Chapter 22 Organism s Development Five essential cellular processes that lead to organism s development o Cell division proliferation Divide make more cells necessary for an individual to develop from an undifferentiated mass of cells Location timing and extent of these cell civisions are tightly controlled by interacting layers of regulation Most cells stop proliferating at maturity but some specialized undifferentiated cells continue proliferating throughout organism s life in plants these cells are called meristems in animals they re stem cells o Cell Cell Interactions During development the most important cell cell interactions involve sending receiving signals Cell cell signals change patters of gene expression and are essential for changing cell activity during development o Cell Differentiation Most cells differentiate in order to become a specialized type of cell Differentiation is a progressive process cells are initially determined or committed to a specific developmental pathway and later becomes differentiated Many plant cells can de differentiate even after they ve specialized Animal cells can de differentiate in cloning experiments Plant meristems and animal stem cells don t become specialized adult cells Instead they remain undifferentiated and therefore retain ability to divide and gives rise to an array of specialized cell types o Meristems can give rise to various structures that develop throughout live o Cell movement Expansion In animals during gastrulation cells in different parts of an early embryo rearrange themselves into three distinctive types of embryonic tissue that later form specific organs Because of plant cell walls plant cells don t move but changes in orientation of cell division control the direction of subsequence cell proliferation and cell expansion o Programmed cell death apoptosis Carefully regulated Abnormal apoptosis can lead to disease or deformation Differential Gene expression Expression of different genes in different cell types the key to cell differentiation during development o If plant cells can de differentiate to form other plant parts each cell must contain the genes required by all different types of plant cells o Occurs during transcription RNA processing translation and post translation Transcription is fundamental level of control in differential gene expression In eukaryotes transcription is controlled primarily by presence of proteins called regulatory transcription factors o Chemical signals trigger differential gene expression Fate of a cell depends on timing current developmental stage of organism its spatial location where it is in the organism s body Spatial location in early development is determined by 3 major body axes anterior posterior ventral dorsal and left right Chemical signals tell cells where they are in time space Activates transcription factors that turn specific genes on off results in differentiation pattern formation series of events that determine the spatial organization of an embryo Some early signals act as morphogens setting up major body axes of embryo This activates a network of genes that sends signals with more specific information about the spatial location of cells As development proceeds a series of signals arrive and activate genes that specify finer and finer control over what a cell becomes Mutant embryos helped two scientists identify 100 genes that played fundamental roles in pattern formation Animal Cloning Mammary gland cells from an adult sheep were fused with enucleated eggs resulting embryos were implanted into surrogate mothers o Then a fertile genetically identical clone of parent sheep was born o This shows that cellular differentiation doesn t involve changes in genetic makeup of cells but instead results from differential gene expression Bicoid Gene Codes for a signal that tells cells where they re located along the anterior posterior body axis o In situ hybridization Process to determine the location of the bicoid mRNA in the egg they found that bicoid mRNA was highly localized in anterior of the egg The bicoid protein is made from mRNA in the anterior end and diffuses away from that end of the embryo Produces a steep concentration gradient from the anterior to the posterior end o The bicoid protein is a regulatory transcription factor showtn to act as a master regulator within drosophila embryos The concentration gradient formed by bicoid proteins provide cells with information about their position along the anterior posterior axis Bicoid also turns on genes responsible for forming anterior structures and absence of bicoid contributes to the formation of posterior structures Morphogen in plants is not transcription factor but rather a hormone o In plant embryos the cell cell signal called auxin enters cells and triggers production of transcription factors that affect differentiation Like bicoids auxins work by forming concentration gradients Regulatory genes provide increasingly specific positional information A Segment is a distinct region of an animal body that contains a distinct set of structures and is repeated along its length o In fruit flies and other animals segmentation genes organize cells tissues into distinct segments Gap Genes early in development gap genes define broad regions that often span several segments Pair rule genes Later in development demarcate individual segments Segment polarity genes Still later these delineate regions within individual segments o After segmentation genes have established the identity of each segment along the anterior posterior axis development continues with activation of Hox genes Hox gene products identify each segment s structural role Triggers development of structure that are appropriate to each type of segment axis Expressed in distinctive pattern along the anterior posterior After segments are established these genes code for regulatory transcription factors that trigger the production of segment specific structures some drosophila mutants have a segment that has been transformed into another segment with its associated structures This homeosis occurs when cells get incorrect information about where they are in the body Regulatory Cascade Interactions among bicoid and segmentation genes Morphogens Trigger the production of other regulatory signals and transcription factors which trigger production of another set of signals and regulatory proteins One singular level in the cascade includes bicoid gene gap genes pair
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