Experiment 4: Eukaryotic Cell Divisions - · PDF fileExperiment 4: Eukaryotic Cell Divisions Learning Objectives: ... position your slide so you have 70-100 cells in your field of

Biol 2281, Spring 2016

E4: Eukaryotic Cell Divisions

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Experiment 4: Eukaryotic Cell Divisions

Learning Objectives:

1. Understand the functional importance of mitosis and meiosis.

2. Describe the different phases of the cell cycle and the main characteristics of each phase.

3. Name and describe the phases of mitosis.

4. Identify cells in various steps of mitosis when they are viewed under a microscope.

5. Name and describe the phases of meiosis.

6. Compare the processes and end products of mitotic and meiotic cell divisions

7. Simulate the division process using the modeling kits.

The following material is adapted from the Student Guide that accompanies “Modeling Mitosis and Meiosis Kit”, with

permission by Carolina Biological Supply Company.

Overview

In animal and most plants, body cells are diploid cells (2n) because they contain two sets of

chromosomes in pairs. The two members of a chromosome pair are called homologous chromosomes

and they contain similar genetic information. A cell with only one set of chromosomes is haploid (n).

Each organism contains a characteristic number of chromosomes in its diploid cells. For example, onion

has 16 (8 pairs), and humans have 46 (23 pairs). Gametes of these organisms are always haploid with

half the total number of chromosomes found in body cells.

In eukaryotic cells, mitosis and meiosis are the means by which genetic information, the DNA

encoded in threadlike structures called the chromosomes, is passed from one generation of cells to the

next. In mitosis, the nucleus of a diploid or a haploid cell divides. The result of mitosis is two cells that

are genetically identical, with the same number of chromosomes as the parent cell. In meiosis, the

nucleus of a diploid cell, containing a complete set of chromosomes, divides twice. Genetic information

is exchanged between homologous chromosomes. The result of meiosis is four genetically diverse

haploid cells, called gametes, each with half the number of chromosomes of the parent cell. The terms

mitosis and meiosis are used to describe the orderly process of segregating and distributing the

replicated chromosomes to the new cells. Cytokinesis means the division of the cytoplasm and it occurs

at the end of the cell division.

In terms of the life of a cell, mitosis and meiosis are crucial but relatively brief part of an

ongoing process known as the cell cycle. The longer period between cell divisions is called interphase.

During interphase, the cell grows, replicates its chromosomes, and produces and assembles the cellular

structures needed for cell division. Interphase is a very busy time in the life of the cell and it is

comprised of three parts: the G1, S and G2 phase.

G1 Phase: occurs immediately following mitosis. It is a growth period. Cells that will not enter

mitosis remain in the G1 stage and carry out their normal functions.

S phase: is for DNA synthesis stage during which DNA replication occurs.

G2 phase: is a second growth stage during which structures directly involved in mitosis, such

as spindle fibers, are synthesized.

For most of the cell cycle, chromosomes are dispersed throughout the nucleus of a cell. During

chromosome replication, however, chromosomes thicken and become tightly coiled structures that can



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be observed under a light microscope. Each replicated chromosome consists of two sister-chromatids

joined at the centromere.

Mitotic Cell Division

For unicellular and a few multicellular organisms, mitotic cell division serves as a means of

reproduction. In higher level eukaryotic cells, it serves as a means of growth and repair. Millions

of new cells are formed in the human body each day in this manner.

The process of mitosis is usually viewed in four phases: prophase, metaphase, anaphase and

telophase/cytokinesis.

A. Prophase

During this stage, each chromosome condenses,

rendering the duplicated chromosomes visible as threadlike

structures. A mitotic spindle forms within the cell, outside the

nucleus, but it is not visible during prophase. The mitotic

spindle helps to separate the identical chromatids later in

mitosis. The cell’s nuclear membrane begins to break down as

the cell transition from prophase to metaphase, allowing the

spindle to attach to each of the chromatids. At about the same

time, the nucleoli disappear.

B. Metaphase:

When the nuclear membrane is no longer distinct, the

cell is in metaphase. This brief phase is characterized by the

chromosome lining up in the center of the cell along an

equatorial plane called the metaphase plate. The spindle

fibers (spindle apparatus) made of microtubules have become

attached to the chromosomes at the kinetochores, groups of

proteins that form the outer faces of the centromeres.

C. Anaphase:

Anaphase begins as the microtubules on both sides of

the chromosome pair pull on the attached chromatid. This pull

separates the sister chromatids at the centromere and begins

the movement of the chromatids to opposite ends of the cell.

The chromatids can now be referred to as daughter

chromosomes. At the end of anaphase, each pole of the cell

has an equal and complete collection of daughter

chromosomes

.



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D. Telophase and Cytokinesis:

In telophase, the cell elongates even more. A nuclear

envelope begins to form, from fragments of the parent cell’s

nuclear membrane, around each set of chromosomes at the

poles of the cell. The nucleolus reappears and the

chromosomes become less condensed and less apparent.

Usually cytokinesis begins prior to the end of telophase,

indicating the end of mitosis. In animal cells, a cleavage

furrow forms during cytokinesis. In plant cells, the rigid cell

wall prevents the formation of a cleavage furrow; instead, a

cell plate forms to separate the parent cell into two daughter

cells, and a new cell wall forms along the cell plate.

Materials per student: one Microscope (Pick up microscope according to your

sign-in number)

one book of lens paper, one bottle of Lens cleaner, one

box of Kim wipes

A box of prepared slides

For today’s activity 3, 4, & 5, obtain one modeling kit

and return the kit back to original container as soon as

you finish each activity.

Lab Activity 1: Mitosis in Onion Root Tip Cells

Cell divisions in plants are, for the most part, localized in specialized regions called meristems. Plants

have two types of meristems: apical and lateral. Found at the tips of stems and roots, apical meristems

contain cells capable of repeated cell divisions and they serve to increase plant organs in length. Lateral

meristems, located beneath the bark of woody plants, increase girth. In this assignment, you will

examine the prepared slides of apical meristem of onion roots.

1. Obtain a slide of a longitudinal section of an Allium (onion) root tip. First focus with the 4X

objective of your compound microscope to get an overall impression of the root morphology.

Then use the 10X objective to scan the apical meristem. Note that most of the nuclei are in the

interphase.

2. Switch to the 40X objective, focusing on a single interphase cell. Note the distinct nucleus, with

one or more nucleoli, and the chromatin dispersed within the bounds of the nuclear envelope.

Draw a typical interphase cell (Part 1, question 1) as it appears on your slide. Use the poster in

lab to help you identify different phases.

3. Find cells in prophase, metaphase, and telophase. Note the cell plate. Draw each phase on your

worksheet/report (Part 1, question 1)

4. Using 40X objective, position your slide so you have 70-100 cells in your field of view. Observe

the phase of each cell and record the totals in the table provided. (Part 1, question 2) This will



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give you an estimate of the percentage of cells in each phase. The frequency of occurrence of a

cell cycle phase is directly proportional to the length of a phase. You can therefore estimate the

amount of time each phase takes. The length of the cell cycle for onion root tip cells is

approximately 24 hours.

Lab Activity 2: Mitosis in Whitefish Blastula

The rapidly dividing cells of whitefish blastula, an early fish embryo, are excellent for studying mitotic

division in animals. With several important exceptions, mitosis in animals is remarkably like that in

plants.

1. Obtain a prepared slide of whitefish blastula. Locate a section for study with the 4X

objective. Then switch to the 10X objective and later to 40X objective to find mitotic phases.

2. Locate a cell in interphase. Note the absence of a cell wall. Note the presence of the nucleus.

Find a prophase cell. The first obvious difference between mitosis in plants and animals is

found in prophase. Whitefish cells contain centrioles (not present in plants). Centrioles

duplicate during interphase. During prophase, one new and one old centriole migrates to each

pole. They may be too small to be seen clearly with your light microscope; however, you can

see a starburst pattern of spindle fibers that appear to radiate from the centrioles.

3. Find a metaphase, an anaphase, and a telophase cell. Also note the cleavage furrow formation

during cytokinesis. Draw prophase and metaphase on your worksheet (Part I, question 3).

Lab Activity 3: Simulate Mitosis Using the Modeling Kit

1. From the chromosome model set labeled with “Mitosis”, obtain four different colored

chromosomes and one piece of yarn. The yarn represents the cell membrane. Place the four

chromosomes inside the cell membrane. This model represents a cell with two pairs of

chromosomes.

2. Modeling chromosome duplication in interphase. Create a duplicate of each chromosome by

attaching a matching chromosome (marked by the same color) to each original chromosome.

The point of attachment represents the centromere that joins the two sister-chromatids.

3. Modeling metaphase: Line up the replicated chromosome end-to-end in the center of the cell.

4. Modeling anaphase: disconnect the identical sister-chromatids from one another at the

centromere and move each chromatid away to the opposite sides of the cell.

5. Modeling cytokinesis: Lift the loop “membrane” and cross it over itself to create a figure

eight. Compare the two sides of the figure eight. Each side should possess the same number

of chromosomes and be identical to the original cell you started with in step 1.

6. Answer Part I, question 4 on your worksheet.

7. Answer Part I, question 5, 6, 7 on your worksheet.



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Meiotic Cell Division

The names of the stages of meiosis and some of the processes involved in meiosis are similar to those in

mitosis, but meiosis has a dramatically different purpose. Meiosis occurs in sexually reproducing

organisms to create sex cells that contain half the number of chromosomes of the parent cell. These sex cells are called gametes in animals and spores in plants. They are haploid cells with increased

genetic variation. When maternal and paternal gametes fuse during fertilization, the offspring of these

organisms then contain a complete set of chromosomes.

Meiotic division consists of two consecutive division, meiosis I and meiosis II, but only one

chromosome replication. Interphase takes place prior to prophase I and is much like the interphase prior

to mitosis: the cell grows, replicates its chromosomes, and prepares itself for cell division. Meiosis I has

five phases: prophase I, metaphase I, anaphase I, telophase I and cytokinesis.

A. Prophase I

During prophase I, the homologous chromosomes come together to form a tetrad, a grouping of

four chromatids. An important step in maintaining genetic variation takes place within the tetrad: along

the lengths of the homologous chromosomes, genes from non-sister chromatids may cross over one

another, exchanging genetic material. This crossing-over mixes maternal and paternal DNA, ensuring



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that daughter cells produced by meiosis I will be genetically different from each other and from the

original parent cell.

B. Metaphase I and anaphase I

During metaphase I, the tetrads line up in the center of the cell along the metaphase plate.

Spindle fibers from each pole of the cell attach to one pair of sister chromatids in each tetrad. In

anaphase I, the homologous chromosomes are separated and pulled to opposite sides of the cells. This is

another important difference between mitosis and meiosis. In mitosis, the spindle pulls one sister

chromatid to opposite side of the cell, but in meiosis I, the spindle pulls a pair of sister chromatids to

opposite sides of the cell.

C. Telophase I and Cytokinesis

These two phases complete the meiosis I resulting in two daughter cells with half the

number of chromosomes in each cell. Each daughter cell is a haploid (n) and is ready to enter

meiosis II.

D. Meiosis II

During meiosis II, the sister-chromatids in each of the two daughter cells move toward the

metaphase plate, and are eventually separated much like what happens in mitosis. Because each cell

entering meiosis II forms two daughter cells, a total of four haploid cells are produced from one diploid

parent cell.

Lab Activity 4: Simulate Meiosis and Crossing-over Using the Modeling Kit 1. Using the chromosome model set labeled with “crossing over”, assemble a cell that has two

autosomes and two sex chromosomes (short) as shown below. The two autosomes have

attached chromosomes on the top, one pink and one orange fragment. Place the small circles

or triangles representing different genes (E and T) on each autosome. The color difference of

the same gene represents different alleles of the gene. (orange circle: E, pink circle: e;

orange triangle: T, pink triangle: t)

2. Obtain two pieces of yarn to make loops. These loops represent cell membranes. Place one

loop around the four chromosomes you have assembled in the previous step, and that will be

your starting parent cell. Set aside the other loop for later.

Figure 4-1: The parent cell



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3. Create a duplicate of each autosome and sex chromosome by connecting matching

chromosome for each at the centromere. Make sure that each replica possesses the same

alleles as the original. Remember to attach fragments to the replicated autosomes. This

simulates which phase of the cell cycle? _________________

4. Prophase I: Line up the homologous chromosomes side-by-side. Each side-by-side pair

forms a tetrad, a paired chromosome structure comprised of four chromatids. Crossing-over

takes place during prophase I of meiosis. Two non-sister chromatids within a tetrad become

close enough to overlap each other and exchange similar pieces of genetic information.

Crossing-over is one of several mechanisms that help ensure genetic variability within a

species. To model crossing-over, exchange the chromosome fragments on the adjacent non-

sister chromatids of the autosomal tetrad. The following figure shows you how it should

look like after crossing over.

5. Metaphase I through Cytokinesis I: Place the tetrads in the center of the cells. Place the

other loop beside the loop containing the chromosomes. Place one chromosome from each of

the two tetrads in the second loop. This represents the completion of the first cellular

division in meiosis. You should now have a model of two daughter cells.

6. As the two daughter cells enter meiosis II, the chromosomes do not go through replication

again. Instead, the sister-chromatids in each chromosome will separate. Simulate this process

by separating the connected sister-chromatids and moving them into two poles of a dividing

cell. This represents the separation of chromosomes during ______________ of meiosis. At

the same time lift one loop “membrane” and cross it over itself to create a figure eight.

Repeat this procedure with the other loop. This represents the division of two cells into four

cells during telophase II and cytokinesis of meiosis II. You should now have four gametes,

each with one autosome and one sex chromosome as shown in Figure 4-3. Show your TA

what you have modeled.

Figure 4-2: The tetrads after crossing-over



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7. Answer Part 2, question 1 in Worksheet/Report.

Lab Activity 5: Simulate Independent Assortment of Genes Located on Different

Chromosomes During Meiosis Using the Modeling Kit.

If two genes are located on two separate pairs of chromosomes, they will segregate

independently of each other during meiosis I. This is due to the random arrangement and separation of

chromosomes during meiosis, giving rise to all possible combinations in equal frequency. Watch the

animation on http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

1. In the modeling kit labeled with “Independent assortment”, you have two pairs of

chromosomes. Assume that one pair of chromosomes contains the genes (A) for flower color,

and another chromosome pair contains the genes (B) for seed color. Also assume that A is

dominant over an allele, and B allele is dominant over b allele.

2. Assemble a set of chromosomes representing a plant that is heterozygous for both flower

color and for seed color.

3. As this plant is producing pollen (male gametes), what are the possible gene combinations

for flower color and seed color in the gamete populations? Use the modeling kit to simulate

independent assortment during meiosis. Answer Part 2, question 2 in Worksheet/Report.

4. Without using the modeling kit, answer Part 2, question 3 in Worksheet/Report.

Figure 4-3: Four gametes at the end of meiosis

http://www.bio-medicine.org/Biology-Dictionary/chromosomes/

http://www.bio-medicine.org/Biology-Dictionary/meiosis/

http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html



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Total Points: 20pts Your Name:___________________

Note: You do not need a title sheet for this report Section: __________________

E4: Cell Division Worksheet/Lab Report

Part 1: Mitotic cell division (13 pts) 1. Lab Activity 1: Draw onion root tip cells in the following stages as they appear on your slide.

(2 pts)

Interphase Prophase Metaphase Telophase

2. Lab Activity 1: Time duration of interphase and mitotic phases in onion root-tip cells. (2 pts)

Phase Number of cells % of all

cells

counted

Duration of

the Phase

Interphase Prophase Metaphase Anaphase Telophase

number of cells of

all phases

The length of cell cycle in actively

dividing onion root tip is about 24 hours.

3. Lab Activity 2: Draw whitefish blastula cells in the following stages. (1 pt)

Prophase Metaphase



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4. Lab Activity 3: Mitosis modeling kit (3 pts)

a. Is this cell a diploid cell or a haploid cell? What is the “n” number?

b. In this modeling process, you used one piece of yarn representing cell membrane. However, the

membrane that actually contained the DNA during interphase is not simulated. Name that

membrane.

c. If the total amount of DNA in G2 phase is 100 picograms, what would be the amount of DNA

in picograms in each of the daughter cell at the end of mitosis?

5. Describe how mitotic cell division in plant cells differs from that in animal cells. (1 pt)

6. Write the term that matches each description: (2 pts)

a) Division of the cytoplasm ____________________________

b) Members of a chromosome pair ____________________________

c) Stage of interphase during which chromosomes replicate______________________

d) The apparatus that pull on the sister chromatids ______________________________

7. Write the mitotic phase that matches each of the following description: (2 pts)

a. Nuclear membrane and nucleolus disappear ____________________________

b. New nuclei are formed ____________________________

c. Sister chromatids are separated ____________________________

d. Chromosomes lined up at the center of the cell ____________________________

Part 2: Meiotic cell division (7 pts) 1. Lab Activity 4:

Suppose that the circular shaped gene has two alleles: “E” (orange colored circle) and “e” (pink

colored circle), and the triangularly-shaped gene has two alleles, “T” (orange colored triangle) and “t”

(pink colored triangle). At the end of your simulated meiosis II in Lab Activity 4, describe the

genotype of the parent cell and the genotypes of each of the four gametes regarding these two genes.

(2 pts)

i. Parent cell: _____________ (refer to Fig.4-1)

ii. Gamete 1: _____________ (refer to Fig. 4-3 for gametes)

iii. Gamete 2: _____________

iv. Gamete 3: _____________

v. Gamete 4: _____________



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2. Lab Activity 5: Independent Assortment (3 pts)

a. Draw the cell containing two chromosome pairs with heterozygous genotype for both the A

(flower color) gene and B (seed color) gene. Draw what you have assembled in step 2. Be sure

to clearly label the alleles.

b. During metaphase I, there are two possible ways for the two tetrads to be aligned at the center of

the cell leading to different assortment of alleles. Draw these two layouts (with metaphase plate).

Possibility 1 Possibility 2

c. Write down the genotypes of all possible gametes. What would be the percentage of each type?

Genotype of gametes

Percentage



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3. Diagram the arrangement of chromosomes in a cell undergoing meiosis as they would appear in the

phases noted below, if the diploid (2n) chromosome number is 8 (2n=8). Note: It is not possible to

use the modeling kit to simulate this question and it is not necessary to label any alleles on the

chromosomes. (2 pts)

Meiosis I: Metaphase I (draw metaphase plate) Meiosis I: Anaphase I

(For the following, draw what happens in only one of the two daughter cells produced at the end of

meiosis I)

Meiosis II: Metaphase II Meiosis II: Anaphase II

Biol 2281: E4 Cell Division

Dr. Elizabeth Pickett & Dr. Wen-ju Lin,

Spring 2016 1

Experiment 4: Eukaryotic Cell Divisions

Part I: Mitotic Cell Division

• What is mitosis? – Process that distributes identical sets of genetic

materials into daughter cells

• Why? – In unicellular organisms: asexual reproduction – In multicellular organisms, cell division allows

• growth and development from the zygote • tissue renewal and repair

• How? – Interphase (DNA replication) – Mitotic phase

The Eukaryotic Cell Cycle

* Duration of the phase: proportional to the

frequency of occurrence of a cell in that phase

Eukaryotic Cell Cycle

• Interphase: the non-dividing phase; with intense biochemical activity during which the cell grows and copies its chromosomes in preparation for the division

– G1 phase: first growth phase

– S phase: DNA duplication phase

– G2 phase: second growth phase

• Mitotic phase: dividing phase • prophase

• metaphase

• anaphase

• Telophase and cytokinesis

Important Terms

• Chromatin

• Chromosome

– Sister-chromatids

– Centromere

– Kinetochore

– Mitotic spindle

• Haploid (n) organism

• Diploid (2n) organism

– Homologous chromosomes

Gene Loci

Dominant allele

Recessive allele

A

a b

b

D

D

Genotype : Aa, bb, DD

Heterozygous:

Homozygous:

Homologous

chromosomes



Spring 2016 2

Prophase • Nuclear membrane breaks

down

• Chromosomes condense

• Mitotic spindles begin to

form between centrioles

• Chromosomes lined

up in the center of the

cell (metaphase plate)

• Spindle fibers attached

to kinetochores on the

chromosomes

Metaphase Anaphase

• Sister-chromatids separate;

• Daughter chromosomes begin to move to

the opposite poles

Telophase

• The cell elongates

• Chromosomes reach poles of

cell

• Nuclear membrane re-forms

• Nucleolus reappears

• Chromosomes decondense

Cytokinesis in animal and plant cells

Part II: Meiotic Division • What is Meiosis?

– A special type of cell division that produces haploid cells (gametes)

– # of chromosomes reduced by half: one DNA replication and two successive divisions

– Involves gene recombination: each gamete is unique

Overview of Meiosis

• Interphase • Meiosis I: segregates

homologous chromosomes and reduces the # of chromosome by half – prophase I – metaphase I – anaphase I – telophase I and cytokinesis

• Meiosis II: separates sister chromatids of each chromosome – prophase II – metaphase II – anaphase II – telophase II and cytokinesis



Spring 2016 3

The stages of meiotic cell division: Meiosis I Crossing Over

• Tetrad Formation during prophase I

• Exchange of genetic material between non-sister chromatids: – Homologous

recombination

• In humans, average of 2 or 3 crossovers per chromosome pair

Animation at:

http://www.sumanasinc.com/webcontent/animations/content/inde

pendentassortment.html

How Much Variation Can

Independent Assortment Cause?

• In humans, 2^23 = 8,388,608 possible

combinations of chromosomes for each

gamete.

The stages of meiotic cell division: Meiosis II Unique Features of Meiosis

• Reduction Division – one DNA replication followed by two cell division – Maintain a species’ chromosome count

– Significantly contribute to genetic variation among offspring

• Contribute to Genetic Variation Among Offspring – Crossing over – Independent Assortment

• Meiosis produces four daughter cells genetically different from the parent cell and from each other



Spring 2016 4

Chromosome Number

and DNA Amount

Diploid

cell

After

interphase

After

mitosis

After

meiosis I

After

meiosis

II

Haploid

cell

(gamete)

# of chromosomes

2n

Amount

of DNA X

picograms

picograms

picograms

picograms

picograms

picograms

The Human Example:

Diploid

cell

After

interphase

After

mitosis

After

meiosis I

After

meiosis

II

Haploid

cell

(gamete)

# of chromosomes

46

Amount

of DNA 6

picograms

picograms

picograms

picograms

picograms

picograms

E4 Activities • Quiz

• Onion root-tip prepared slide

• Whitefish blastula prepared slide

• Modeling mitosis and meiosis

• Modeling crossing-over and

independent assortment

• Practice finding anaphase cells

• Complete worksheet/report

questions. Report must be

submitted before you leave.

Sample Quiz Questions • What is the purpose of mitosis?

• What is the purpose of meiosis?

• Which phase of cell cycle tends to take the longest amount of time?

What occurs during this time?

• What is the difference between sister chromatids and homologous

chromosomes?

• A diploid cell has a total of 24 chromosomes. What is the n number?

• Reduction of chromosome number occurs in ______.

• Name two mechanisms that generate genetic variation during

meiosis.

• How do you calculate the number of possible chromosome

combinations in gametes due to independent assortment?

• Name one difference between animal cell division and plant cell

division.

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Experiment 4: Eukaryotic Cell Divisions - · PDF fileExperiment 4: Eukaryotic Cell Divisions Learning Objectives: ... position your slide so you have 70-100 cells in your field of