Transcript Chapter 21
Chapter 21 The Genetic Basis of Development (a) Fertilized eggs of a frog AP Biology (b) Tadpole hatching from egg Embryonic Development Through a succession of mitotic cell divisions The zygote gives rise to a large number of cells In cell differentiation Cells become specialized in structure and function Morphogenesis encompasses the processes That give shape to the organism and its various parts AP Biology Totipotency in Plants Totipotent: able to reverse their differentiation and then give rise to all the cell types in a mature plant. Transverse section of carrot root EXPERIMENT 2-mg fragments Fragments cul- Single cells tured in nutrient free in medium; stirsuspension ring causes begin to single cells to divide. shear off into liquid. Embryonic Plantlet is culplant develops tured on agar from a cultured medium. Later single cell. it is planted in soil. A single RESULTS Somatic (nonreproductive) carrot cell developed into a mature carrot plant. The new plant was a genetic duplicate(clone) of the parent plant. AP Biology Adult plant Nuclear Transplantation in Animals Cloning Is using one or more somatic cells from a multicellular organism to make another genetically identical individual “Copy Cat Was the first cat ever cloned AP Biology Nuclear Transplantation in Animals The nucleus of an unfertilized egg cell or zygote is replaced with the nucleus of a differentiated cell The nucleus of the donor contains all genetic information It should be able to direct development into proper tissues and organs AP Biology Egg cell donor Mammary cell donor 1 2 Egg cell from ovary Nucleus Nucleus removed 3 Cells fused removed Cultured mammary cells are semistarved, arresting the cell cycle and causing dedifferentiation Nucleus from mammary cell 4 Grown in culture Early embryo 5 Implanted in uterus of a third sheep 6 Embryonic development Surrogate mother Lamb (“Dolly”) genetically identical to mammary cell donor Reproductive Cloning of Mammals In 1997, Scottish researchers http://learn.genetics.utah.edu/content/clonin g/whatiscloning/ http://learn.genetics.utah.edu/content/clonin g/clickandclone/ http://www.dnalc.org/view/16992-Cloning101.html AP Biology Problem associated with cloning In most nuclear transplantation in animals thus far Only a small percentage develop normally to birth Frog embryo Donor nucleus transplanted Frog tadpole Fully differentiated (intestinal) cell Less differentiated cell Most develop into tadpoles AP Biology Frog egg cell Enucleated egg cell Donor nucleus transplanted <2% develop into tadpoles The Stem Cells of Animals A stem cell Embryonic stem cells Early human embryo at blastocyst stage (mammalian equivalent of blastula) Is a relatively unspecialized cell Reproduces indefinitely Can differentiate into specialized cells of one or more types, given appropriate conditions Can be isolated from embryo at blastocyst stage Adult stem cells are pleuripotent AP Biology Adult stem cells From bone marrow in this example Totipotent cells Pluripotent cells Cultured stem cells Different culture conditions Different types of differentiated cells Liver cells Nerve cells Blood cells Cytoplasmic Determinants and CellCell Signals in Cell Differentiation Cytoplasmic determinants in the cytoplasm of the unfertilized egg Regulate the expression of genes in the zygote that affect the expression of tissue specific proteins in embryonic cells AP Biology Unfertilized egg cell Molecules of a a cytoplasmic determinant Fertilization Molecules of another cytoplasmic deterZygote minant (fertilized egg) Mitotic cell division Two-celled embryo Nucleus Transcriptional Regulation of Gene Expression During Development Nucleus Master control gene myoD Other muscle-specific genes DNA OFF Embryonic precursor cell 1 Myoblast (determined) 2 Determination. Signals from other cells lead to activation of a master regulatory gene called myoD, and the cell makes MyoD protein, a transcription factor. The cell, now called a myoblast, is irreversibly committed to becoming a skeletal muscle cell. Differentiation. MyoD protein stimulates the myoD gene further, and activates genes encoding other muscle-specific transcription factors, which in turn activate genes for muscle proteins. MyoD also turns on genes that block the cell cycle, thus stopping cell division. The nondividing myoblasts fuse to become mature multinucleate muscle cells, also called muscle fibers. OFF OFF mRNA MyoD protein (transcription factor) mRNA MyoD AP Biology Muscle cell (fully differentiated) mRNA Another transcription factor mRNA mRNA Myosin, other muscle proteins, and cell-cycle blocking proteins