Topic 6 - anterior-posterior axis formation PDF

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Kevin Friesen ...

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Topic 6: Anterior-Posterior Axis Formation Molecular Parsimony -

Conservation of molecules that affect development o Simplest explanation is the most likely one

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Orthologues  homologous sequences across species

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Paralogues  homologous sequences within the same species; result of a duplication event

Bilaterians and Body Plan -

Hom-C complex emerged 550 mya in common bilaterian ancestor o Establishes A-P/L-R axes o Necessary for segmentation to occur

Hox Genes/Homeotic Selector Genes -

Encode for body regions

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Hox selectors contain highly conserved homeobox sequence which code for the homeodomain  functional end of the gene that binds to specific enhancers

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Insects o Activated by segmentation genes o 8 genes on one chromosome

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Vertebrates o >8 genes on 4 chromosomes

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Expressed in embryo according to position on chromosome o 3’ end has more anterior hox genes

Homeotic Genes Activate Gene Cascades -

Selector genes = 1st type of gene activated (e.g. hox gene)

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Regulator gene = slightly more specific in what they are activating (e.g. TFs or PFs)

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Realisator genes = function genes (e.g. particular enzymes)

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Result or process (e.g. morphogenesis)

Fly Homeotic Genes legs -

Antennapedia complex o Contains genes that give thoracic segment their identities (AntP) 

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Ectopic expression of AntP = legs growing where they should not

Bithorax complex o Ubx is responsible for the identity of the 3rd thoracic segment 

Reduced expression of Ubx = 2nd set of wings develop (2nd thoracic segment w/ wings)

Segmentation in Vertebrates -

Hox genes determine the type of vertebrae in particular location of spine

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Knockout mutants lack specific vertebrae that are specific to particular hox genes

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Molecular modules vs. anatomic modules o Molecular = group of molecules that work towards the development of anatomical modules o Anatomical = distinct anatomical structure forming dependently/as a unit

Early Morphogens -

Help establish expression patterns of hox genes

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Gradient present o E.g. Wnts, BMPS, and FGFS decrease as they near the anterior portion due to mRNA being degraded

Retinoic Acid -

Derivative of vitamin A

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Bind to nuclear receptors to regulate gene transcription

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In vertebrates, it is high in the posterior cells and low in anterior cells

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Signalling molecule

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Excess RA = posterior phenotype is extended...