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Chapter 20.1-20.3, 28.3

BIOLOGY 172 Chapter 20.1-20.3, 28.3: Reading 30: Homeotic Genes

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Stephen Clark

Biology 172 Reading 30 20.1-20.3, 28.3 Fertilized egg is a totipotent cell  totipotent means it can give rise to a whole organism Zygote is the fertilized egg which undergoes successive mitotic cell divisions as it moves along a fallopian tube  morula (ball of cells)  No growth between these divisions Blastocysts are hollow spheres produced by cells in the morula that move in relation to one another; has an inner cell mass and only occurs in mammals Inner cell mass is a mass of cells in one region of the inner wall of blastocyst from which the body of embryo develops Gastrula is a layered structure formed when the inner cell mass of blastocyst migrates and reorganize into three germ layers which are sheets of cells including the ectoderm, mesoderm, and endoderm  form differentiated cells Ectoderm includes outer layer of skin and nerve cells in brain Mesoderm includes cells that make up inner layer of skin, muscle cells, red blood cells Endoderm includes cells of lining of digestive tract and lung, liver cells, pancreas cells Cellular differentiation increasingly restricts alternative fates  Lose potential to develop into any kind of cell at each successive stage in development Pluripotent describes embryonic stem cells whose inner mass can give rise to any of the three germ cells layers, and any cell of body; cannot give rise to whole organism Multipotent cells can form a limited number of types of specialized cells; include cells of germ layer Together; totipotent, pluripotent, and multipotent cells are called stem cells Why do differentiating cells increasingly lose their development potential?  Hyp A: genes no longer needed are repressed  Hyp B: as cells differentiate, delete DNA for genes they do not need Used nuclear transfer to test the hypotheses Clone is an individual that carries an exact copy of the nuclear genome of another individual  result when nuclear transfer succeeds CopyCat  X-inactivation; same genome, but different expression Regenerative medicine aims to use natural processes of cell growth and development to replace diseased or damaged tissues Induced pluripotent stem cells (IPS Cells) are cells that have been reprogrammed to become pluripotent by activation of certain genes, most of them encoding transcription factors or chromatin proteins Many genes are activated and repressed at different times during development of a complex organisms  genes expressed early in an organism’s development control the activation of other groups of genes that act alter in development HIERARCHAL: genes at one step control the genes at the next step Drosophila has played a prominent role in our understanding of genetic control of early development Cellular blastoderm is the structure in Drosophila development formed by nuclei in single-cell embryo when they migrate to periphery of embryo and each nucleus becomes enclosed in its own cell membrane Gastrulation is the process in which the cells of the blastoderm migrate inward, creating layers of cells within the embryo  3 germ layers Segmentation is the formation of discrete parts or segments in the insect embryo  3 main segments Egg is a highly polarized cell Oocyte
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