Chapter 20 Summary
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Core Concepts Summary
20.1 In the development of humans and other animals, stem cells become progressively more restricted in their possible pathways of cellular differentiation.
The fertilized egg can give rise to a complete organism. page 400
At each successive stage in development, cells lose developmental potential as they differentiate. page 401
Embryonic stem cells can give rise to any of the three germ layers, those further along in differentiation can form only a limited number of specialized cell types, and those still further along can form only one cell type. page 401
Stem cells play a prominent role in regenerative medicine, in which stem cells—
20.2 The genetic control of development is a hierarchy in which genes are activated in groups that in turn regulate the next set of genes.
Hierarchical gene control can be seen in fruit fly (Drosophila) development. page 404
The oocyte of a fruit fly is highly polarized, with gradients of maternal mRNA that set up anterior–
These gradients in turn affect the expression of segmentation genes in the zygote, including the gap, pair-
The segmentation genes direct the expression of homeotic genes, key transcription factors that specify the identity of each segment of the fly and that are conserved in animal development. page 408
20.3 Many proteins that play key roles in development are evolutionarily conserved but can have dramatically different effects in different organisms.
Many proteins important in development are similar in sequence from one organism to the next. Such proteins are said to be evolutionarily conserved. page 410
The downstream targets of homeotic genes are different in different animals, allowing homeotic genes to activate different developmental pathways in different organisms. page 410
Although animals exhibit an enormous diversity in eye morphology, the observation that the proteins involved in light perception are evolutionarily conserved suggests that the ability to perceive light may have evolved once, early in the evolution of animals. page 410
The Pax6 transcription factor is a master regulator of eye development. Loss-
20.4 Combinatorial control is a developmental strategy in which cellular differentiation depends on the particular combination of transcription factors present in a cell.
By analyzing mutants that affect flower development in the plant Arabidopsis, researchers were able to determine the genes involved in normal flower development. page 412
The ABC model of flower development invokes three activities (A, B, and C) present in circular regions (whorls) of the developing flower, with the specific combination of factors determining the developmental pathway in each whorl. page 413
20.5 Ligand–
Cell signaling involves a ligand, an extracellular molecule that acts as a signal to activate a membrane receptor protein, which in turn activates molecules inside the cell. page 415
Activation of a receptor sets off a pathway of signal transduction, in which a series of proteins in the cytoplasm become sequentially activated. page 416
Because signal transduction can amplify and expand a developmental signal, a single ligand–
An example of signal transduction in development is differentiation of the nematode vulva, which is determined by means of an EGF ligand and receptor. page 416
Self-Assessment
Distinguish among totipotent, pluripotent, and multipotent stem cells, and give an example of where you would find each type of cell.
Self-Assessment 1 Answer
Totipotent cells can give rise to a complete organism and can be found in a fertilized egg, before it develops into a blastocyst. Pluripotent cells are able to give rise to any of the three germ layers, and therefore to any cell of the body, but cannot give rise to an entire organism. Embryonic stem cells are pluripotent and can be found in the inner cell mass of the blastocyst. Multipotent cells are further along in differentiation than pluripotent cells and can form a limited number of specialized cell types. The cells of the mesoderm are multipotent, and differentiate into skin, muscle, bone, and red blood cells, and can be found in the gastrula.
Explain how an individual’s own cells might be used in stem cell therapy.
Self-Assessment 2 Answer
Stem cell therapy for an individual might someday consist of taking that person’s own adult cells and reprogramming them into pluripotent and multipotent stem cells. These cells could be programmed to replace burned tissue, defective heart muscle, regenerate nerve cells, and so on. The possibilities of this technology are great.
Draw a diagram to illustrate how a concentration gradient of a transcription factor along the anterior–
posterior axis of a Drosophila embryo can create a region in the middle in which transcription of a target gene takes place without being transcribed in either the anterior or posterior region. Self-Assessment 3 Answer
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Expression of a homeotic gene in the wrong tissue in Drosophila results in the development of an inappropriate body part from that tissue. Explain why this happens and how it shows that homeotic genes are positive regulators of developmental pathways.
Self-Assessment 4 Answer
A homeotic gene is a gene that specifies the identity of a body part or segment during embryonic development. The gene activates downstream genes in a developmental pathway, and therefore is a positive regulator of the pathway. This is why expression in the wrong tissue results in development of an inappropriate body part from that tissue.
Explain why master regulatory genes tend to be more strongly conserved in evolution than are the downstream genes they regulate.
Self-Assessment 5 Answer
Some genes are evolutionarily conserved because they are master regulatory switches that turn on a whole pathway of development. While the downstream genes may be free to evolve in different ways in different organismal lineages, the master gene at the beginning needs to be conserved in order to activate the downstream genes.
Define combinatorial control in the context of the ABC model of floral development.
Self-Assessment 6 Answer
Combinatorial control describes the regulation of gene transcription according to the mix of transcription factors in the cell. In the floral development ABC model, “A” transcription factors are involved in the development of whorl 1 and 2, “B” transcription factors are involved in whorl 2 and 3, and “C” transcription factors are involved in whorl 3 and 4 development. In whorl 3, for example, both sets of transcription factors (“B” and “C”) have to be present for the formation of the stamen of the flower.
Diagram a pathway of signal transduction including a ligand, receptor, and ultimately a transcription factor that activates a gene that inhibits the receptor.
Self-Assessment 7 Answer
