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Chapter 1 Questions1. Identify major animal models and their strengthsC. elegans- Easy to breed and keep; freeze viable organisms- Entire anatomy and cell lineage known- Develop in 2-3 daysDrosphila- Cheap and easy to breed- 50% human homologues (Pax6)- Embryogenesis complete in 24 hours; life cycle complete 2 weeks- Useful for forward and backward geneticsXenopus- Large, robust eggs- Microsurgery allows grafting experiments- Helps understand how manipulation changes cell fates- Helps understand how neighbors effect cellsGallus gallus- Large eggs easy to store and collect- Embryos large enough for surgical manipulations- mRNA reverse genetics methods- Quick developmentMus musculus- Well studied bred for certain phenotypes- Resemble human neural structures with same functions- Pluripotency of cultured stem cells allow for chimera experiments- Trangenic mice allow knock ins and knock outs2. What is a forward genetic screen?- When you start with a phenotype, which is the outward expression of a gene and work backwards to discover what gene causes the phenotype. Drosophila is used to do such experiments. Male flies are fed mutagens, which induces mutant germ cells. They mate with female flies and produce offspring. Just in case there is a recessive trait they breed offspring of flies out twice to ensure all possibilities have a chance to show.3. What is a backward genetic screen and how is RNAi used in one?- A backward genetic screen is when you start with an interesting looking gene and either activate it or knock it out. RNA interference is used to stop certain genes from encoding. Antisense RNA molecules bind with mRNA and stop the ribosomes from being able to translate the mRNA. Antisense molecules can also use enzymatic degradation to interfere with RNA.Chapter 2 Questions1. What cells in C. elegans eventually divide into the nervous system?- AB cells2. Define gastrulation- Gastrulation is the rearrangement of cells to from the three germ layers; involves the movement from outer surface to the inside of the embryo3. Gastrulation of C. elegans- The process of the derivatives of the AB cells which give rise to the ectoderm surrounding the derivatives of the P1 cells which give rise to the mesoderm and endoderm4. Gastrulation of Drosophila- There is an invagination along ventral side. Future mesoderm cells enter interior and outer ectoderm cells come together to close furrow made by invagination.- Ectoderm internalizes through delamination to form neuroblasts o Give rise to mother ganglion cells5. Gastrulation of Frog- Involves the inward movement of cells from embryos outer layer at the blastopore. The first cells move in anteriorly at the dorsal lip; this movement displaces the blastocoel which eventually disappears. These inner cells become the mesoderm and endoderm as well as the notochord (from mesoderm) Cells derived from animal pole spread to cover with ectoderm6. Gastrulation of Chick- Involves the ingression of epiblasts at Koller’s sickle gives rise to three germ layers which at the end of this process form the primitive streak and Hensen’s node7. Gastrulation of Mouse- Epiblast begins as a concave sheet the movement of epiblasts towards and through primitive streak form mesoderm between outer layer of endoderm cells and inner layer of ectoderm cells8. Define Neurulation and describe in frog- There are two types of neurualtion which are primary and secondary neurulation. Primary neurulation is the process by which neuroectoderm folds to form the neural plate which then rolls up into the neural tube. Secondary neurulation is the process by which an initially solid structure is hollowed out.- Neurulation in the frog involves the neuroectoderm folding to form the neural plate which then rolls up in the dorsal direction to form the neural tube. This also gives rise to the neural crest. The anterior portions of the neural tube form the eyes, optic tectum, and brain structures.9. Neurulation in Chick- Primitive streak lengthens anteriorly and a structure called Hensen’s node bulges at the top. Hensen’s node moves posteriorly to signal changes in cells and eventually will elongate to form the notochord by intercalation.Chapter 3 Questions1. Describe the Organizer region in the frog embryo and the experiment that led to its discovery.- The organizer region in the frog is at the dorsal lip of the blastopore; it generates and releases BMP inhibitors. This was discovered by Spemann and Mangold’s experiment where they grafted the dorsal lip of one embryo to an area in a second host embryo in a different location. This led to the formation of two body axes, two neural plates, two neural tubes , and eventually a second mature nervous system. It is important to note that the second body axis was derived from cells of the host cell andnot the cell from which the area was grafted from.2. Describe the Default Model of embryogenesis and the experiment that led to its discovery.- The Default Model explains neural induction and that if left alone the cells of the animal cap will have a neural fate by default. The experiment that led to this discovery was one in which they took the animal cap of a frog embryo and put it in culture the cells became epidermis cells. When they dissociated the cells of the animal cap, neural cells were produced. When they cultured the dissociated animal cap cells and added Bone-Morphogenetic Proteins the cells became epidermis cells. When they cultured animal caps intact ad added BMP inhibitors, neural cells were produced. 3. Describe how signal gradients develop originally from gastrulation.- The Organizer of the cell lets off signal to differentiate cells into the three germ layersectoderm, mesoderm and endoderm. For example, in Drosophila the protein Dorsal is released at the ventral side of the embryo and it diffuses into the intracellular space.4. Describe another way gradients of a signal molecule may be developed.- After one gradient is established, receptors of that signaling molecule are activated in certain affinities and produce signaling molecules based on the previously laid gradient. For example, the Dorsal gradient gives rise to two other signaling moleculesat specific thresholds. At high concentrations Snail is produced which diffuses and atintermediate levels of Dorsal, SOG is activated. From here lateral inhibition takes over to establish boundaries of these signaling gradients.5. What problem do cell surface receptors and

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