ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Chapter Outline		AP BIOLOGY: Chapter Seventeen Review Answers 1. Cleavage is the initial division of a fertilized egg. It proceeds so rapidly that the zygote does not increase in size. Fate of cells immediately post-cleavage can often be determined by where they are located (e.g., animal or vegetal pole). 2. A blastula is a hollow ball of cells formed from the rapid cleavage divisions of the zygote. As cell division progresses, the blastula invaginates to form a gastrula. 3. A gastrula is an invaginated blastula that continues in rapid cell division. The wall of cells that invaginates and becomes an internal fold is the endoderm, destined to become the gut. The external surface of cells is the ectoderm, which will ultimately form skin and a nervous system. The region of cells between the endoderm and the ectoderm is the mesoderm, which will give rise to the notochord, bones, blood vessels, connective tissue, and muscles. 4. A thickened band of ectoderm overlying the notochord develops during neurulation, immediately following gastrulation. The thickening, triggered by the notochord beneath the ectoderm, continues until a long tube overlying the dorsal side of the body pinches off forming the neural tube. 5. In vertebrates, development occurs directly as a result of rapid cell division and migration of cells during gastrulation and subsequent developmental stages. Insect cells during development are not discrete, having no cell membrane between adjoining nuclei. Plant cells do not move at all during development; instead, different body parts develop from continuously produced meristematic tissues. 6. Cell movement is important in animal development because cells have to travel often great distances to specific locations in the embryo whereupon they stop and differentiate into the "designated tissue." Cells migrate these distances principally through crawling, or making cell-to-cell contacts via cadherins and integrins and pulling themselves past each other. 7. In mosaic development, cells have specific developmental destinies at the outset. In regulative development, all cells have equal capacity to develop into different tissues. 8. Induction is the switching of a cell from one developmental path to another as a consequence of interaction with an adjacent cell. Organizers produce diffusible signal molecules (morphogens) that convey positional information to other cells. 9. Determination is the commitment of a totipotent cell to a specialized developmental path; it is critical to development in that it insures the organism will have all the tissues and organs it needs to survive. Differentiation is an end-stage phenomenon whereby a cell (which will have been previously determined to be what and where it is) develops into its specific structure and function. 10. Maternal mRNA is concentrated in the cephalic end of a Drosophila embryo. When fertilization stimulates production of a protein from one of the maternal genes (bicoid), the result is the creation of a morphogen gradient which regulates development. Gap genes, pair-rule genes, and segment polarity genes represent a cascade of increasingly specific body development plans in Drosophila. 11. Homeotic genes determine what each body segment during development will ultimately turn into (i.e., wings, legs, etc.). Homeotic genes have been identified in everything from plants to insects to humans. The homeobox is a DNA domain that codes for transcription factor proteins critical to pattern formation in a developing organism. 12. Apoptosis is programmed cell death-death that is supposed to occur and has been specified for a specific time. It is critical to development as, at specific points during development, certain tissues need to be removed (such as the webbing between fingers, etc.). Necrosis is simply death of a tissue-usually not as a result of "planning ahead," (i.e., the limb of a frog is twisted during development and the limb tissue dies).