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Ch21WordLectureOutline - CHAPTER 21 THE GENETIC BASIS OF...

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CHAPTER 21 THE GENETIC BASIS OF DEVELOPMENT Introduction The application of genetic analysis and DNA technology to the study of development has brought about a revolution in our understanding of how a complex multicellular organism develops from a single cell. For example, in 1995 Swiss researchers demonstrated that a particular gene functions as a master switch that triggers the development of the eye in Drosophila . A similar gene triggers eye development in mammals. Developmental biologists are discovering remarkable similarities in the mechanisms that shape diverse organisms. While geneticists were advancing from Mendel’s laws to an understanding of the molecular basis of inheritance, developmental biologists were focusing on embryology. Embryology is the study of the stages of development leading from fertilized eggs to fully formed organism. In recent years, the concepts and tools of molecular genetics have reached a point where a real synthesis has been possible. The challenge is to relate the linear information in genes to a process of development in four dimensions, three of space and one of time. A. From Single Cell to Multicellular Organism In the development of most multicellular organisms, a single-celled zygote gives rise to cells of many different types. Each type has different structure and corresponding function. Cells of similar types are organized into tissues, tissues into organs, organs into organ systems, and organ systems into the whole organism.
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Thus, the process of embryonic development must give rise not only to cells of different types but to higher-level structures arranged in a particular way in three dimensions. 1. Embryonic development involves cell division, cell differentiation, and morphogenesis An organism arises from a fertilized egg cell as the result of three interrelated processes: cell division, cell differentiation, and morphogenesis. From zygote to hatching tadpole takes just one week. Cell division alone would produce only a great ball of identical cells. During development, cells become specialized in structure and function, undergoing differentiation . Different kinds of cells are organized into tissues and organs. The physical processes of morphogenesis, the “creation of form,” give an organism shape. Early events of morphogenesis lay out the basic body plan very early in embryonic development. These include establishing the head of the animal embryo or the roots of a plant embryo. The overall schemes of morphogenesis in animals and plants are very different. In animals, but not in plants, movements of cells and tissues are necessary to transform the embryo. In plants, morphogenesis and growth in overall size are not limited to embryonic and juvenile periods.
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