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York University (12,350)
Biology (939)
BIOL 1001 (162)
Tamara Kelly (110)


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York University
BIOL 1001
Tamara Kelly

2.3 - Cell-Cell signals Trigger Differential Gene Expression • Suppose that you were one of the 100s or 100s of cells in a developing animal embryo • Your fate- whether you end up as a part of an arm or a kidney would depend on 4 axes 1) Anterior - toward the head & Posterior - toward the tail 2) Ventral- toward the belly & Dorsal - toward the back 3) One axis runs left to right ★Cell-cell signals tells cells where they are in time and space. This information activates transcription factors that turn specific genes on or off, resulting in differentiation Master Regulators Set up the Major body Axes • Biologists use the term pattern formation to describe events that determine the spatial organization of an embryo • If a molecule signals that a target cell is in the embryo’s head or tail or dorsal side or ventral side, that molecule is involved in pattern formation • Genes activated by these master regulators send signals with more specific information about the cells “physical location” • As growth continues, the process repeats: new signals arrive and activate genes that specify finer and finer control over what a cell becomes The Discovery of Bicoid • The mutant embryo were missing all of the structure normally found in the anterior end • Instead, the anterior end contained some structures normally found in the posterior • The gene responsible for this phenotype was dubbed bicoid, meaning two tailed • Based on this phenotype, Volhard and Wieschaus suspected that the bicoid gene’s product must provide positional information • In other words, they hypothesized that the bicoid gene coded for a signal that tells cells where they are located along the anterior- posterior body axis The importance of concentration gradients When Volhard’s group treated eggs and early embryos with labelled copies of a probe that • bound to bicoid mRNA, they found that hte mRNA to be highly localized at the anterior end • Follow-up work showed that when these mRNAs are translated, the protein product forms a steep concentraition gradient: Bicoid protein is abundant in the anterior end but declines to progressively lower concentrations in the posterior end • Bicoid protein is a regulatory transcription factor • It binds to DNA and activates genes required for the formation of anterior structures • In effect, a high concentration of Bicoid is a signal that says “You’re in the head region” • A medium concentration means “you’re in the middle” and a low concentration means “you’re in posterior”. Auxins role in plant development • To capture Bicoid’s role in development of a fruit fly embryo, biologists refer to it as a master regulator The idea is that Bicoid gets the cell differentiation process underway extremely early in • development, by proving information on where the cells are in the body • Plants also have a master regulator. Instead its a hormone: a signaling molecule that travels through the body of the organism and acts on distant target cells • In plant embryos, the cell-cell signal called auxin enters cells and triggers the production of transcription factors that affect differentiation • Auxin is produced in meristematic cells at the tip, or apex of the growing embryo - what will become the top of the stem and is transported towards the base - what will become the top of the stem - and is transported toward the base - what will become the root • In the process, a concentration gradient forms. A high concentration of auxin is a signal that means “your near the top of the shoot” ; when • auxin accumulates at the root it signals “you’re near the bottom of the root” • In both plants and animals, molecules that provide spatial information during early embryonic development via concentration gradient are called morphogens ★ Bicoid and auxin are morphogens that have a fundamental impact on early development ★ They provide cells with information about their position along the anterior-posterior or apical-basal body axis • Other initial signals are found in concentration gradients that tells the cell where they are along the other body axes Regulatory Genes Provide increasingly Specific positional Information • Along with the “two posteriored monsters” they named bicoid mutants, Volhard and Wieschaus found an array of embryos that had normal anterior - posterior patterning but defects in how their body segments became organized later in development • A segment is a region of an animal body that contains a distinct set of structures and is repeated along its length. The mutants had defective segment
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