s
.\,
ffi*ffi
&
CELL REPRODUCTION. MEIOSIS
BACKGROUND READING - Pages 550 - 555 in Netson Biotogy
PURPOSE - To develop an understanding of meiosis and its role in genetic variation.
INTRODUCTION
All new cells come from previously existing cells. New cells are formed by the process of cell
division which involves both replication of the cell's genetic material and division of the cytoplasm to
form daughter cells.
There are two types of nuclear division: mitosis and meiosis. Mitosis typically results in new
body (somatic) cells. Formation of an adult organism from a fertilized egg, asexual reproduction,
regeneration, and maintenance and repair of body parts is accomplished through mitotic cell division.
Meiosis, on the other hand, results in the formation of either gametes (in animals) or spores (in
plants). These cells have half the chromosome number of the parent cell.
Meiotic Cell Division - A Simulation
Objective
Meiosis involves two successive nuclear divisions that produce four haploid cells. The first
division (meiosis l) is the reduction division while the second division (meiosis ll) separates the
chromatids. Meiosis increases genetic variation in a given population. Each diploid cell undergoing
meiosis can produce 2n different chromosomal combinations, where n is the haploid number. In
humans the number is 223 which is more than 8 million combinations. The goal of this activity is to
become familiar with the meiotic process.
Materials
(,r.1
r 2 strands of ydery yarn ^
o 2 strands of b€r#n yarn 5{or-,tt
r 2 strands of
yarn (3 l -2-tt./t /
o 2 strands of fliafr
yesevv yatn fix (i
. 4 paper clips
. I piece of plain paper
. 1 felt marking pen
_
n
1. The sheet of paper will represent the cell. All other cell parts will be ignored except for those
involved in meiosis. Draw a line from the top of the piece of paper to the bottom dividing the sheet in
half . Make a dot half way down each edge of the sheet. These will represent the poles and the line will
represent the equator.
2. Make sure the red and pink strands are roughly the same length and the brown and yellow are of
the same length. The red/pink will represent one homologous pair and the yellow/brown will
represent a second homologous pair meaning the cell has two types of chromosomes.
Q1 - Define the term homologous pair. Where did each member of the pair originally come from?
Just prior to lhe beginning of meiosis each chromosome in the nucleus forms a replica during
lnterphase. To simulate this replication, use a paper clip to attach pairs of strands of identical
colours at the center of the strands, This will give you four "double" chromosomes which are simply
called chromosomes. Place each of the four chromosomes in any position on the sheet of paper. This
will represent PROPHASE I of meiosis.
Q2 - What are the strands of each double chromosome known as?
Q3 - Define the term diploid. What is the diploid number of this cell ?
Q4 - Define the term haploid. What is the haploid number of the cell you are working with?
Q5 - Why is this stage called Prophase I instead of just prophase ?
Cell Reproduction Unil
3. Pair up the homologous chromosomes (red with pink, brown with yellow) by overlapping the paper
clips to form "tetrads". Move the tetrads to line up at the equator of the cell to simulate METAPHASE
l. The paper clips (centromeres) should be on the equatorial line.
Q6 - What special activity often goes on during the tetrad phase that is responsible for mixing the
genetic material?
4. To simulate ANAPHASE l, hold the paper clips of each tetrad and pull the double chromosomes
apart until they are half way to the poles of the cell.
5. ln TELOPHASE I the chromosomes are moved to the opposite poles of the cell. To simulate the
division of the cytoplasm forming two new cells, use a felt marker to darken the equatorial plate line.
Q7 - How many chromosomes are in the daughter cells? How does this compare with the mother cell?
QB - Are the new cells haploid or diploid ?
6. Draw a pencil line joining the two poles of the original cell. This line will be at right angles to your
first line. This line will represent a new equatorial plate in each of the daughter cells. Mark new poles at
the top and bottom of each new cell. Randomly spread the double chromosomes in the cenlral region
of each daughter cell.
This will represent PROPHASE ll.
Q9 - What does not occur in Prophase ll that occurred in Prophase l?
7. Simulate METAPHASE ll by moving the chromosomes to the equatorial plate, placing the
centromeres on the lines.
8. Unclip each strand of the double chromosome and separate them to half way from the poles.
This will represent ANAPHASE ll.
Q10 - How does the process of separation in Anaphase I differ {rom that of Anaphase ll?
9. To simulate TELOPHASE ll, move the chromosomes to the poles of each new daughter cell. Use
the marker to darken the equatorial plate, representing the division of the two cells.
Q1 1 - How many chromosomes are in each new daughter cell? ls this diploid or haploid?
Q12 - What is the total number of cells produced in meiotic celldivision?
Q12 - An organism has 20 types of chromosomes. How many chromosomes would be found in a
body cell of that organism? ln a sex cell ? Explain.
Q13 - Using the original chromosome number of your yarn cell, determine the number of possible
combinations (2n) or red, pink, brown and yellow that may have occurred in the final cell ?
Q14 - ln the chart found below, compare the processes of mitosis and meiosis by using the provided
headings.
TABLE
I.
MITOSIS AND MEIOSIS COMPARISON
NUMBER OF CELL DIVISIONS
NUMBER OF CELLS PRODUCED
DO HOMOLOGOUS CHROMOSOMES
PAIR-UP? (YES/NO)
NUMBER OF CHROMOSOMES IN MOTHER
CELL
NUMBER OF CHROMOSOMES IN
DAUGHTER CELL
BODY CELL PRODUGED
?
(YES/NO)
Cell Reproduction Unit