CHAPTER 19: CELLULAR MECHANISMS OF DEVELOPMENT
WHERE DOES IT ALL FIT IN?
Chapter 19 applies the concepts of mitosis, gene regulation, and genomics to explain development in
eukaryotic organisms. A good understanding of Chapters 16 and 18 is essential for satisfactory
comprehension of Chapter 19. A brief synopsis of the information from these earlier chapters is
helpful before discussing development. This chapter is helps explain many of the aspects of animal
and plant structure mentioned later in the book.
SYNOPSIS
Multicellular eukaryotes depend on cell specialization and have evolved complex developmental
processes to ensure that the adult is formed properly. Plant development is flexible and highly
influenced by the environment, while animal development is rigidly controlled and much less
dependent on the environment. The ties to the environment between plants and animals are
greatly a result of their mobility. Plants are immobile, they need to work within the strict
confines of a changing habitat. Animals are mobile and can leave their habitat if it is not to their
liking, they do not have to adjust to it to survive in the same manner as plants.
After cleavage of the initial zygote, vertebrate animals go through an involved series of stages of
cell movement and tissue formation. Although a greater number of cells are produced, each cell
is progressively smaller in size and the embryo stays roughly the same size through the
formation of the blastula and the gastrula. Neurulation is unique to vertebrate animals as only
they possess the neural tube formed in this process. Insect development begins before the egg is
even fertilized. The actions of maternal genes control early development since nurse cells
distribute mRNA to the egg. The syncytial blastoderm is formed as a result of nuclear divisions
without production of intervening cell membranes. These two events are paramount in terms of
pattern formation and the development of segment polarity in insects. Insect larvae have a single
purpose, to accumulate nutrients. Upon formation of the hard-shelled pupa, the cells that were
the larva break down and reconstitute themselves into the adult tissues. This metamorphosis is
controlled by the insect’s imaginal disks.
In the first division of a plant, one cell is destined to become the embryo while the other becomes
the suspensor. The embryo differentiates into three germ layers (epidermis, ground tissue, and
vascular tissue), like animals, but there is no ensuing cell migration. Development halts with the
formation of the seed and resumes with germination. The two events may be separated by many
years. The seed enables a plant to be dispersed a great distance from the parent and allows it to
wait out unfavorable environmental conditions that would only end in death. Plants produce
various meristems, which become the components of the adult plant. These meristems are
influenced by hormones, which are, in turn, controlled by the environment.
Cell migration is of great importance in animal development. What a cell becomes is greatly
influenced by where it has been and where it is going. These movements occur as a result of
interactions with cadherins and integrins. Cells can be switched from one developmental path to
another via the process of induction. Inducing cells secrete proteins, intercellular signals that
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determine what kind of tissue a cell will become. The same signal can have different results
through variations in concentration. A cell is said to be determined when it has become
committed to a particular developmental path. Differentiation is the cell specialization that exists
at the end of that path. Therefore, cells can be determined, but not yet differentiated. Partial
commitment may reflect a cell’s location in an embryo and is associated with positional labels.
Until recently, the process of determination was thought to be irreversible. Numerous nuclear
transplant experiments were performed and failed unless the nucleus came from an early,
undetermined embryo cell. A sheep has been successfully cloned using a nucleus from an adult
mammary cell (thus the name “Dolly” for Dolly Parton). The key to this attempt was getting
both donor and recipient cells in the beginning stage of the cell cycle through starvation of the
cell cultures.
Although first discovered in Drosophila, homeotic genes have been found in mice and humans
and similarly functioning genes also exist in plants. Homeotic genes contain a homeobox
sequence of amino acids that ensure certain genes are transcribed at the proper time. These genes
are found in clusters and are aligned in the same order as the segments that they control. The
ordered nature of these gene clusters is highly conserved over the course of evolution. Many
cells produced in the process of development are ultimately destined to die. It may not make sense
in the isolated context of the end result, but there is a logical, progressive reason for it when the
developmental process is examined in greater depth.
It is said that the only certainties in life are death and taxes. Substantial research is being done to
investigate the process of cell aging and death. There are currently four strong hypotheses involved
with four different cell processes. These include accumulated mutations, telomere depletion, cell
wear and tear, and the gene clock hypothesis. This last hypothesis suggests that many aspects of
aging are regulated by genes in much the same way that development is gene regulated.
LEARNING OUTCOMES
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In general terms, compare and contrast plant and animal developmental processes.
Understand the stages of development, from cleavage to organogenesis, in a model vertebrate.
Describe the stages of development in a representative insect from the influence of maternal
genes to metamorphosis.
Explain the stages of development in a typical plant from early divisions through morphogenesis.
Know how cell movement and cadherins are involved in the developmental process.
Understand the process of induction and the importance of cell-cell interactions.
Explain the role of organizers in animal development.
Define and compare determination, differentiation, and commitment.
Explain the reversibility of determination using the cloned sheep Dolly as an example.
Understand pattern formation in Drosophila and the involvement of positional labels and the
syncytial blastoderm.
Know the importance of homeotic genes.
Know why programmed cell death is necessary and differentiate between necrosis and apoptosis.
Describe the four model developmental systems and their representative specimen.
Compare the four major theories of aging.
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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 19 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 all genes program for proteins
Students are unfamiliar with the exact nature of regulatory genes
Students believe that phenotype can be fully by knowing the genotype
Students do not fully understand the role of genetics and environment on determining
observable variation in organisms
Students believe all of development is driven by genes
Students do not understand the roles of growth factors and hormones on development
Students believe that all mutations are deleterious
Students believe that only animals undergo embryological development
Students believe that all stem cells can clone a complete organism
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
Many texts relegate plant development to a minuscule part of one chapter in the midst of the
“plant biology” section. Here plant development is discussed right in context with animal
development making it very easy to compare and contrast the two.
HIGHER LEVEL ASSESSMENT
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 19.
Application
Analysis
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Have students explain the effects of a chemical that stimulates meristems.
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Have students explain why causing a mature cell to become mitotic is
important in stem cell research.
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Ask students to explain the effects of pollutants that inhibit morphogens
in insects.
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Have students to explain how mutations to homeotic genes can affect the
development of an organism.
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Ask students to explain any evidence of segment identity in adult humans.
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Synthesis
Evaluation
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Ask students to assess what would happen during development of an
animal if programmed cell death did not occur.
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Ask students to find a possible medical application for a chemical that
stimulates programmed cell death.
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Ask students explain why certain plants produce chemicals that act as
endocrine hormones in animals.
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Ask students describe an agricultural or commercial use for a chemical
that controls the plane of cell division in plants.
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Ask students to explain the pros and cons of using stem cells to replace
lost body functions.
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Ask evaluate the safety and value of a drug that alters the sequential
activation of genes associated with arm and leg bone elongation.
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Ask student to debate the ethical issues associated with cloning human
beings.
VISUAL RESOURCES
Transparencies, slides, and videos are essential in presenting this chapter. Note the three Insight
Media videos listed in Media Resources: Videos, Part Five: Molecular Genetics.
Douglas Green and Ben Jones have developed two computer simulations associated with this
material entitled Morphogenic Construction Kit and Diffusion Laboratories. The former is
designed to facilitate exploration of classical experiments in biological pattern formation. The
latter combines two simulations, Particle Diffusion and Pattern Formation, which explore
mathematical models of embryological pattern formation. See Media Resources: Computer
Programs, Part Five: Molecular Genetics.
Programmed cell death is like constructing a scaffold while building a house. It’s needed for the
building process, but eventually will be disassembled since it isn’t meant to be incorporated into
the final design of the house.
Think of other analogies using the building trade. Auto and other mechanical analogies don’t
work as well with regard to this topic because most auto companies completely retool for each
new production year. Parts are pretty specific as to their use. With houses you can create an
innumerable amount of different structures from nails and a few two-by-fours. Borrow your kids
Legos™ (or buy sets for labs)!
Along the same lines, there is the old story about cooking a turkey for thanksgiving. The
granddaughter cuts off the turkey’s legs, stuffs it, puts it in the roaster, and slides it in the oven.
When her new husband asks her why she replies, “Mom did it this way, silly, it’s delicious!” One
day he asked his mother-in-law why she cooked turkey in such a unique way and got the same
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answer “My mother did it this way!” Still curious, he finally asked grandmom. Apparently, her
oven wasn’t large enough for the whole turkey and she had to chop off its legs to get it in.
Similarly, developmental processes simply build upon themselves, often with no immediately
apparent reason.
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. Virtual Cloning
Introduction
Clever and accurate animations are an effective tool for teaching the steps involved in
therapeutic cloning. An interactive animation called Interactive Animations can be used to
demonstrate the steps used in cloning vectors for gene therapy.
Materials
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Computer with live access to Internet
LCD projector attached to computer
Web browser with bookmark for Interactive Animations at
http://www.wiley.com/legacy/college/boyer/0470003790/animations/cloning/cloning.htm
Procedure & Inquiry
1. Review the concept of therapeutic cloning.
2. Tell students they will be viewing an animation showing the steps of cloning a gene that
can be used in therapeutic cloning
3. Show the Cloning sequence starting with the DNA digestion through transformation
4. Have the class answer brief questions describing what they saw at each major step
5. Have the class discuss the value of genomic libraries and DNA screening
B. Visual Chromosomes
Introduction
The complexity of information on each human chromosome is amazing to see
particularly in context of the gene regulation that takes place during development. The
Department of Energy provides a website called “Human Chromosome Launchpad” which has
up-to-date cartoon images of the genes on each of the human chromosomes. The website is a
useful demonstration tool for hypothesizing about the regulatory mechanisms needed during
human development.
Materials
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Computer with live access to Internet
LCD projector attached to computer
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Web browser with bookmark to Human Chromosome Launchpad at:
http://www.ornl.gov/sci/techresources/Human_Genome/launchpad/
Procedure & Inquiry
1. Provide students with a brief discussion about the complexity of gene regulation needed
for development.
2. Have the students hypothesize about the organization of genes needed to streamline the
expression of polygenic and pleiotropic traits.
3. Then go to the Human Chromosome Launchpad website.
4. Click on one of the chromosomes and then click on Images to investigate its details and
see an image of the identified genes. The chromosome can be zoomed for viewing using
the instructions on the website. Chromosomes can be printed for students to use in group
work.
5. Take time to ask the students to review particular features including the assortment of
organization of traits of each chromosome.
6. This demonstration can be expanded to a take home activity in which students write a
“resume” for a chromosome.
USEFUL INTERNET RESOURCES
1. Animations are a valuable classroom resource for reinforcing concepts cloning and
organismic development.. The Dolan DNA Learning Center at Cold Spring Harbor
Laboratory provides an animated lesson on cloning. It can be used to supplement a lesson
based on Chapter 19. This website can be found at http://www.dnalc.org/cloning.html.
2. The National Institutes of Health Stem Cell Information webpage is an excellent source
of information on stem cell research and policy for faculty and students. It is a valuable
reference for expounding on the stem cell and cloning information covered in Chapter 19.
The website can be found at http://stemcells.nih.gov/.
3. Stanford University provides a series of embryology animations useful for lectures
related to the topics in this chapter. They can be used as lecture enhancers or points of
classroom discussions. The website is available at
http://www.stanford.edu/group/Urchin/ani-plus.htm.
4. Cases studies are an effective tool for reinforcing the concepts related to cellular
mechanisms of development. The University of Buffalo provides a teaching case study
called “Saving Superman: A Look into Stem Cell Research.” It encourages student to
evaluate the medical value and ethical issues associated with stem cell research. The case
study can be found at http://www.sciencecases.org/superman/superman1.asp.
LABORATORY IDEAS
Plants are excellent models for investigating the roles of hormones, chemical treatments, and
environmental factors on embryological development. This activity asks students to
investigate the effects of an unknown plant hormone on tissue differentiation.
a. Students should be provided with the following materials to perform open-ended
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b.
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experiments on plant development.
a. Covered plant tissue culture tubes purchased from biological supply companies.
b. Murashige and Skoog basal salt macronutrient solid medium purchased from
biological supply companies
i. The medium should be added to the tubes before the activity
c. 70% ethanol in a small beaker
d. 10% bleach with a drop of liquid soap in a small beaker
e. Sterile distilled water in a covered container
f. Scalpel
g. Forceps
h. Sterile metal spatulas
i. Plant rooting powder from a garden supply store or florist (keep in a container
labeled “Plant Hormone” for student use)
j. Rex Begonia leaves with petioles
Tell the students that they will be asked to identify the action of an unknown plant
hormone on differentiation.
Ask the students to design a controlled experiment on Rex Begonia petioles to see how
the hormone influences regeneration of plant tissues.
Students should use the following steps after designing the experiment:
a. Cut a small section of Rex Begonia petiole using the scalpel
b. Use the forceps to dip the petiole into the 70% ethanol solution for one minute
c. Then transfer the petiole to the 10% bleach with soap solution for 20 seconds
d. The petiole can now be placed into the sterile water for one minute
e. Now the petioles can be added carefully to the culture tube in a manner that
reduces contamination from dust and fingers
i. A petiole should be dipped and coated in the rooting powder before being
added to the culture tube.
ii. A control with no rooting powder should be set up
f. Students should place the plants in a warm area out of direct sunlight and observe
the growth over time.
Have students investigate the probable hormone based in their results. Information aout
plants hormones can be found on the Plant Hormones website at http://www.planthormones.info/
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.
1. Have students organize a stem debate for the community on campus with a science club
or a debate team.
2. Have students tutor high school students covering genetics in a biology class.
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3. Have students do a short PowerPoint presentation on cloning for high school teachers.
4. Have students do a display on cloning for the science hallway or building.
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
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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,
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