Blank Figures for Labeling c b 1 a L q 6 4 2 m 5 3 d e r 7 n o i f g p j k h Chapter 1 Questions Identify the major animal models we are working with in chapter 2 and what their major strengths are Drosophila they have life cycles of only two weeks and they are cheap and easy to breed in large numbers Embryogenesis is complete in 24 hours one of the best studied model organisms it is the standard model for genetics research Pax6 is found in virtually all animals including humans about 50 of fly protein sequences have mammalian homologues it is used for both forward and reverse genetic screens C elegans Cells are stereotypical the cell s fates are mapped out and they always go the same way every time Very simple organism transparent and the anatomy is completely known All cell lineages are completely known fate mapping Live in soil and feed on bacteria and fungi can be stored frozen They are simpler than fruit flies development is completed in 2 3 days Xenopus African clawed frog aimed at answering the question could the normal developmental fates be altered by manipulation They provided some of the earliest insights into embryogenesis They have large robust eggs and embryos used to understand how different groups of cells are committed to fates Microsurgery on their embryos helped answer the question what effects cells have on their neighbors portions could be grafted from one embryo to another Chick Ease of acquisition and storage for eggs quick development large enough for surgical manipulations and mRNA mediated reverse genetic methods Zebrafish Eggs are fertilized and develop externally useful for surgical manipulations translucent embryos forward genetic screens Mouse Very well studied organism bred for specific phenotypes Pluripotency of cultured stem cells made researchers able to study candidate genes from other models The pluripotency of stem cells implies that these cells were able to generate most maybe all cell types when reintroduced to the embryo Researchers were able to manipulate these cells add or subtract genes then return them to embryos They can insert sequences of DNA using homologous recombination Recombined cells were reintroduced to the embryo to make a chimera some chimeric cells make it to germ cells breeding the chimera gives rise to some transgenics knock outs or knock ins What is a forward genetic screen A forward genetic screen attempts to find variations for a specific phenotype to attempt to identify genes that control that phenotype In fruit flies when mutations are identified that affect the phenotype of interest various crosses with wild type animals and with known mutations to genes that affect the same phenotype are performed This way it can be characterized and identified as affecting the same gene or different genes that affect the same phenotype Furthermore it can be identified as being a different mutation of the same gene or of a different gene by doing complementation tests 1 Pick a phenotype through observation the way it looks and functions 2 Must be specifically defined in a way that we know if something is changed when manipulated Attempt to find variations for a specific phenotype to attempt to identify genes that control that phenotype Identify a control 3 4 Mutate a fly using a mutagen i e radiation chemicals most common type of radiation is concentrated X rays which can damage DNA and cause mutations 5 You have to get an average of one mutation per organism you need the mutation to occur in the gametes because otherwise its offspring won t display the mutation the parent may not necessarily express the mutation RNAi can be used for a backward genetic screen What is a backward genetic screen and how is RNAi used in one Reverse genetics allows the experimenter to manipulate specific genes in a mammalian species so as to learn about their functions For example a normal gene might be replaced with a non functional version to generate a knock out To do this the genome of embryonic stem cells is manipulated in culture The experimenter constructs DNA molecules that have stretches at each end identical to sequences in and or around the gene that is to be mutated and a central portion whose incorporation into the target gene will prevent its functions To enable the embryonic stem cells to take up these DNA molecules they are put into the solution around the embryonic stem cells and a current is passed through the cells In RNAi mediated reverse genetic screens RNAi is introduced into the system to interact with the mRNA and it binds to it thereby preventing the mRNA from associating with the ribosomes because the RNAi has a complementary sequence of the mRNA so they pair together and that is how RNAi blocks the function of a gene RNA interference By blocking the mRNA from being translated we prevent the gene from being expressed Chapter 2 Questions Keep in mind a blastomere is a cell derived from cleavage in an early embryo A blastula is an embryonic structure composed of blastomeres The cavity in the blastula is the blastocoel If the blastula lacks a blastocoel it is a stereo blastula A mammalian blastula is called a blastocyst and the invagination where gastrulation begins in the blastopore What Cells in c Elegans eventually divide into the nervous system AB cells give rise to the ectoderm which then gives rise to the nervous system Define Gastrulation Formation of the three germ layers Ectoderm Mesoderm and Endoderm Mesoderm includes muscles and skeletons Endoderm includes gut and associated organs Ectoderm includes neuroectoderm and gives rise to skin and the brain Describe the process of Gastrulation in c Elegans The blastocoel opens for a very short period of time Cells migrate inward tiny opening forms and it is very brief AB cells will form the nervous system AB cells gradually surround the entire developing embryo Blastopore is very tiny and also very transient Don t need to know this in too much detail Describe the process of Gastrulation in Drosophila Gastrulation begins on the ventral surface of the blastula those cells pinch and get involuted into the inside of the sphere Mesoderm becomes internalized some cells of the ventral ectoderm also internalize to form neuroblasts Then delamination occurs the neuroblasts give rise to mother ganglion cells And then axonal development begins motor and sensory neurons form Overall the blastula forms with a clear blastocoel once the blastula is formed an invagination of cells to form