Chapter 1 Questions 1 Identify the major animal models we are working with in chapter 2 and what their major strengths are Invertebrate model organisms C Elegans The nematode worm lives in soil and feeds on bacteria and fungi Major strengths Can be stored frozen Development complete in 2 3 days Transparent anatomy has been completely identified Entire cell lineage and fates have been identified meaning that all cells have been identified as well as their daughter cells and the fates of all of them from zygote to mature c elegans Drosophila The fruit fly is the standard model for genetics research Major Strengths Cheap and easy breeding Short 2 week life cycle Embryogenesis complete in just 24 hours Can be used for both forward and reverse genetics About 50 of fly protein sequences have mammalian homologues Pax6 eyeless gene found in pretty much all animals including humans Vertebrate model organisms Xenopus The frog was a platform for some of the earliest insights into embryogenesis Major Strengths Large robust eggs and embryos Allows for physical manipulation on the part of the researcher Microsurgery on the embryo Taking a portion of the embryo and grafting it to another area on another embryo Helped answer questions regarding the effects that cells have on each other as well as what happens when they are secluded and allowed to grow on their own Addressing cell to cell communication addressing the origin of signals that assign fates Answering are the signals originated in the cell itself or from neighboring cells Chick The chick holds good homology to other organisms and has a prime strength through early development of anatomy Major strengths Easily accessible eggs and little difficulty in storage of eggs Ability to physically manipulate the embryo through surgery Good for mRNA reverse genetic methods Develop quickly Mouse The mouse is a largely studied organism used by many research facilities around the world which allows for easier translation of findings between scientific fields The mouse can be bred for specific phenotypes Major strengths Pluripotency of cultured stem cells These cells can generate most if not all cell types when they are reintroduced to the embryo These cells can be manipulated to add or remove genes and then may be returned to the embryo Can insert sequences of DNA using homologous recombination This makes a chimera breading knock outs or knock ins 2 What is a forward genetic screen Going from phenotype to gene The goal of a forward genetic screen is to distinguish variations for a specific phenotype in an attempt to identify which genes control the specific phenotype 1 Mutations induced by x ray irradiation on male fly which induces mutations on the male germ cells 2 Mutagenized males breed with wild type females to produce the F1 population a F1 contains many offspring heterozygous for a random mutation m1 m2 i Note at this stage experimenters will only know of flies carrying dominant mutations that generate phenotypes in the heterozygotes 3 Each F1 fly is bred with a wild type female to generate population F2 4 Sibling mating occurs between each population to produce population F3 a F3 will have some offspring homozygotes for each mutation allowing phenotypes due to recessive mutations to be identified Allows the gene to be characterized and identified as affecting the same gene or different genes that affect the same phenotype Also it can be identified as being a different mutation of the same gene or of a different gene by doing further testing 3 RNAi can be used for a backward genetic screen What is a backward genetic screen and how is RNAi used in one A backward genetic screen flows from gene to phenotype So backward genetics aims to find the phenotype associated with a specific DNA sequence It can be done through either activating or inhibiting certain genes of interest RNAi RNA interference works as loss of function approach in reverse genetics meaning it inhibits the production of the gene s of interest How it is done In a normally functioning process of transcription and translation mRNA produced through transcription machinery in the nucleus would exit and move out into the endoplasmic reticulum At this point the mRNA would bind with ribosomes which in turn would produce proteins encoding a certain gene through translation In a cell that has been targeted with RNAi the mRNA that exits the nucleus is not able to bind with ribosomes Instead the RNAi binds to the mRNA blocking the binding sites that would normally interact with ribosomes Through doing this the ribosome cannot bind with the mRNA and therefore cannot translate it into a protein As a final result the gene that was supposed to be coded for is not and cannot be expressed This would allow the researcher to observe if the blocking of the gene results in a change in phenotype 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 1 What Cells in c Elegans eventually divide into the nervous system AB cells AB blastomeres give rise to ectoderm which gives rise to the nervous system Some of the ectodermal cells located ventrally migrate with in the embryo where they differentiate to form neurons 2 Define Gastrulation Gastrulation is the process of the formation of the three germ layers They are the ectoderm the mesoderm and the endoderm 3 Describe the process of Gastrulation in c Elegans Gastrulation in c Elegans begins early and is not as easily observed as other models The embryo s first cleavage generates a large cell the AB blastomere and a smaller cell the P1 cell AB blastomere gives rise to ectodermal cells that spread to form the outer wall of the embryo the epidermis and nervous system nervous system arises almost entirely from the AB blastomere ectodermal cells situated ventrally migrate inside the embryo and diffrentiate into neurons P1 cells internalize to form the ectoderm and mesoderm 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 4 Describe the process of