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Lab 10

MEIOSIS: BASIS OF SEXUAL REPRODUCTION

INTRODUCTION

Sexual reproduction occurs only in eukaryotes. During the formation of gametes, the number of

chromosomes is reduced by half, and returned to the full amount when the two gametes fuse during

fertilization. Haploid aond diploid are terms referring to the number of sets of chromosomes in a cell. Gregor

Mendel determined his peas had two sets of alleles, one from each parent. Diploid organisms are those with

two (di) sets. Human beings (except for their gametes), most animals and many plants are diploid. We abbreviate diploid as 2n. Ploidy is a term referring to the number of sets of chromosomes. Haploid

organisms/cells have only one set of chromosomes, abbreviated as n. Organisms with more than two sets of

chromosomes are termed polyploid. Chromosomes that carry the same genes are termed homologous

chromosomes. The alleles on homologous chromosomes may differ, as in the case of heterozygous

individuals. Organisms (normally) receive one set of homologous chromosomes from each parent.

Meiosis is a special type of nuclear division which segregates one copy of each homologous chromosome

into each new "gamete". Mitosis maintains the cell's original ploidy level (for example, one diploid 2n cell

producing two diploid 2n cells; one haploid n cell producing two haploid n cells; etc.). Meiosis, on the other

hand, reduces the number of sets of chromosomes by half, so that when gametic recombination (fertilization)

occurs the ploidy of the parents will be reestablished. Most cells in the human body are produced by mitosis.

These are the somatic (or vegetative) line cells. Cells that become gametes are referred to as germ line cells.

The vast majority of cell divisions in the human body are mitotic, with meiosis being restricted to the gonads.

Indicate the differences and similarities between meiosis and mitosis;

Describe the basic differences between the life cycles of higher plants and higher animals;

Describe the process of meiosis, and recognize events that occur during each stage;

Discuss the significance of crossing over, segregation, and independent assortment;

Identify the meiotic products in male and female animals;

Describe the process of non-disjunction and chromosome number abnormalities in resulting

gametes and zygotes.

OBJECTIVES

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Table 10-1 summarizes the differences between mitosis and meiosis.

Table 10-1 Comparison of Mitosis and Meiosis

MITOSIS MEIOSIS

Equatorial division: amount of Reduction division: amount of genetic

genetic material remains constant material is halved

Completed in one division Requires two divisions for completion

Produces 2 genetically identical Produces 2-4 genetically different nuclei

Nuclei

Generally produces cells not directly Produces cells for sexual reproduction

involved in sexual reproduction

Life Cycles

Life cycles are a diagrammatic representation of the events in the organism's development and reproduction. When

interpreting life cycles, pay close attention to the ploidy level of particular parts of the cycle and where in the life cycle meiosis

occurs. For example, animal life cycles have a dominant diploid phase, with the gametic (haploid) phase being a relative few cells.

Most of the cells in your body are diploid, germ line diploid cells will undergo meiosis to produce gametes, with fertilization closely

following meiosis. Plant life cycles have two sequential phases that are termed alternation of generations.

The sporophyte phase is "diploid", and is that part of the life cycle in which meiosis occurs. However, many plant species are

thought to arise by polyploidy, and the use of "diploid" in the last sentence was meant to indicate that the greater number of

chromosome sets occur in this phase. The gametophyte phase is "haploid", and is the part of the life cycle in which gametes are

produced (by mitosis of haploid cells). In flowering plants (angiosperms) the multicelled visible plant (leaf, stem, etc.) is sporophyte,

while pollen and ovaries contain the male and female gametophytes, respectively. Plant life cycles differ from animal ones by

adding a phase (the haploid gametophyte) after meiosis and before the production of gametes. Many protists and fungi have a

haploid dominated life cycle. The dominant phase is haploid, while the diploid phase is only a few cells (often only the single celled

zygote, as in Chlamydomonas ). Many protists reproduce by mitosis until their environment deteriorates, then they undergo

sexual reproduction to produce a resting zygotic cyst.

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I. DEMONSTRATION OF MEIOSIS USING POP BEADS

Within the nucleus of an organism, each chromosome bears genes, which are units of inheritance. Genes

may exist in two or more alternative forms called alleles. Each homologue bears genes for the same traits;

these are the gene pairs. However, the homologues may or may not have the same alleles. An example will

help here.

Suppose the trait in question is flower color and that a flower has only two possible colors, red or white

(Figure 10-1). The gene is coding (providing information) for flower color. There are two homologues in the same

nucleus, so each bears the gene for flower color. However, on one homologue, the allele might code for red

flowers, while the allele on the other homologue might code for white flowers. There are two other possibilities.

The alleles on both homologues might be coding for red flowers, or they both might be coding for white flowers.

Note that these three possibilities are mutually exclusive.

RED FLOWER WHITE FLOWER

or

Nuclei with one pair of homologous chromosomes.

The alleles are R and r.

Figure 10-1 Chromosomal control of flower color.

R r r r

R R

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Procedure

1. Build the components for two pairs of homologous chromosomes by assembling strings of pop beads as

follows:

a. Assemble two strands of pop beads with 8 pop beads of one color on each arm, with a magnetic centromere

connecting the two arms.

b. Repeat step a, but use pop beads of a second color.

c. Assemble two more strands of pop beads, but with 4 pop beads of one color (used in a) on each arm.

d. Repeat step c, using pop beads of a second color (used in b).

You should have four long strings, two of each color, and four short strings, also two of each color. Each pop-

bead string should have a magnetic centromere at its midpoint by which pop-bead strings can attach to each

other. Each pop-bead string represents a single molecule of DNA plus proteins, with each bead representing a

gene.

2. Place one of each kind of strand in the center of your workspace, which represents the interphase nucleus of a

cell that will undergo meiosis. You have created a nucleus with four “chromosomes,” two long and two short.

The long strands represent one homologous pair, and the short strands represent a second homologous pair of

chromosomes.

We start by assuming that these chromosomes represent the diploid condition. The two colors represent the

origin of the chromosomes: One homologue (color ) came

from the male parent, and the other homologue (color ) came from the

female parent.

3. The four single-stranded chromosomes represent four unduplicated chromosomes. Now simulate DNA

duplication during the S-phase of interphase, whereby each DNA molecule and its associated proteins are copied exactly. The two copies, called sister chromatids, remain attached to each other at their centromeres

(Figure 10-2). During chromosome replication, the genes also duplicate. Thus, alleles on sister chromatids are

identical.

Figure 10-2 One pair of homologous pop-bead chromosomes

One duplicated

chromosome made of

two identical sister

chromatids (one

homologue)

One duplicated

chromosome made of

two identical sister

chromatids (one

homologue)

One pair of homologues

(homologous chromosomes

Centromere

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Questions

1. How many sister chromatids are there in a duplicated chromosome?

2. How many chromosomes are represented by four sister chromatids? By eight?

3. What is the diploid number of your starting (parental) nucleus? (Hint: Count the number of

homologues to obtain the diploid number.)

A. Meiosis Without Crossing Over

Although crossing over is a nearly universal event during meiosis, we will first work with a simplified

model to illustrate chromosomal movements and separations during meiosis. Refer to the diagrams as you

manipulate your model.

Figure 10-3 Meiosis without crossing over.

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spindle tetrad

1. Late Interphase–During interphase the nuclear envelope is intact, and the chromosomes are

randomly distributed throughout the nucleoplasm (semi-fluid substance within the nucleus). Both

duplicated chromosomes (eight chromatids) should be in the parental nucleus, indicating the DNA

duplication has taken place. The sister chromatids of each homologue should be attached by their

magnetic centromeres, but the four homologues should be separate. Your model nucleus contains

a diploid number (2n) = 4.

The pop-bead chromosomes should appear during interphase in the parental nucleus as shown in

Figure 10-3. Be sure to mark the location of the alleles. Use different pencil or pen colors to

differentiate the homologues on your drawings.

2. Meiosis I–During meiosis I, homologues are separated from each other into different nuclei.

Daughter nuclei created are thus haploid.

a. Prophase I–During the first prophase the parental nucleus contains two duplicated

homologous chromosomes, each made up of two sister chromatids joined at their centromeres.

The chromatin condenses to form discrete, visible chromosomes. The homologues pair with

each other. This pairing is called synapsis.

• Chromosomes become visible

• Nuclear membrane disappears and spindle forms

• Synapsis of two homologous chromosomes to forms a tetrad

• Crossing over (exchange of genetic material) occurs

i. Slide the two homologues together. Twist the chromatids about one another to simulate

synapsis. The nuclear envelope disorganizes at the end of prophase I.

nuclear envelope Centromere

Sister chromatids

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b. Metaphase I–Homologous chromosomes now move toward the spindle equator, the centromeres

of each homologue coming to lie on either side of the equator. Spindle fibers, consisting of

aggregation of microtubules, attach to the centromeres. One homologue attaches to microtubules

extending from one pole, and the other homologue attaches to microtubules extending from the

opposite spindle pole.

• Tetrads align on the spindle equator

i. To simulate the spindle fibers, attach one piece of string to each centromere. Then lay the free

ends of strings from two homologues toward one spindle pole and the ends of the other

homologues toward the opposite pole.

c. Anaphase I–During anaphase I, the homologous chromosomes separate, each homologue

moving toward opposite poles. The movement of the chromosomes is apparently the result of

shortening of some spindle fibers and lengthening of others. Each homologue is still in the

duplicated form, consisting of two sister chromatids.

• Homologous chromosomes separate from each other (disjunction)

• Sister chromatids (dyads) move toward opposite poles of cell

i. Pull the two strings of one homologous pair toward its spindle pole and the other toward the

opposite spindle pole, separating the homologues from one another. Repeat with the second

pair of homologues.

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d. Telophase I–Each homologue is now at its respective pole.

• Nuclear membrane reforms and spindle breaks down

• Cytokinesis is completed

• Formation of 2 haploid (1 N) daughter cells

i. Continue pulling the string spindle fibers until each homologue is now at its respective pole.

The first meiotic division is now complete. There should be two nuclei, each containing two

chromosomes (one long and one short) consisting of two sister chromatids.

ii. Draw your pop-bead chromosomes as they appear after meiosis I on the two nuclei labeled

“After Meiosis I” of Figure 10-3.

Depending on the organism involved, an interphase (interkinesis) and cytokinesis may precede the

second meiotic division, or each nucleus may proceed directly into meiosis II. The chromosomes

decondense into chromatin form.

It is important to note here that DNA synthesis does not occur following telophase I (between

meiosis I and meiosis II).

Before meiosis II, the spindle is rearranged into two spindles, one for each nucleus.

3. Meiosis II–During meiosis II, sister chromatids are separated into different daughter nuclei. The

result is four haploid nuclei.

a. Prophase II–At the beginning of the second meiotic division, the sister chromatids are still

attached by their centromeres. During prophase II, the nuclear envelope disorganizes and the

chromatin recondenses.

Daughter cells from Meiosis I

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• Chromosomes become visible (not replicated)

• Nuclear membrane disappears and spindle forms

b. Metaphase II–Within each nucleus, the duplicated chromosome aligns with the equator, the

centromeres lying on the equator. Spindle fiber microtubules attach the centromeres of each

chromatid to opposite spindle poles. Note that each nucleus contains only one duplicated

chromosome consisting of two sister chromatids.

• Dyads align on the spindle equator

i. Your string spindle fiber should be positioned so that the two spindle fiber strings from sister chromatids lie toward opposite poles. Note that each nucleus contains only two duplicated

chromosomes (one long and one short) consisting of two sister chromatids each.

c. Anaphase II–The sister chromatids separate, moving to opposite poles.

• Sister chromatids separate and single

chromatids move to opposite poles

i. Pull on the string until the two sister chromatids separate.

ii. After the sister chromatids separate, each is an individual (not duplicated) daughter

chromosome.

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d. Telophase II–The two daughter chromosomes are at opposite poles. The nuclear envelope re-

forms around each chromosome. Four daughter nuclei now exist. Note that each nucleus

contains one individual chromosome (formerly a chromatid) originally present within the parental

nucleus. Note that each nucleus contains two individual unduplicated chromosomes (formerly a

chromatid) originally present within the parental nucleus. These nuclei and the cells they’re in

generally undergo a differentiation and maturation process to become gametes (in animals) or

spores (in plants).

• Nuclear membrane reforms and spindle breaks down

• Cytokinesis occurs

i. Continue pulling on the string spindle fibers until the tow daughter chromosomes are at

opposite poles.

ii. Draw your pop-bead chromosomes as they appear after meiosis I on the two nuclei labeled

“After Meiosis II” of Figure 10-3. Your diagram should indicate the genetic (chromatid) complement before meiosis and after each meiotic division, note the stages of each division.

Remember that meiosis takes place in both male and female organisms.

Questions about Meiosis (Without Crossing Over)

1. If the parental nucleus was from a male, what is the gamete called?

2. If female?

3. Is the parental nucleus diploid or haploid?

4. Are the nuclei produced after the first meiotic division diploid or haploid?

5. Are the nuclei of the gametes diploid or haploid?

6. What is the genotype of each gamete nucleus after meiosis II? (The genotype is the genetic

composition of an organism, or the alleles present. Another way to ask this question is, what alleles

are present in each gamete nucleus? Write in the format: AFE, afe, and so on.)

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B. Meiosis With Crossing Over

A very important event that results in a reshuffling of alleles on the chromatids occurs during prophase I.

Recall that synapsis results in pairing of the homologues. During synapsis, the chromatids break, and

portions of chromatids bearing genes for the same characteristic (but perhaps different alleles) are exchanged between non-sister chromatids. This event is called crossing over, and it results in combination

(shuffling) of alleles.

Procedure

1. Look again at Figure 10-2. Distinguish between sister and non-sister chromatids. Now look at Figure

10-4 below, which demonstrates crossing over in one pair of homologues.

2. Return your chromosome models to the nucleus format with two pairs of homologues entering

prophase I.

Maternal chromosom

Paternal chromosomes

A pair of duplicated homologous

chromosomes.

Cross over between nonsister chromatids

of the two chromosomes.

Nonsister chromatids exchange segments.

Homologues have new combinations of

alleles.

Figure 10-4 Crossing over in one pair of homologues.

3. To simulate crossing over, break four beads from the arms of two nonsister chromatids in the long

homologue pair, exchanging bead color between the two arms. During actual crossing over, the

chromosomes may break anywhere within the arms.

Crossing over is virtually a universal event in meiosis. Each pair of homologues may cross over in

several places simultaneously during prophase I.

4. Manipulate your model chromosomes through meiosis I and meiosis II again and watch what

happens to the distribution of the alleles as a consequence of the crossing over. Fill in Figure10-5 as

you did before, but this time show the effects of crossing over. Again, use different colors in your

sketches.

es

chiasma

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Figure 10-5 Meiosis with crossing over.

Parental nucleus after crossing over

Two cells after Meiosis I (Anaphase I)

Gamete nuclei after Meiosis II (Anaphase II)

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Questions about Meiosis (With Crossing Over)

1. What are the genotypes of the gamete nuclei?

2. Is the distribution of alleles present in the gamete nuclei after crossing over the same as that

which was present without crossing over?

3. Is the distribution of alleles present in the gamete nuclei after crossing over the same as that

in the nuclei after the first meiotic division?

4. Crossing over provides for genetic recombination, resulting in increased variety. How many

different genetic types of daughter chromosomes are present in the gamete nuclei without

crossing over (Figure 10-3)?

5. How many different types are present with crossing over (Figure 10-5).

6. What is the difference between a gene and an allele?

6. Alleles A and a are present in the parental nucleus, how many are present in the gametes? (This

illustrates Mendel’s first principle, segregation. The Law of Segregation means that during gamete

formation, pairs of alleles are separated (segregated) form each other and end up in different

gametes.)

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C. Independent Assortment Procedure

1. Manipulate your model chromosomes again through meiosis with crossing over (Figure 10-5),

searching for different possibilities in chromosome distribution that would make the gametes

genetically different.

Questions

1. Does this distribution of the alleles for enzyme production to different gametes on the second set of

homologues have any bearing on the distribution of the alleles on the first set (alleles for skin

pigmentation and ear-lobe condition)?

This distribution demonstrates the principle of independent assortment, which states that segregation

of alleles into gametes is independent of the segregation of alleles for other traits, as long as the genes

are on different sets of homologous chromosomes. Genes that are on different (non-homologous)

chromosomes are said to be non-linked. By contrast, genes for different traits that are on the same

chromosome are linked.

Parental nucleus before crossing over

Because the genes for enzyme production and those for skin pigmentation and earlobe attachment

are on different homologous chromosomes, these genes are , while

the genes for skin pigmentation and earlobe attachment are

because they are on the same chromosome.

In reality, most organisms have many more than two sets of chromosomes. Humans have 23 pairs

(2n = 46), while some plants literally have hundreds!

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II. MEIOSIS IN ANIMAL CELLS

In animals, as mentioned previously, meiosis results in the production of gametes, ova in females and

sperm in males.

A. Spermatogenesis in Male Animals

In male animals, meiosis occurs in the seminiferous tubules within the testes. Examine Figure 10-6. A

diploid reproductive cell, the spermatogonium, first enlarges into a primary spermatocyte. The primary

spermatocyte undergoes meiosis I to form two haploid secondary spermatocytes. After meiosis II, four

haploid spermatids are produced, which develop flagella during differentiation into four sperm cells. This

process is called spermatogenesis.

Procedure

1. Examine the demonstration slide of spermatogenesis in animal testes. Under low power, note the many

circular structures. These are the seminiferous tubules, where spermatogenesis takes place.

2. Switch to high power and focus on one seminiferous tubule for closer observation.

3. Mature sperm appear as fine dark lines in the center of the tubule. Cells in progressively earlier stages of

meiosis are seen as you move toward the outer wall of the tubule. Adjacent to the tubule wall, you can

see diploid primary spermatocytes and spermatogonia.

Oogonium (diploid

reproductive cell)

Growth

Growth

Meiosis

I

Meiosis II

Spermatogonium (diploid

reproductive cell)

Primary sporocyte

(diploid 2n)

Secondary

sporocytes

(haploid 1n)

Meiosis I

Meiosis II

Primary oocyte (diploid 2n)

n

Differentiation

Spermatids

(haploid 1n)

Sperm

(haploid

gametes)

OOGENESIS

Mature ovum (haploid

gamete) & 3 polar

bodies

SPERMATOGENESIS

Figure 10-6 Gametogenesis in animals.

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B. Meiosis in Female Animals

In the ovaries of female animals, ova (eggs) are produced by meiosis during the process

called oogenesis (see Figure 10-6). Unlike spermatogenesis, only one of the meiotic products

becomes a gamete.

The diploid reproductive cell, called an oogonium, grows into a primary oocyte. The primary

oocyte undergoes meiosis I, one product being the secondary oocyte, the other a polar body.

Notice the difference in size of the secondary oocyte and the polar body. This is because the

secondary oocyte ends up with nearly all of the cytoplasm after meiosis I.

In humans and other animals, secondary oocytes are released from the ovary. If fertilization

occurs, a sperm penetrates the secondary oocyte, which then continues through meiosis II.

Following meiosis II, only the secondary oocyte becomes a mature, haploid ovum; depending on

the species, the polar body may or may not undergo meiosis II. In any case, the polar bodies are

extremely small and do not function as gametes.

Procedure

1. Examine Figure 10-6, which depicts oogenesis in female animals.

2. Examine the demonstration slides of oogenesis in an animal. Identify the follicles

within which oogenesis begins.

3. Also identify oocytes that may be in various stages of development.

4. Study the following model illustrating oogenesis in the roundworm, an organism that has

only two pair of homologous chromosomes (2n = 4). Meiosis in the primary oocyte does

not begin until a sperm penetrates the cytoplasm. In this model, the oogonium’s nucleus is

intact.

a. Prophase I–The nuclear envelope has disorganized. How many chromatids are there?

How many chromosomes does this represent?

b. Late Metaphase I (or Early Anaphase I)–This is a transition between metaphase I

and anaphase I). During metaphase I, the homologous chromosomes become located

on either side of the spindle equator. The spindle is distinct, the component fibers

seemingly attached to the centrioles. The homologous chromosomes are beginning to

separate. Note the sperm nucleus within the cytoplasm of the primary oocyte.

c. Later Anaphase I (or Early Telophase I)–The homologous chromosomes move

toward opposite spindle poles. Remember, each homologous chromosome consists of

two sister chromatids. The sperm nucleus remains in "lying in wait."

d. Formation of the first polar body–Cytokinesis takes place, separating the

homologous chromosomes. One set of homologues resides in a small cell with

relatively little cytoplasm. This is the first polar body.

Two non-homologous chromosomes (four chromatids) remain in the larger cell, which

is now called the secondary oocyte. A nuclear envelope does not form about these

chromosomes, so essentially the secondary oocyte is in prophase II.

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e. Late Metaphase II (or Early Anaphase II)–Now these sister chromatids of the

chromosomes within the secondary oocyte line up on the spindle equator. A new

spindle with centrioles is present as the sperm nucleus remains in wait. In the

roundworm, the polar body does not undergo meiosis II.

f. Telophase II and cytokinesis–A thin line represents cytokinesis occurring to form the

second polar body. How many unduplicated chromosomes (formerly sister chromatids)

does the mature haploid ovum contain?

g. Fertilization–Fertilization is the fusion of the ovum nucleus with the sperm nucleus.

With fertilization, the large ovum becomes first diploid cell, the zygote. How many

chromosomes does the zygote contain?