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 developmento 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 typeso Meristems can give rise to various structures that develop throughout liveo 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 expansiono 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 developmento If plant cells can de-differentiate to form other plant parts, each cell must contain the genes required by all different types of plant cellso 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 majorbody 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 organizationof 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 motherso Then a fertile genetically identical clone of parent sheep was borno 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 axiso 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 asa 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 hormoneo 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 ofstructures 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 segmentso 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 eachtype of segment Expressed in distinctive pattern along the anterior-posterior axis After segments are established, these genes code for regulatorytranscription 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
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