CHAPTER 10: HOW CELLS DIVIDE
WHERE DOES IT ALL FIT IN?
Chapter 10 begins a new conceptual theme by addressing the cell cycle and replication. It provides
students the principles of asexual reproduction in prokaryotes and eukaryotes. It is important to
briefly review the basic cell structure information in Chapter 4 before proceeding with Chapter 10.
The information in Chapter 10 is crucial for students to understand the principles of sexual
reproduction and embryology covered later in the book.
SYNOPSIS
Cellular division in bacteria is simple since the genome is one double-stranded circle of DNA
attached to the interior of the cell at a single point. Duplication is an enzyme mediated process
that begins at that point and continues around the circle resulting in two side-by-side DNA
circles. Physical division occurs when the cell attains a certain size. New membrane materials are
laid down between the points of attachment of the DNA circles and pinch inward, binary fission.
Eukaryotic cell division is more complicated because the eukaryotic genome is larger and more
complex. Eukaryotic chromosomes are linear structures composed of chromatin, mostly DNA
and protein with a small amount of RNA. Eukaryotic DNA is a long double-stranded fiber.
Every 200 nucleotides it coils around a core of eight histone polypeptides forming a nucleosome.
The string of nucleosomes is further wrapped into supercoils. Heterochromatin is highly
condensed chromatin while euchromatin is relatively uncondensed. Some portions of the DNA
are permanently heterochromatic to prevent DNA expression; the remainder is uncondensed at
the proper time to facilitate transcription.
The number of chromosomes in eukaryotic organisms varies widely from species to species.
Human cells possess a diploid complement of 23 homologous pairs of chromosomes each with a
characteristic appearance. Prior to cell division each homologue replicates producing two
identical sister chromatids joined by a common centromere. The process of growth and division
in a typical eukaryotic cell is called the cell cycle and is composed of five phases. The G1 phase
is the cell’s primary growth phase while the genome is replicated during the S phase. During the
G2 phase, various organelles are replicated, the chromosomes start to condense, and
microtubules are synthesized. All of these are preparatory for mitosis or M phase. Actual cell
division occurs in the final C phase, cytokinesis.
Mitosis is a continuous process that is divided into four stages for ease of examination: prophase,
metaphase, anaphase, and telophase. Much of the preparation for mitosis occurs during
interphase, a collective stage that includes G1, S, and G2. Preparations include chromosome
replication, centriole replication (in animals only), and tubulin synthesis. Chromatin
condensation begins near the end of interphase and continues through prophase when individual
chromosomes become visible. At the same time, the nuclear envelope breaks down and the
centrioles of animal cells move apart. One set of microtubules assembles between the nucleolar
organizing regions while another set grows outward from each centromere toward the poles.
Metaphase begins when the pairs of sister chromatids align across the center of the cell at the
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metaphase plate. The end of this phase is signalled by the division of the centromeres. During
anaphase, each chromatid moves toward the pole to which it is attached. Separation occurs when
the central spindle fibers slide past one another, moving the poles farther apart. The chromatids
also move toward the poles as the microtubules to which they are attached shorten. The nucleus
begins to reform around the uncoiling chromosomes during telophase. The spindle apparatus
breaks down and the nucleolus reappears as rRNA genes are again expressed.
There are significant differences in cytokinesis in animals and plants. Animal cells are pinched in
two by a belt of constricting microfilaments at the cleavage furrow. Rigid plant cells are not
easily deformed and divide from the inside outward. This expanding partition is called the cell
plate. The final addition of cellulose to either side of the membrane results in two separate cells.
Cell cycle control is based on a check-point feedback system. When certain conditions at a
checkpoint are met, the cell proceeds to the next stage of activity or division. Cyclin-dependent
kinases (Cdk’s) and cyclins are intimately associated with these control processes. Unicellular
organisms make independent decisions on whether or not to divide. Multicellular organisms
must limit independent cell proliferation to maintain the integrity of the whole. Eukaryotes
utilize various growth factors to do this. Disruption of these control mechanisms is characteristic
of cancer.
CHAPTER OBJECTIVES
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Discuss the molecular composition of eukaryotic chromosomes and their association with RNA,
histones, and nucleosomes.
Describe the structure of a condensed eukaryotic chromosome and identify the structure that
most accurately indicates the number of chromosomes present in a given cell.
Understand the differences between heterochromatin and euchromatin.
Understand the genetic composition of individual eukaryotic chromosomes, chromosome pairs,
and sex chromosomes.
Identify the five phases of the cell cycle and describe the events that highlight each stage.
Identify the four stages of mitosis and describe the most characteristic events of each stage.
Understand the composition and function(s) of the spindle apparatus.
Understand the purpose of mitosis in terms of the genetic composition of progeny cells and the
survival of a given cell line.
Explain how mitosis differs in plant and animal cells.
Describe the process of cytokinesis in both animals and plants.
Understand how cyclin-dependent kinases and cyclins control the cell cycle normally and in
cancer.
Identify several growth factors and describe how they affect cell division.
CONCEPT MAP
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Concept mapping is a structured graphical presentation of the concepts covered in a particular
topic. The following concept map represents the links between the information covered in this
chapter. It is important to tell students to develop their own concept maps after covering the
particular information covered in class.
<<Attachment>>
COMMON STUDENT MISCONCEPTIONS
There is ample evidence in the educational literature that student misconceptions of information
will inhibit the learning of concepts related to the misinformation. The following concepts
covered in Chapter 10 are commonly the subject of student misconceptions. This information on
“bioliteracy” was collected from faculty and the science education literature.
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Students believe that binary fission is the same as mitosis
Students do not distinguish between the cell cycle and mitosis
Students believe asexual reproduction is restricted to microorganisms only.
Students conceptualize all DNA as being X-shaped
Students do not distinguish between the terms chromatin and chromosomes
Students believe that spindles work like rubber bands during replication
Students are not aware that endosymbionts are attached to spindles
Students are not fully aware that mitochondria and chloroplasts self-replicate
Students believe that asexual reproduction always produces identical offspring cells
Students believe asexual reproduction results in weakness and sexual reproduction
always produces stronger individuals
Students think haploid cells have half the traits needed to make an organism
Students have the idea that cancer is merely a condition of uncontrolled cell division
Students believe that all tumors are cancerous
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
It is more efficient to move by packing your belongings in boxes and bags than to move each
item individually. Similarly, condensing the chromatin into discrete chromosomes makes it
easier to separate them during mitosis.
Remember the order of mitotic stages via PMAT (or IPMAT if interphase is included). Any
student named Matthew deserves apologies on this mnemonic!
Stress that the purpose of mitosis is to produce many identical copies of a cell.
Most students merely memorize when the nucleolus disappears and reappears. If they associate
its presence with its function synthesizing rRNA, it is obvious when the transient organelle will
be present and when it will be absent.
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Higher level assessment measures a student’s ability to use terms and concepts learned from the
lecture and the textbook. A complete understanding of biology content provides students with the
tools to synthesize new hypotheses and knowledge using the facts they have learned. The
following table provides examples of assessing a student’s ability to apply, analyze, synthesize,
and evaluate information from Chapter 10.
Application
Analysis
Synthesis
Evaluation
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Have students explain how drugs that alter cytoskeleton function would
affect mitosis in animal cells.
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Have student explain why food poisoning is likely to occur if foods such
as meats are sitting at room temperature for 30 to 60 minutes.
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Ask students why bacterial infections spread more quickly on the skin
than yeast infections.
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Ask students to explain what would happen to offspring cells if the
centromeres did not separate easily during anaphase.
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Ask students to explain why diabetes, a condition in which glucose is not
taken up readily by cells, slows down mitosis.
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Have students explain how amino acid deficiencies can affect the
progression of the G1 phase of the cell cycle.
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Ask students to think about the properties of a drug that would selectively
harm cancer cells without causing death or injury to normal body cells
undergoing cell division.
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Have students develop a rationale for the use of a chemical that causes
telomeres, the tips of chromosomes, to shorten rapidly during mitosis.
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Ask students come up with a strategy that would inhibit binary fission
without affecting the mitosis of microorganisms.
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Ask students to evaluate the effectiveness of an anticancer drug that
inhibits the formation and growth of blood vessels.
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Ask students to support or debate the claim that nicotine, which affects
cytoskeleton function, reduces the body’s ability to repair damaged body
parts.
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Have the evaluate why using stem cell treatments that replace dead cells
are more likely an effective treatment for repairing brain damage than for
treating wounds to the skin.
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VISUAL RESOURCES
1. Bring in a ball of yarn to simulate DNA as chromosomes and some unraveled yarn to
represent DNA in chromatin form. Question the likelihood of knitting a scarf with the
yarn in a ball. This is like trying to transcribe DNA as chromosomes. Also question the
ease of separating two bunches of identically colored yarn when unraveled as compared
to the same yarn when rolled into two separate balls.
2. In a small classroom, use clay or plastic foam and colored straws to represent
chromosomes. In a large classroom with an overhead projector, cut rod-shaped
chromosomes out of colored acetate. Make a second set to show chromatid replication
during the S phase and hold the two chromatids together with overlapped post-it-note
centromere circles. Cut similar-shaped, but different-colored chromosomes to show
homologues.
3. Use colored beads and two sets of spaghetti to simulate chromosomes and spindle
microtubules in a cell bounded by yarn. The pieces of spaghetti anchored at the poles
push the yarn boundary apart as they slide past one another. Shorten the spaghetti
attached to each chromosome to move the chromosomes to the poles. (One might want to
use string instead of spaghetti, but the latter is more accurate.
4. The DNA content of bacteria can be illustrated using an audiocassette. The cassette
represents a single bacterium. Pulling out all of the tape (without tearing it away from
the cassette) represents the amount of uncoiled DNA in a single bacterium.
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. Name That Phase
Introduction
Laboratory sessions on animal and plant cell mitosis are often confusing adventures for
students. In addition, it is difficult for instructors to troubleshoot every student’s microscope
issues in large laboratory section. This demonstration assists students with recognizing the stages
of mitosis before a laboratory session. It can also be used as a quick review strategy for tests the
ask students to recognize or describe the stages of mitosis.
Materials
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Computer with internet access
Downloaded PDF images found at the Jdenuno website:
http://www.jdenuno.com/PDFfiles/Mitosis.pdf#search=%22mitosis%20images%22
LCD projector
Laser pointer
Procedure and Inquiry
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1. Show the low power image of the onion root tip slide. Ask students to name the structure
and tell if the tissues making up the structure are growing or mature. Have them explain
their answers.
2. Then show the close-ups of the dividing cells and use the laser pointer to select various
cells at different stages of mitosis.
3. Ask the students to identify the stages and explain what features of the cells gave them a
clue to their answers.
4. Show the low power image of the whitefish blastula slide. Ask students to name the
structure and tell if the tissues making up the structure are growing or mature. Have them
explain their answers.
5. Then show the close-ups of the dividing cells and use the laser pointer to select various
cells at different stages of mitosis.
6. Ask the students to identify the stages and explain what features of the cells gave them a
clue to their answers.
B. Modeling Cell Division
Introduction
This fun activity asks students to be model of cell division using various craft and hobby
materials. It reinforces retention of the cell features and cell events involved in binary fission and
mitosis.
Materials
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Small paper plates
Scissors
Assorted dried noodles & spaghetti
Assorted color pipe cleaners
Glue
Colored markers or crayons
Cellophane tape
Wrapping twine
Assorted buttons
Procedure & Inquiry
1. Have students break up into teams of two.
2. Assign them to a particular stage of mitosis or cell cycle
3. Tell them they must make a accurate model of the that stage of
binary fission, mitosis or cell cycle
4. Have the students show the model to the class and explain each
feature including the justification for using a particular craft or
hobby material to represent a cell structure.
USEFUL INTERNET RESOURCES
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1. Case studies are excellent for reinforcing scientific concepts. The University of Buffalo
produced a study using corneal surgery as a means of understanding the role of mitosis in
body maintenance.. This case study can be done in class or be given as a take-home. The
case study can be found at http://www.sciencecases.org/cornea/cornea.pdf.
2. Mature nerve cells are not capable of undergoing mitosis making it difficult to heal
organs in which nerve cells are killed. A case study produced for the University of
Buffalo uses a case study about the implications of brain death. This helps reinforce the
important of mitosis in body healing and maintenance. The case study can be found at
http://www.sciencecases.org/brain_death/brain_death.asp.
3. Cancer is a relevant topic that fosters an interest in knowing the intricacies of cell
replication. A website provided by the Cancer Research UK organization has valuable
information on the role of cell cycle research in understanding and treating cancer. The
website can be found at http://www.cancerhelp.org.uk/help/default.asp?page=85.
4. Animations are a wondeful tool for supplementing lectures on mitosis. The Cells Alive
website provides a useful cell cycle animation for supplementing a lecture on mitosis.
The website can be found at http://www.cellsalive.com/mitosis.htm.
LABORATORY IDEAS
Influencing Mitosis: Onion Roots as a Model
Have students perform a simple long-term experiment using onion root tip growth as a
model for investigating factors that affect mitosis.
a. Tell the class that they will be using onion root elongation as a model for investigating
factors that affect mitosis.
b. They will be growing onions on shallow bowls containing water.
c. Let students know that they can grow the onions under different environmental
conditions or add various chemicals to the water.
d. Provide students with the following materials:
a. Fresh onion with intact roots
b. One shallow bowl for onion
c. Soap water
d. Toothpicks
e. Small metric rulers
f. Access to microscopes
g. Access to microscope slides and cover slips
h. Access to methylene blue
i. Access to water supply
j. Access to incubators
k. Access to refrigerator
l. Chemicals for testing affects on mitosis
i. Nicotine solution (cigarettes soaked in a 100 ml per cigarette solution of
50% V/V alcohol water solution
ii. Broadleaf weed killer
iii. Grass weed killer
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iv. Caffeine – dark coffee of caffeine tablets dissolved in a 100 ml per
cigarette solution of 50% V/V alcohol water solution
v. Plant fertilizer solution
vi. Other chemicals can be selected at the discretion of the instructor or
student
e. Tell the students to carry out the following procedure.
a. Ask the students to use the soap water to gently rinse any growth inhibitors off of
the base of the onion and the roots.
b. Have the students use the toothpicks and bowls to make a set up in which the
onions are suspended over the water. The bottom of the onion must be preserved.
c. Ask the students to design an experiment in which they use mitosis of onion root
tips cells as a indicator of chemicals that inhibit mitosis.
d. Let this experiment run until the roots of a control onion have grown at least 5
cm.
e. Ask the students to explain their results after the period of time it takes the roots
grow 5 cm.
f. Have students explain the mechanism by which the growing conditions or
chemicals specially affect mitosis in the root tip.
LEARNING THROUGH SERVICE
Service learning is a strategy of teaching, learning and reflective assessment that merges the
academic curriculum with meaningful community service. As a teaching methodology, it falls
under the category of experiential education. It is a way students can carry out volunteer projects
in the community for public agencies, nonprofit agencies, civic groups, charitable organizations,
and governmental organizations. It encourages critical thinking and reinforces many of the
concepts learned in a course.
Students who have successfully mastered the content of Chapter 10 can apply their knowledge
for service learning activities in the following ways:
1. Have students do a presentation about the biology of cancer to scout groups or
elementary school students.
2. Have students design prepare an electronic presentation of cell division for school
teachers.
3. Have students tutor middle school or high school biology students studying cell
replication.
4. Have students work with a cancer awareness organization at a health fair.
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This project is funded by a grant awarded under the President’s Community Based Job Training Grant as implemented by the U.S.
Department of Labor’s Employment and Training Administration (CB-15-162-06-60). NCC is an equal opportunity employer and
does not discriminate on the following basis:
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against any individual in the United States, on the basis of race, color, religion, sex, national origin, age disability, political
affiliation or belief; and
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against any beneficiary of programs financially assisted under Title I of the Workforce Investment Act of 1998 (WIA), on
the basis of the beneficiary’s citizenship/status as a lawfully admitted immigrant authorized to work in the United States, or
his or her participation in any WIA Title I-financially assisted program or activity.
This workforce solution was funded by a grant awarded under the President’s CommunityBased Job Training Grants as implemented by the U.S. Department of Labor’s Employment
and Training Administration. The solution was created by the grantee and does not
necessarily reflect the official position of the U.S. Department of Labor. The Department of
Labor makes no guarantees, warranties, or assurances of any kind, express or implied, with
respect to such information, including any information on linked sites and including, but not
limited to, accuracy of the information or its completeness, timeliness, usefulness, adequacy,
continued availability, or ownership. This solution is copyrighted by the institution that
created it. Internal use by an organization and/or personal use by an individual for noncommercial purposes is permissible. All other uses require the prior authorization of the
copyright owner.
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