modeling meiosis

advertisement
BIOLOGY – Activity
Modeling Meiosis
Period 1 2 3 4 5 6 7 8
Date: _____________
Station # _____
Names _____________________
_____________________
_____________________
_____________________
_____________________
INTRODUCTION
The body cells of plants and animals are diploid. A diploid (2n) cell has two sets of
chromosomes in its nucleus. A cell with only one set of chromosomes in its nucleus is termed
haploid (n). Egg and sperm, gametes, are examples of haploid cells. When gametes fuse at
fertilization, a diploid zygote is formed. The zygote contains one set of chromosomes from
each parent.
The processes that produces haploid (n) cells such as gametes from diploid (2n) cells is
called meiosis. Before meiosis begins, DNA replication occurs. Following replication, each
chromosome consists of two chromatids that are joined by a centromere. Meiosis involves
two successive divisions of the nucleus. The first of these divisions is called meiosis I. During
meiosis I, the homologous chromosomes (chromosomes that carry the same genes and are
similar in size and shape) come together, or pair up, and then separate. The nuclei that result
from meiosis I contain only one set of chromosomes, or one chromosome from each pair of
homologous chromosomes. Therefore, meiosis I is also known as reduction division because
each of the resulting nuclei contains half the number of chromosomes of the original cell. The
second division of the nucleus is called meiosis II. During meiosis II, the chromatids separate.
forming 4 haploid nuclei.
During meiosis I, the chromatids of a homologue (member of a pair of homologous
chromosomes) may exchange parts. This exchange of segments between chromatids is called
crossing over. Crossing over, as well as the fusion of two gametes during sexual reproduction,
is a type of genetic recombination, which is the regrouping of genes into new combinations.
OBJECTIVES
To model the stages of meiosis in an animal cell
To demonstrate genetic recombination
To relate the events of meiosis to the formation of haploid gametes
MATERIALS
4 pieces of string (1 meter long)
scissors
paper clips (8)
4 pieces of string (40 cm long)
metric ruler
tape
8 pieces of string (10 cm long)
4 strips of paper (2cm x 6 cm), one each of light blue, dark blue, light green, dark green
PROCEDURE
1. Using a 1 meter piece of string, make a circle on the lab table to represent the cell
membrane of a cell. Using a 40 cm piece of string, make another circle inside the
cell to represent the nuclear membrane.
2. Fold each of 4 strips of paper ( one light blue, one dark blue, one light green, and one
dark green) in half lengthwise. Then place each of these folded strips inside the nucleus
to represent the four chromosomes before replication. The light and dark strips of the
same color represent homologous chromosomes. The light strips represent chromosomes
from one parent and the dark strips, chromosomes from the other parent.
3. Interphase. To represent DNA replication, unfold each paper strip and cut each in half
lengthwise. The two pieces that result from cutting each homologous strip represent the
chromatids. Attach the two identical chromatid strips at the center with a paper clip so
that an X is formed (see Fig. 1 below). Each paper clip represents a centromere.
What process did you model when you cut the paper strips in half?
__________________________________________________________________
What is the function of the centromere?
__________________________________________________________________
4. Prophase I. Remove the nuclear membrane (the 40 cm string). Place the blue
chromatid pairs next to each other and the green chromatid pairs next to each other.
Simulate crossing over by measuring and cutting a 1 cm piece from the tip of a light
blue strip and a dark blue strip. Tape the light blue piece to the dark blue strip and the
dark blue piece to the light blue strip (see Fig. 1). Repeat this procedure with the green
strips.
light blue
dark blue
light green
dark green
Fig. 1
What is the purpose of placing the light and dark strips of the same color side by side?
____________________________________________________________________
5. Metaphase I. Place four 10 cm pieces of string inside the cell so that two strings
extend from one side into the center of the cell and the other two strings extend from the
opposite side into the center of the cell. These strings represent the spindle fibers. Using
a small piece of tape, attach one string to the centromere of each of the four chromatid pairs.
Move the chromatid pairs to the center of the cell so that they line up in a double file of
X’s, blue next to blue and below them, green next to green. Make sure that strings
attached to similar colors come from opposite sides of the cell.
6. Anaphase I. To simulate anaphase I, gather the loose ends of the two strings on each
side of the cell and gently pull the strings in opposite directions so that the homologous
pairs of chromosomes are moved to opposite sides of the cell.
7. Telophase I. Carefully remove the tape from each of the centromeres. Place a 40 cm
piece of string around each group of chromatids, forming two nuclei. Remove the
original 1 meter piece of string and place new 1 meter pieces of string around each of
the nuclei thus forming two cells.
How many chromosome pairs are in each of the cells you formed? ____________
List the materials used to make these two cells and what each represents.
__________________________________________________________________
__________________________________________________________________
8. Prophase II. Remove the strings that represent the nuclear membranes of each cell.
Attach a 10 cm piece of string to each chromatid (not the centromere).
What must happen to the centromeres before the chromatids can separate?
__________________________________________________________________
9. Metaphase II. Move the chromatid pairs to the center of each cell and line them up in a
column with the blue X above the green X. Make sure the strings attached to each of the
chromatids come from opposite sides of each cell.
10. Anaphase II. Gather the strings on both sides of each cell and gently pull in opposite
directions, separating the paper strips (chromatids) and pulling them to opposite
sides of each cell. Note: only one strip in each pair should have the paper clip attached.
11. Telophase II. Remove all of the strings and the paper clips. Each strip of paper
now represents a chromosome. Place a 40 cm piece of string around each of the 4
groups of chromosomes, thus forming 4 nuclei. Place a 1 meter piece of string around
each of the nuclei thus forming 4 cells.
How many chromosomes are in each of the cells you formed? Are these cells
haploid or diploid? ______________________________________________
12. Save the paper clips and dispose of all the strings and paper strips you cut. Make
sure your work area is returned to the way you found it.
ANALYSIS
1. What is the diploid number of the original cell you modeled? How many homologous
pairs does this represent?
________________________________________________________________
2. If a cell with a diploid number of 6 undergoes meiosis, what will the cell look like
after Telophase I? Draw it in the space below and label all parts.
3. Give two reasons why meiosis is important in sexual reproduction.
_______________________________________________________________
_______________________________________________________________
4. Why is meiosis I known as reduction division?
_______________________________________________________________
_______________________________________________________________
5. List two ways that meiosis is different from mitosis.
_______________________________________________________________
_______________________________________________________________
Download
Related flashcards
Create Flashcards