STUDENT LAB GUIDE I

MODELING MEIOSIS

INTRODUCTION and BACKGROUND

In this lab you will investigate the cellular basis of organism variations by exploring the processes involved in sexual reproduction. This lab has two parts: Meiosis and Eggs and Sperm. Start by investigating how sex cells, or gametes, are produced in males and females. A specialized process called meiosis produces the gametes. This meiotic cell division results in the formation of sperm and egg in humans and other animals and pollen and egg in plants. Meiotic cell divisions occur in specialized cells in the reproductive structures of plants (male anther and female ovulary) and animals (male testes and female ovary) and other sexually reproducing organisms.

The second part of the lab is an exploration of cytokinesis (cytoplasmic division) in human sex cells. There is equal distribution of the cytoplasm during spermatogenesis (formation of sperm) which results in the production of four viable sperm cells of approximately the same size. There is an unequal distribution of the cytoplasm during oogenesis (formation of egg or ovum) which results in the production of only one large, viable egg cell and three much smaller cells, called polar bodies, which die off.

Work through the cellular events involved in meiosis step by step using the chromosomes of Mr. Dudley Dibble, an imaginary creature. Mr. Dibble has a diploid chromosome number of four (two pairs of homologous chromosomes). One homologous pair of Mr. Dibble’s chromosomes is represented by strands of red and white pop beads. The other pair of homologous chromosomes is represented by strands of orange and navy blue pop beads. For the remainder of this lab the red/white pop beads will be referred to as PAIR 1 and the orange/navy blue will be referred to as PAIR 2.

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The second part of the lab uses two different colors of Play Dough to demonstrate the process of cytokinesis in the formation of eggs and sperm. In human sex cells, nuclear division (division of the chromosomes) occurs exactly the same way in males and females through meiotic cell division. A diploid parent cell (cell with 46 chromosomes) gives rise to four haploid daughter cells (cells with 23 chromosomes). However, the process of cytokinesis differs between male and female cells. As you learned from modeling meiosis during the first part of this lab, cytokinesis occurs on two separate occasions: Telophase l and II.

Recall from the Modeling Mitosis Lab that chromosomes are composed of DNA and contain the genetic blueprint for every organism. Each species has its own special set of chromosomes but all individuals of a particular species typically have the same number of chromosomes. All normal humans have 46 chromosomes or 22 homologous pairs and 1 pair of sex chromosomes for a total of 23 pairs. The domestic dog has 78 chromosomes, the domestic cat has 38 chromosomes and the mouse has 40 chromosomes.

Within each individual of a species every somatic (body) cell contains the same number of chromosomes as every other individual of that species. Humans (and most other animals) are diploid meaning that each cell contains 2 complete chromosome sets.

ANSWER THE FOLLOWING QUESTIONS

1. Where did the two chromosome sets that are initially inherited come from?

2. Human gametes (sex cells) are a. haploid b. diploid.

3. Human gametes have a. one complete set of chromosomes b. ½ of a complete set of chromosomes.

4. If human somatic cells have 46 chromosomes, how many chromosomes do human gametes have?

5. When fertilization occurs, the haploid sperm cell and haploid egg join producing the zygote (fertilized egg). This “restores” the _________ chromosome number.

6. How many chromosomes do fertilized eggs contain?

7. Are fertilized eggs haploid cells or diploid cells?

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A: SETTING UP THE CELL

1. Lay out on a table the bulletin board paper on which we will represent a cell.

2. Tape the piece of black yarn to the paper in an oval shape to form the cell membrane.

3. Tape the piece of purple yarn to the paper to form the nuclear membrane inside the cell membrane.

B: SETTING UP THE GENOME

1. Using four different colors of beads, create a diploid nucleus containing two pairs of generic homologous chromosomes. Make a strand of 10 red pop beads and a strand of 10 white pop beads. These two strands represent one pair of homologous chromosomes. They will be referred to as PAIR 1 for the remainder of this lab. Make a strand of 20 orange pop beads and a strand of 20 navy blue pop beads. These two strands represent another pair of homologous chromosomes. They will be referred to as PAIR 2 for the remainder of this lab. Each bead represents a gene on the chromosome and the bead color indicates the particular trait specified by that gene. These pairs will be modified as the lab progresses.

FIGURE 1: BEAD CHROMOSOME MODELS and COLOR KEY

PAIR 1

2. Recall from studying mitosis that a chromosome is formed from a tightly wound strand of DNA. Along each chromosome there are many hundreds of genes which are

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Red

White

Navy Blue

Orange

PAIR 2

segments which code for certain proteins that can produce the expression of observable traits in organisms (phenotypes). Chromosomes within each pair are said to be homologous—which means similar but not identical. Homologous chromosomes contain genes for the same genetic traits but not necessarily the same alleles (differing forms of the same gene). For example, Mr. Dibble might have two homologous chromosomes with genes that code for fur color, but the alleles might be different; for example, the allele on one chromosome might code for lime green fur and the other allele might code for yellow fur.

3. You will now modify the generic homologous chromosomes in Pair 1 and Pair 2 (Figure 1) by replacing some of the beads by other beads which represent genes that code for certain traits in our imaginary creature, Mr. Dibble. Use beads of the colors given in Table 1 to replace some of the beads in the generic Pair 1 and Pair 2 to obtain the modified Pair 1 and Pair 2 shown in Figure 2. These pairs of homologous chromosomes now demonstrate the genotype (the genetic makeup) of Mr. Dibble. This model looks at only two genes (two pairs of alleles) on each chromosome and ignores hundreds of other genes. Note also that Mr. Dibble is completely heterozygous, i.e. has one dominant gene (which is expressed) and one recessive gene (which is not expressed) for each trait.

TABLE 1: IMAGINARY TRAITS OF MR. DIBBLE

GENES ON PAIR ONEFur Color Bead

ColorFur Type Bead

ColorRed Chromosome

G (green fur) Lime Green

c (straight fur)

Hot Pink

White Chromosome

g (yellow fur) Yellow C (curly fur) Turquoise

GENES ON PAIR TWOEye Color Ear Shape

Navy Blue Chromosome

B (baby blue eyes)

Baby Blue

P (pointy ears)

Peach

Orange Chromosome

b (pink eyes) Pink p (flat ears) Mint Green

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In genetics, the dominant and recessive genes that code for a particular trait, fur color, for example, are represented by a capital and lower case of the same letter such as G (dominant) and g (recessive).

FIGURE 2: MAP of MR. DIBBLE’S GENES

KEY: Four Genes/Eight Alleles

Lime Green Bead = G = Lime Green Fur

Yellow Bead = g = Yellow Fur

Turquoise Bead = C = Curly Fur

Hot Pink Bead = c = Straight Fur

Baby Blue Bead = B = Blue Eyes

Pink Bead = b = Pink Eyes

Peach Bead = P = Pointy Ears

Mint Green Bead = p = Flat Ears

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PAIR 1

PAIR 2

ANSWER THE FOLLOWING QUESTIONS

1. What is Mr. Dibble’s diploid number?

2. If capital letters represent dominant genes (those that are expressed) and lower case letters represent recessive genes (those that are not expressed), describe the outward, physical appearance (phenotype) of Mr. Dibble, i.e. the color of his fur, the texture of his fur, the color of his eyes and the shape of his ears.

C: INTERPHASE

Chromosomes are duplicated by DNA replication.

1. Make an additional matching chromosome for each strand (same bead color in same location). This simulates what occurs during replication. Construct each chromatid with genes so they are exact copies of one another (sister chromatids).

2. Connect sister chromatids together at the center with a piece of pipe cleaner (represents centromere). (Figure 3)

FIGURE 3: MR. DIBBLE’S REPLICATED CHROMOSOMES SHOWING SISTER CHROMATIDS

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Centrome

PAIR 1 Sister Chromatids

Sister Chromatids

PAIR 2

Centromere

ANSWER THE FOLLOWING QUESTIONS

1. How many chromatids are present in the nucleus after DNA replication?

2. How many chromosomes are present?

D. PROPHASE I

The nuclear membrane begins to break down; imagine this happening at this point.

1. Pair each newly created chromosome with its homologous chromosome partner(called a tetrad). By pairing them, two tetrads containing 4 sister chromatids each will result. There should be 8 chromatids in all. (Figure 3)

2. With the formation of tetrads crossing over can occur. Crossing over occurs when chromatids break into segments of matching regions which are then exchanged with one another. In each tetrad, overlap chromosome arms to form a cross over so that one allele can be traded between two non-sister (non-identical) chromatids. (Crossing over shown in Figure 4 for Pair 1)

3. Break the beads to separate the chromosomes again and join the exchange parts to their new chromosome. (Shown in Figure 5 for Pair 1) Do this for homologous chromosomes Pair 1 and Pair 2. (Result shown in Figure 6)

PAIR 1 PAIR 1

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FIGURE 4: CROSSING OVER for PAIR 1

FIGURE 5: THE RESULT of CROSSING OVER for PAIR 1

Crossing over is important because it results in new combinations of genes that are different from either parent and contributes to genetic diversity and variability in offspring.

E: METAPHASE I

1. The nuclear membrane has now broken down in Mr. Dibble’s cell. Remove the string representing the nuclear membrane and set it aside.

2. Line the 2 tetrads of the homologous pairs (Pair 1 and Pair 2) end to end across the center of the cell. (Figure 6)

3. Attach a piece of green yarn (spindle fiber) stretching from the centromere (pipe cleaner) of each chromosome to the end of the cell nearest to it.

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Spindle Fibers

Cell Membran

Spindle Fibers

Imaginary Line

Figure 6:

Tetrads Pair 1

Tetrads Pair 2

ANSWER THE FOLLOWING QUESTION

1. Was there a reason why the original white chromosome of Pair 1 (Figure 1) ended up on the same side of the equatorial plate (cell center) as the original navy blue chromosome of Pair 2 (Figure 1) or was it random? Why or why not? Explain your answer.

F: ANAPHASE I

In Anaphase I of meiosis, only the tetrads of homologous chromosomes separate. NOTE!! The sister chromatids of each chromosome are still attached at the centromere to one spindle fiber.

1. Separate each tetrad of Pair 1 and Pair 2 by pulling the spindle fiber to move the homologous chromosomes to opposite poles (sides) of the cell. (Figure 7)

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FIGURE 7:

Spindle Fibers attached to centromere of chromosomesSpindle Fibers attached to centromere of

chromosomes

G: TELOPHASE I

Cytokinesis (cell division) occurs and forms two daughter cells. At this phase of meiosis these daughter cells contain only one chromosome out of each of the original pairs of homologous chromosomes. Reduction division has occurred. These daughter cells are not considered at this point to be haploid or diploid; instead, they exist in some intermediate stage.

1. Pinch in the cell membrane (black yarn) in the center to form two cells. Cut the black yarn and tape it to form two new cells of approximately the same size. (Figure 8)

ANSWER THE FOLLOWING QUESTION

1. Are the two daughter cells identical in genetic make-up? Why or why not? (Figure 8)

FIGURE 8: TELOPHASE I

H: PROPHASE II + METAPHASE II

There is no Interphase II of Meiosis II because DNA does not replicate and therefore the chromosomes are not duplicated. Pretend the chromosomes and spindle fibers have disappeared and the other activities of the cell cycle are occurring.

1. Remove the beads (chromosomes) and the yarn (spindle fibers) from the cell.

During Prophase II both the chromosomes and the spindle reappear.

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2. Line up the chromosomes in each cell end to end along the center line. (Figure 9) Fold each piece of green yarn in half to make spindle fibers for the two new daughter cells.

3. Attach a spindle fiber to the centromere (pipe cleaner) of each chromatid.

I. ANAPHASE II

1. Cut the centromere in half and leave each half attached to a chromatid (bead strand) which was attached to the spindle fiber in the previous phase.

2. Using the spindle fibers attached to the centromeres separate the two center sister chromatids and move them to opposite poles. (Figure 10)

J: TELOPHASE II

Cytokinesis occurs again producing a total of four daughter cells.

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Spindle fibers

Imaginary Center LineFIGURE

Chromatids FIGURE 10: ANAPHASE II

Chromatids

Spindle fibers

1. Cut the black yarn (cell membrane) into four pieces of approximately equal length. Tape them to the paper to make four cells.

2. The spindle fibers disappear. Remove the pieces of green yarn.

3. The nuclear membrane reappears. Place four pieces of purple yarn around the two chromosomes in each of the four daughter cells. (Figure 11)

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Daughter Daughter Daughter Daughter

FIGURE 11:

Prepared by: Dr. Debbie Payne, Ruth H. Liddell, Shirley K.

At the end of Telophase II, the four daughter cells produced all have similar combinations of genetic information which came from the original parent cell. The original parent cell contained chromosomes, half of which were contributed by the mother and half contributed by the father. All sexually reproducing plants and animals generate their reproductive or germ cells through a process of cell division called MEIOSIS. Similarly, all multicellular plants and animals are able to grow because their body or somatic cells undergo another process of cell division called MITOSIS.

ANSWER THE FOLLOWING QUESTIONS

1. Are the cells produced in Telophase II haploid or diploid?

2. Are the genotypes (combination of alleles) of any of the daughter cells identical? Why or why not?

3. Write the genotype of the following:

Daughter Cell A: __________

Daughter Cell B: __________

Daughter Cell C: __________

Daughter Cell D: __________

4. What color fur trait would:

Daughter Cell A contribute: __________

Daughter Cell B contribute: __________

Daughter Cell C contribute: __________

Daughter Cell D contribute: __________

5. What texture fur trait would:

Daughter Cell A contribute: __________

Daughter Cell B contribute: __________

Daughter Cell C contribute: __________

Daughter Cell D contribute: __________

6. What color eye trait would:

Daughter Cell A contribute: __________

Daughter Cell B contribute: __________

Daughter Cell C contribute: __________

Daughter Cell D contribute: __________

7. What shape ear trait would :

Daughter Cell A contribute: __________

Daughter Cell B contribute: __________

Daughter Cell C contribute: __________

Daughter Cell D contribute: __________

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