Chapter 21 1 Shared Development Processes a a few fundamental principles are common to all developmental seqs observed in multi cellular organisms 2 Cell Proliferation a Dividing of cells to make more cells b location timing extent of cell division have to be tightly controlled c There undifferentiated cells that keep proliferating throughout an organism s life in plants these cells are grouped into meristems which give rise to structures that develop throughout life stems leaves etc d In animals these cells are called stem cells embryonic cells have give rise to any cell type in the body in adults they are found in areas where constant cell division is necessary 3 Programmed Cell Death Apoptosis a Occurs as certain tissue take shape cells bet toes of chicken must die in order for separate toes to form b Ellis and Horvitz investigated cells that cause apoptosis compared to those that dont uncovered 2 genes essential for apoptosis c When genes causing apoptosis in mice were disrupted result was a malformation of the brain d Normal apoptosis is imp in development of human embryos e inappropriate activation of programmed cell death involved in neurodegenerative diseasees such as Lou Gehrigs Disease 4 Cell Movement or Cell Growth a Many cells have to move to a new location in order for normal development to occur b During gastrulation cells in diff parts of the mass rearrange themselves into 3 layers which give rise to skin gut and other parts of the body all of this comes from mass of similar looking cells that develop early in development c certain animal cells break away and move to new location where they give rise to other kinds of cells d Plant cells are encased in stiff cell walls and dont move changes in direction of cell growth result in proper formation of straight and bent stems leaf veins and other structures e differential cell growth key part of plant development 5 Cell Differentiation a process of becoming a specialized type of cell b initially cells can differentiate into any cell type but most commit to certain function early in development c Some plant cells can change structure and function cells like these are called totipotent cells d Totipotent cells highlight key diff bet animal and plant cells 6 Cell Cell Interactions a During development most important cell cell interactions involve sending and receiving signals b embryonic cells grow move differentiate in response to signals from other cells c Signal Transduction Pathways trigger production of the transcription factors d cell cell signals can change patterns of gene expression and thus the embryonic cells structure and behavior Role of Differential Gene Expression In Development 1 Differentiated Plant Cells Are Genetically Equivalent a Proof cells from stem of cedar tree can de differentiate to form roots cells must contain genes for root cells as well b All plant cells contain the same gene c Processes that control gene expression such as chromatin structure Regulatory transcription factors RNA processing miRNA activity etc get reprogrammed in de differentiation 2 Differentiated Animal Cells Are Genetically Equivalent Draw figure 21 2 a Ian Wilmut and colleagues reported results of nuclear transfer experiment b removed mammary gland cells from pregnant female sheep grew them in culture and fused them w eggs whose nuclei had been removed resulting embryos were placed in uteri of surrogate mothers in one of the transfer attemps Dolly a clone was born c majority of nuclear transplant experiments fail d This has proven that differentiation doesnt involve change in genetic make up of cell but rather results from differential gene expression 3 How Differential Gene Expression Occurs a Transcriptional responsible for gene differentiation only mRNA genes needed for a muscle cell will be produced for a muscle cell b In eukaryotes transcription controlled by presence of regulatory transcription factors 1 Cell Cell Signals Trigger Differential Gene Expression a whether a cell becomes certain kind of cell muscle nerve etc depends on location along 4 axes Organism s stage of development and 3 spatial dimensions Draw 1 One axis runs anterior twd the head to posterior twd the tail 2 One axis runs ventral twd the belly to dorsal twd the back 3 One axis runs left to right 2 Master Regulators Set up the Major Body Axes a Pattern Formation describes events that determine spatial organization of an embryo b Early signals act as master regulators that set up the axes Master Regulators c As growth continues new signals arrive and activate genes that specify finer control over what a cell becomes 3 Discovery of BICOID a Experiment with fruit flies and mutants one mujtation caused embryos missing structures found in anterior end instead anterior end contained structures normally found in the posterior b gene responsible for this phenotype called Bicoid gene which must provide positional info c bicoid gene coded for signal that tells cells where they are located along the anterior posterior body axis 4 The Importance of Concentration Gradients a Nusslein Volhard used Situ Hybridization Draw from figure 21 5 to find where bicoid mRNAs are located in embryos b probes were designed to bind to bicoid mRNA inside the embryo c mRNA for bicoid was found to be highly localized in anterior end d protein product forms steep concentration gradient bicoid protein abundant in anterior medium amount means in between poster and anterior low amount means in posterior 5 Auxin s Role in Plant Development a Bicoid called a master regulator b plants also have master regulator that is a hormone cell cell signal in plants called auxin enters cells and triggers the production of transcription factors that affect differentiation c Auxin produced in meristematic cells at the tip or apex of embryo d If high concentration of auxin in root it means the cell is in the root and etc e molecules that provide spatial information during embryonic development via concentration gradient are called morphagens both bicoid and auxin are morphagens 6 Regulatory Genes Provide Specific Positional Information a Differentiation is a progressive step by step process b Segment a region of organism that contains a distinct set of structures and is repeated along its length c Defects in how body segments are organized attributed to defective segmentation genes d 3 Classes of segmentation genes Draw 1 gap genes expressed first in broad regions along head to tail axis define
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