RAVEN 9/e

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 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

·  Explain the importance of cell division to early development.

·  Describe the use of C. elegans to track cell lineages.

·  Distinguish differences in cell division between animals and plants.

·  Describe the progressive nature of determination.

·  List the ways in which cells become committed to developmental pathways on the molecular level.

·  Differentiate between the different types of stem cells.

·  Define nuclear reprogramming and describe ways in which it has been accomplished.

·  Differentiate between reproductive and therapeutic cloning.

·  Describe A/P axis formation in Drosophila.

·  Describe D/V axis formation in Drosophila.

·  Explain the importance of homeobox-containing genes in development.

·  Discuss the importance of cell shape changes and cell migration in development.

·  Explain how cell death can contribute to morphogenesis.

·  Describe the role of the extracellular matrix in cell migration.

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.

·  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 / ·  Have students explain the effects of a chemical that stimulates meristems.
·  Have students explain why causing a mature cell to become mitotic is important in stem cell research.
·  Ask students to explain the effects of pollutants that inhibit morphogens in insects.
Analysis / ·  Have students to explain how mutations to homeotic genes can affect the development of an organism.
·  Ask students to explain any evidence of segment identity in adult humans.
·  Ask students to assess what would happen during development of an animal if programmed cell death did not occur.
Synthesis / ·  Ask students to find a possible medical application for a chemical that stimulates programmed cell death.
·  Ask students explain why certain plants produce chemicals that act as endocrine hormones in animals.
·  Ask students describe an agricultural or commercial use for a chemical that controls the plane of cell division in plants.
Evaluation / ·  Ask students to explain the pros and cons of using stem cells to replace lost body functions.
·  Ask evaluate the safety and value of a drug that alters the sequential activation of genes associated with arm and leg bone elongation.
·  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 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

·  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

·  Computer with live access to Internet

·  LCD projector attached to computer

·  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.

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.

  1. Students should be provided with the following materials to perform open-ended experiments on plant development.
  2. Covered plant tissue culture tubes purchased from biological supply companies.
  3. Murashige and Skoog basal salt macronutrient solid medium purchased from biological supply companies
  4. The medium should be added to the tubes before the activity
  5. 70% ethanol in a small beaker
  6. 10% bleach with a drop of liquid soap in a small beaker
  7. Sterile distilled water in a covered container
  8. Scalpel
  9. Forceps
  10. Sterile metal spatulas
  11. Plant rooting powder from a garden supply store or florist (keep in a container labeled “Plant Hormone” for student use)
  12. Rex Begonia leaves with petioles
  13. Tell the students that they will be asked to identify the action of an unknown plant hormone on differentiation.
  14. Ask the students to design a controlled experiment on Rex Begonia petioles to see how the hormone influences regeneration of plant tissues.
  15. Students should use the following steps after designing the experiment:
  16. Cut a small section of Rex Begonia petiole using the scalpel
  17. Use the forceps to dip the petiole into the 70% ethanol solution for one minute
  18. Then transfer the petiole to the 10% bleach with soap solution for 20 seconds
  19. The petiole can now be placed into the sterile water for one minute
  20. Now the petioles can be added carefully to the culture tube in a manner that reduces contamination from dust and fingers
  21. A petiole should be dipped and coated in the rooting powder before being added to the culture tube.
  22. A control with no rooting powder should be set up
  23. Students should place the plants in a warm area out of direct sunlight and observe the growth over time.
  24. 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.plant-hormones.info/

LEARNING THROUGH SERVICE