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Development AP Biology Big Questions: 1. How does a multicellular organism 2. 3. 4. 5. develop from a zygote? How is the development of an animal different or similar from the development of a plant? How is the position of the parts of an organism determined? How does differentiation of cell type occur in animals and plants? How are genetics and development connected? Environment? Essential Knowledge: Timing and coordination of specific events are necessary for the normal development of an organism, and these events are regulated by a variety of mechanisms. AP Biology Gamete formation Fertilization Four processes of development: Cleavage • Determination sets the fate of Gastrulation the cell. Organogenesis • Differentiation is the process by which different types of cells Growth arise. • Morphogenesis shapes differentiated cells into organs, etc. • Growth is an increase in body size by cell division and cell expansion. A program of differential gene expression leads to the different cell types in a multicellular organism Embryonic development--fertilized eggs give rise to different cell types Tissuesorgansorgan systemswhole organism Gene expression orchestrates development; zygoteadult results from cell division, cell differentiation, morphogenesis and then growth (a) Fertilized eggs of a frog (b) Newly hatched tadpole Fertilization Formation of zygote – union of male & female gametes Purpose? Variation/creates a diploid cell Triggers development of the egg Cytoplasmic Determinants and Inductive Signals Unfertilized egg—uneven distribution of RNA, proteins, other substances Cytoplasmic determinants—molecules in oocyte that influence early development – Zygote dividescells have different determinantsdifferential gene expression Another source of developmental info--environment (e.g. signals from nearby embryonic cells) Induction--signal molecules from embryonic cells cause transcriptional changes in nearby target cells Interactions between cells induce differentiation of specialized cell types blastomere-cytoplasmic determinants distributed/ create first “totipotent” cells Cleavage Embryo divides into cells called “blastomeres” Hollow ball forms of somatic cells called a blastula These cells are also called germ cells (totipotent stem cells!!!) **NO growth – just cell division Morula….Blastula Gastrulation “conversion of blastula into germ layers” Invagination – blastula forms horseshoe shape -Gastrocoel forms (stomach cavity) -Blastopore forms (“germ hole”) Gastrula stage forms 2 germ layers: Endoderm & Ectoderm (incomplete gut) “Diploblastic” Comb Jelly Gastrulation: Formation of complete gut Cavity extends through animal (endoderm and ectoderm layers join) Gastrulation: Formation of Mesoderm (3rd germal layer) Usually formed by pieces of endoderm breaking away Animals are termed “Triploblastic” Triploblastic animals can be Protostomes (blastopore forms mouth first) or Deuterostomes (blastopore forms the anus 1st and mouth is formed 2nd) Endoderm – inner layer forms the digestive system, inner lining of respiratory system Ectoderm – outer layer forms the skin and nervous system Mesoderm – middle layer forms muscle, bone, reproductive system, circulatory system, etc. Acoelomate-no body cavity Pseudocoelomatefalse body cavity Coelomate-true body cavity (cavity lined with mesoderm) Organogenesis Cell differentiation Formation of coelom (cavity) Body organs form Development is Regulated The development of an organism is coordinated by sequential changes in gene expression. One Cell Organism Cellular Differentiation Accomplished by the expression of cell typespecific proteins. Stem Cells Transcriptional Regulations Determination vs. Differentiation Determination sets the fate of the cell. Differentiation is the process by which different types of cells arise. Sequential Regulation of Gene Expression During Cellular Differentiation Myoblasts—make muscle-specific proteins, form skeletal muscle cells. MyoD--“master regulatory genes”; makes proteins that commit cell to becoming skeletal muscle (50+) MyoD—TF--binds to enhancers of target genes Pattern Formation Cells need to establish their position in the developing organism. This is established through protein gradients. Embryonic Induction Local signals communicated among populations of cells to control their development Apoptosis “Programmed cell death”: Important role in defining borders and openings in the developing organism. Environmental Cues The presence of particular molecules and conditions in the local environment is required for development to proceed properly. Ex. The role of temperature and moisture in seed development. Experimental Evidence Mutations in normal developmental pathways lead to malformations in embryonic development Experimental Evidence Transplantation Experiments: Moving regions of a developing embryo affects normal pattern formation. Essential Knowledge: Interactions between external stimuli and regulated gene expression result in specialization of cells tissues and organs. AP Biology Control of Differentiation Differentiation is regulated by internal and external cues. These cues “switch” specific genes “on” and “off” at specific times. Turning On AND Turning Off Genes must be both turned on and turned off at appropriate times and locations in the developing organism. Transcription factors, and micro RNA’s both function in regulating gene expression. Pattern Formation: Setting Up the Body Plan/Axis Establishment Pattern formation-development of spatial organization of tissues and organs—studied in fruit flies Animal pattern formation starts with formation of major axes Positional information-molecular cues that control pattern formation- gives cell location relative to the body axes and neighboring cells The Life Cycle of Drosophila Head Thorax 0.5 mm Pre-fertilization--cytoplasmic determinants determine axes After fertilization--embryo develops into segmented larva with 3 larval stages Edward Lewis, Christiane NüssleinVolhard, and Eric Wieschaus--Nobel 1995 Prize--decoding pattern formation in Drosophila discovered that homeotic genes direct the developmental process; studied segment formation – discovered embryonic lethals with lethal mutations – found 120 genes essential for normal segmentation Antenna Abdomen Dorsal BODY Anterior AXES Left Right Posterior Ventral (a) Adult Follicle cell 1 Egg cell Nucleus developing within ovarian follicle Egg cell Nurse cell Egg shell 2 Unfertilized egg – Depleted nurse cells Fertilization Laying of egg 3 Fertilized egg Embryonic development 4 Segmented embryo 0.1 mm Body segments Leg Wild type 5 Larval stage Leg Hatching Eye (b) Development from egg to larva Mutant Hox Genes Hox Genes A family of related genes that serve as master regulators of animal development in all animals on the planet. AP Biology Internal Cues Various transcription factors must be present inside a cell to allow for specific genes to be expressed. Transcription factors can be either stimulatory or inhibitory. External Cues External cues signal to cells, causing cells to alter their gene expression. Differentiation leads to Divergence As cells differentiate, the proteins that they express commit them to particular “fates”. These fates are normally irreversible. Don’t Forget The Environment! The environment of the cell can also play a major role by contributing to gene expression in mature cells.