October26 - Biology 442 Developmental Biology Lecture 2...

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Biology 442 Developmental Biology Lecture 2 Anterior-Posterior Pattern Formation in the Drosophila Embryo II
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Learning objectives for this class Organize the different events involved in anterior-posterior patterning in sequential order and name the main genes implicated Relate the different mechanisms employed to pattern the embryo back to the developmental stage when they occur Predict how loss of other patterning genes perturb anterior-posterior patterning Explain how cis and trans regulatory elements interact to regulate gene transcription during development Describe how regulatory elements can be identified using transgenic animals expressing reporter gene constructs Memorize the key steps involved in Hedgehog and Wnt signaling
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Cell fate commitment along the A-P axis is a two step process Cell fate commitment in Drosophila involves 2 steps: specification and determination - Specification : loose commitment; ie, cells can be altered in response to signals from one another. Specification is mediated by the maternal genes. - Determination : unlike specification, this is irreversible. Mediated by segmentation genes
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Cad Bcd Nos hkb gt hb Kr kni hkb eve runt hairy ftz odd en Maternal Gap Pair rule Segment polarity Genetic hierarchy during A-P patterning in Drosophila
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Segmentation genes
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Segmentation genes: Gap, Pair rule and Segment Polarity • Segmentation genes are TFs that use the gradients established by maternal effect genes during early cleavage to transform the embryo into periodic parasegmental units • Mutations in segmentation genes cause embryos to lack certain segments or parts of segments. Often these mutations affect parasegments : regions of the embryo that are separated by mesodermal thickenings and ectodermal grooves • The segmentation genes divide the embryo into 14 parasegments.
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Relation between segments and parasegments Parasegments do not become the segments (Ma, Mx, Lb, T1-T3 and A1-A8) of the larva or adult. They are shifted one compartment forward in relation to segments.
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Gap genes • Gap genes include: hunchback , giant, kruppel and knirps • These genes encode DNA-binding proteins that can activate or repress the transcription of other gap genes • Gap genes divide the embryo into broad regions and mutations in gap genes cause gaps in the segmentation pattern Example: kruppel: expressed in parasegments 4-6, which are missing in kruppel mutants
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