STUDENT LAB GUIDE II – EGG AND SPERM
INTRODUCTION AND BACKGROUND
In human sex cells nuclear division (meiosis) follows an identical series of steps in both
males and females. Through meiotic cell division a diploid (contains two copies of each
type of chromosome --- 23 pairs) parent cell will produce four haploid (contains only one
copy of a chromosome --- 23 single) daughter cells. However, the cytoplasmic division
(cytokinesis) is significantly different between male sperm cells and female ova.
Recall from modeling meiosis there are two separate cytoplasmic divisions, Telophase I
and Telophase II. This lab will model both oogenesis (formation of the ovum or egg)
and spermatogenesis (formation of sperm cells).
PART A: FEMALES
1. Select one color of play dough and roll it into a ball of about one inch in diameter.
Think of this as DNA (genetic material) instead of play dough and that it is an entire
cell. It is large so that as a parent sex cell in human females it is filled with enough
nutrients to nourish a developing embryo. (Figure 1)
Figure 1: Female Parent Sex Cell
Recall the phases of Meiosis I from Lab 1 which included the following (imagine they
are occurring in the female parent sex cell):
a. Chromosome replication
b. Chromosome pairing to form tetrads
c. Crossing over and homologous chromosome “gene swapping”
d. Tetrads line up at the cell center tetrads
e. Tetrads (homologous chromosomes) separate
f. First nuclear division occurs
The cell now undergoes the first cytoplasmic division (cytokinesis) in Telophase I.
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2. Unequally divide the one female sex cell (play dough) into one small cell and one
large cell. (Figure 2)
FIGURE 2: CYTOKINESIS IN TELOPHASE I
Recall from Lab 1 the next phases from Meiosis II which included the following
(imagine each of these are occurring in the two cells that were produced in Telophase
I):
a. The chromatid pairs line up in the center of each cell.
b. The two sister chromatids in each pair now separate.
In Telophase II the second cytoplasmic division (cytokinesis) occurs.
3. Divide the large cell unequally into one large cell and one small cell. (Figure 3)
4. Divide the small cell into two small cells of approximately equal size. (Figure 3)
FIGURE 3: CYTOKINESIS IN TELOPHASE II
Ovum (egg)
Polar Bodies
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The second unequal cytoplasmic division produces the one large ovum (egg cell) and
the three small nonfunctional cells called polar bodies. The polar bodies usually die off.
The result of the unequal division means that the fertilized egg, because of its size,
inherits nearly all of its internal organelles and cytoplasm from the female sex cell.
PART B: MALES
1. Select another color of play dough to represent your male parent sex cell.
2. Start with one ball of play dough approximately one inch in diameter. As in the
model of the female sex cell, this represents the genetic material (DNA) of the male
parent sex cell.
3. Divide the male cell (play dough) into two cells that are approximately equal for the
first cytoplasmic division at the end of Telophase I. (Figure 1)
FIGURE 1: TELOPHASE I
Recall from modeling Meiosis in Lab 1, at the end of Telophase I, the homologous
chromosome pairs (tetrads) had separated and the chromosome number had been
reduced.
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4. At the end of the second Meiotic division (nuclear division), the cytoplasm divides
again. Each of the two cells produced in Telophase I divide again producing four equal
sized cells. Divide your play dough into four equal balls. (Figure 2)
FIGURE 2: TELOPHASE II
Each of the four daughter (son?) cells then grows a long tail (flagellum) to help it swim
around.
5. Attach a red piece of yarn by pushing it into each of your four balls of play dough.
This represents four active sperm cells. (Figure 3)
FIGURE 3: SPERM WITH FLAGELLUM
Any of the four viable (able to live, survive) sperm produced from one male parent cell is
capable of fertilizing an egg cell. At first the sperm swim in circles. (Figure 4)
FIGURE 4: SPERM SWIMMING IN CIRCLES
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Gradually the sperm begin to swim in a straight line.
FIGURE 5: SPERM SWIMMING IN STRAIGHT LINES
After the formation of the sperm and egg, the diploid number is restored at fertilization
when the haploid egg and haploid sperm fuse. The fertilized egg is called the zygote.
Each parent gamete contributes one chromosome of a homologous pair to the zygote
thus restoring the 23 pairs of chromosomes in human cells.
INTERESTING FACT: A woman is born with all the eggs she will ever have in her
ovaries. One egg typically matures each month after puberty until after menopause.
However, the male can continue to produce new sperm throughout his lifetime.
ANSWER THE FOLLOWING QUESTIONS
1. Based on what you know about the function of the male gamete (sperm) do you think
it would be better for it to be small or large and full of nutrients like an egg cell? Why or
why not? Explain your answer.
2. Why must gametes be haploid? What might happen if gametes were produced by
mitosis and were diploid?
Prepared by: Dr. Debbie Payne, Ruth H. Liddell, Shirley K. Scarbrough
Alabama State University, Math, Science Partnership, Spring 2013
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