Lecture 21 - HMB265: Human & General Genetics Lecture...

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Unformatted text preview: HMB265: Human & General Genetics Lecture 21: Using Genetics to Understand Development Prof. Maria Papaconstantinou Lecture Outline Model organisms Reverse genetics Forward genetics Drosophila development Reading Griffiths et al., 10th edition, Chapter 13 (emphasis on pages 449-470). See animation link at: http://www.dnaftb.org/dnaftb/37/concept/index.html YOU Two-cell stage human embryo HOW? Regulated gene expression Model organisms are used to study development Why use model organisms? Easy to grow Rapidly reproduce Small size Large collections of mutants with developmental defects maintained in stock centers Genes and pathways are conserved Genomes sequenced easier for geneticists to identify genes with mutant phenotypes Is information gained useful to understand human development? Yes! All living forms are related Hypomorphic or null mutations in eyeless Hypomorphic or null mutations in Pax-6 Evolution has conserved basic strategies of development across multicellular eukaryotes. eyeless, Pax-6 and Aniridia (humans) genes are homologous Pax-6 and Aniridia wild-type genes can direct eye development in Drosophila highly conserved Hartwell et al. (2007) Genetics: From Genes to Genomes Individual with 1 defective copy of Aniridia heterozygote Aniridia is the loss of iris (only 1 defective copy) 2 defective copies lead to complete failure of the entire eye formation -- fetuses die Genetic techniques used to study development Reverse genetics genotype-->phenotype Lindsay, M. (2003) Nature Reviews Drug Discovery 2: 831-838 Genetic techniques used to study development Forward genetics phenotype-->genotype Identify mutated gene Lindsay, M. (2003) Nature Reviews Drug Discovery 2: 831-838 Model organism: Drosophila Relationship between adult & embryonic body axes 3rd class: Pair-rule genes act at every 2 segments. Mutations lead to missing parts of each pair of segments. E.g. Even-skipped gene Odd-skipped gene 4th class: segment-polarity genes affect patterning within genes. 5th class: Hox genes 5 classes of genes are required for proper organization of the anteroposterior body axis. 1st class consists of the maternal effect genes, and sets up the anteroposterior axis. Embryos from Bicoid mutant mothers are missing anterior region of the embryo. E.g. Bicoid gene 2nd class: gap genes affect the formation of a contiguous block of segments. Mutations leads to large gaps in segmentation. Genes required for anteroposterior pattern formation Christiane Nüsslein-Volhard Eric Wieschaus Edward B. Lewis Nobel prize in Physiology or Medicine 1995: for their discoveries concerning “the genetic control of early embryonic development” See animation link at: http://www.dnaftb.org/dnaftb/37/concept/index.html http://nobelprize.org/nobel_prizes/medicine/laureates/1995/press.html Screening for Drosophila Mutants maternal-effect genes: genes with products provided by the female to the egg depend only on the genotype of the female Classes of Drosophila segmentation-gene mutants Summary of Embryonic Patterning Bicoid mRNA in anterior translated into proteins. TLL Griffiths (2005) Introduction to Genetic Analysis Expression of anteroposterior-axis-patterning proteins Kruppel: gap gap genes are required for a continuous block of segments Hairy: pair-rule 7 stripes in total, 1 stripe every 2 segments Engrailed: segment polarity Many of the identified genes code for transcription factors or components of signaling pathways Gap genes are activated by specific maternally-provided proteins Hunchback (gap gene) is one of the targets of Bicoid (maternal effect). Multiple regulatory regions need to be occupied to have a strong effect Gene targets of Bicoid Bicoid activates expression of the zygote's hunchback gene Bicoid also represses translation of the caudal mRNA =posterior Therefore, Caudal protein only appears at the posterior end and defines that end Bicoid has the properties of a DNA-binding transcription factor, regulates gap genes. Expression pattern of maternal-effect & gap proteins Set combinations of concentration of proteins. Combinations of maternal-effect and gap proteins control individual pair-rule stripe formation Combinations of maternal-effect and gap proteins control individual pair-rule stripe formation Kruppel and Giant are repressors Bicoid & Hunchback are activators Analysis of cis-acting regulatory elements with reporter genes Modular organization of regulatory DNA of eve gene Summary of Embryonic Patterning TLL Griffiths (2005) Introduction to Genetic Analysis Genes required for anteroposterior pattern formation Christiane Nüsslein-Volhard Eric Wieschaus Edward B. Lewis Nobel prize in Physiology or Medicine 1995: for their discoveries concerning “the genetic control of early embryonic development” See animation link at: http://www.dnaftb.org/dnaftb/37/concept/index.html http://nobelprize.org/nobel_prizes/medicine/laureates/1995/press.html Homeotic mutants of Drosophila melanogaster Clustering of these Hox genes had arisen by tandem duplications of an ancestral gene. All 8 Hox genes are similar enough to hybridize, because there is a short region in each gene = homeobox The protein domain that homeobox encodes = homeodomain Hox proteins are DNA binding proteins that control the expression of genes within developing segment and appendages. Hox genes encode transcription factors. Hox genes are expressed in spatially restricted domains Hox genes regulate the identity of body parts Hox genes are clustered on 2 gene complexes located on 3rd chromosome of drosophila. Bithorax complex: 3 Hox genes Antennapedia complex: 5 Hox genes The order of these genes corresponds to the order of body parts. No Hox genes: Segments can form, but they all have the same identity. Limbs can form, but they are all antennae. Wings can form, but they are all forewings. Hox genes control the identity. Other genes control formation of segments. Hox genes are found throughout the animal kingdom Mouse Hox genes are clustered into 4 complexes. Each cluster contains 9 - 11 Hox genes. The order of genes in the mouse Hox complexes parallels the order of their counterparts in the fly Hox complexes. This indicates a deep, common ancestry of all bilateral animals. Hox genes regulate the identity of serially repeated structures in vertebrates Serially reiterated structures are repeating parts of similar structures: fore wings and hind wings, segments, antennae , legs, mouthparts. Hox mutations transform identities within these sets. Mammalian homologues exist for several of these genes Evx-1 - patterning of ectoderm and mesoderm Pax-3 - neural and mesodermal patterning Osf/Cbfa - bone development ...
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This note was uploaded on 04/03/2012 for the course HMB 265 taught by Professor French during the Fall '09 term at University of Toronto.

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