they are covered, switch reciprocal portions of two homologous
chromatids.
•
What stage of the cell cycle does this happen in?
__________________________
K. Now repeat steps D through I moving the recombinant and non-recombinant chromatid in each
tetrad with their attached centromeres.
•
When we reach telophase II again with groups of students
in each of the four corners of the room, are any of the
arrays of chromosomes identical?
__________________________
Part 2: Simulations of Mendelian inheritance
Introduction
The purpose of this laboratory is to show you that genetic transmission is a kind of sampling.
In lecture, we have discussed Mendel's rules of segregation and assortment. Mendel derived his
rules based on experimentally observed ratios. Today we will simulate the process of segregation by
allowing simulated 'gametes' from hybrid individuals (heterozygotes) to meet by chance.
Activity 4- The rule of segregation
Mendelian ratios with 40 progeny - For this exercise, you should work in pairs. Assume that you
and your partner are F1 hybrids from two 'pure' parental lines (Aa). Each of you will produce
gametes for fertilization. The identity of the gametes will be determined by a coin toss with a penny.
If Abraham Lincoln's head is visible (heads), give it a score of 'A'. If it is tails, score it as 'a'. If your
partner tosses differently than you do, then your progeny is a 'Aa' heterozygote. If you both obtain
heads, then your offspring is a 'AA' homozygote. Record the genotype of your offspring, and repeat
the toss 20 times. When you are done, calculate the frequency of occurrence of the different
genotypic classes. Fill in Table 1 while doing so.
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Bio 122
Cell Cycles and Inheritance
Spring 2007
Table 1- Observed segregation ratios at one genetic locus
number of
Progeny
(tosses)
trial
20
1
20
2
frequency/proportion
AA
Aa
aa
ratio
What would the expected ratios be if Mendel's rules apply? __________________
Do your observed values conform to these expectations?
__________________
Take the sum of your total of 40 coin tosses and write them in blank sheets on the chalk board in
front of the room. When the class is finished putting their data on the board, take the total of all the
tosses and fill in the table below.
number of
offspring
(tosses)
AA
frequency/proportion
Aa
aa
ratio
Do the observed values come close to Mendelian expectations when the class data are pooled? If so,
how would you explain the difference and what would you recommend to an investigator that plans
to repeat Mendel's experiment?
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