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Biology II Development ? ? Muscle Intestinal Nerve EGG Blood Development • How do we go from an egg to an adult? • How do cells of one type change or produce cells of another type? • How is a cell lineage constrained? • How does the genetic code cause these changes? OUTLINE 2 II. Cell Differentiation A. What do we know? B. The developmental landscape model C. Two hypotheses for how cells become differentiated 1. Nuclear alteration 2. Nuclear differentiation D. Experimental evidence 1. Seward’s totipotent carrots 2. Gurdon - nuclear transplantation in toads Waddington’s Developmental Landscape Undifferentiated cell Differentiated cells muscle lung brain skin Nuclear Alteration (progressive loss) Which One? Nuclear Differentiation (differential expression) Fig. 21.5 Seward’s carrot experiment Fig. 21.6 Nuclear transplantation 2% successful Fig. 21.6 Nuclear transplantation Serial transfers of nucleus 17% successful Issues related to totipotency Stem cell research Dolly and “mom” Noah - an endangered guar Fig. 21.8 New York Times: November 21, 2007 “Scientists Bypass Need for Embryo to Get Stem Cells Two teams of scientists reported yesterday that they had turned human skin cells into what appear to be embryonic stem cells without having to make or destroy an embryo — a feat that could quell the ethical debate troubling the field. All they had to do, the scientists said, was add four genes.” Place DNA modified by addition of four genes into skin cells – appear to act as stem cells. But added genes are cancer genes Clonal reproduction is not an evolutionary novel concept Some plants and animals do so naturally Clonal reproduction in plants …and animals Chordata Cnidaria Echinodermata Development Nuclear Differentiation has been supported by experimental evidence, Nuclear Alteration has been rejected. Development occurs because the same genetic blueprint is expressed in different ways in different cells and at different times. Development In order to understand the process of development, we must understand what controls the expression of genes (how genes are turned on and off). OUTLINE 3 III. Control of Gene Expression in Prokaryotes A. Regulatory proteins B. The operon model C. Examples 1. the lac operon (substrate induction) 2. the tryp operon (end product repression) 3. the lac operon (positive control)