Ch. 13_Developmental Genetics

Ch. 13_Developmental Genetics - Ch. 13 : Genetic Control of...

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Ch. 13 : Genetic Control of Development In a multicellular organism, in general, every cell has the same genetic material but different genes are expressed to create a variety of cell types and tissues through a number of gene expression regulatory development of various organs from a single cell, the fertilized egg. The key event is called “ pattern formation ”. The fertilized egg is totipotent ”. It is capable of making all possible organs and tissues. As it divides into 2,4,8,…. . cells, the descendent cells become less and less capable. Their fates become more and more defined. Fates that a cell can take are determined partly by what genes are active (and which ones are silent) and partly by its location in the developing embryo ( positional information; Fig. 13.2). Even before fertilization, the egg is not an uniform sphere. It has polarity. The cytoplasm in the front end is different from that in the rear end ( Fig. 13.1 ). In some cases, the 2 poles of the egg are visibly different ( Fig. 13.5 ). Lot of proteins needed for early development of an animal embryo are pre-stored in the egg (oocyte) by the mother. Their distribution is polarized. After fertilization, the zygote undergoes a series of rapid divisions creating a ball of cells with a hollow space inside (the blastula stage ). Then the surface undergoes invagination (folding) creating multiple layers and some cells move into the space inside creating a gastrula (Fig. 13.1 ). While all this is visible under the microscope, a lot is happening at the molecular level unseen by the microscope. As cell division occurs, protein molecules are being partitioned unevenly among the daughter cells. These proteins are activators/inhibitors of gene expression. They activate and suppress different cells in various daughter cells depending upon the concentration of the transcription factors. As a result, the genetic program of each daughter cell is rapidly diverging ( Fig. 13.1) . In addition to the changing gene expression patterns due to different transcription factors within a cell, signals are being received from neighboring cells, which are also influencing the fate choices a cell can make ( Fig. 13.2, 13.3 ). Cell transplantation studies show the effect of a cell’s location in the developing embryo. Depending upon how plastic/fixed the fate of a cell has become, transplantation may/may not have any effect ( Fig. 13.2 ). When the fate has already been determined, transplantation to a new location in the developing embryo will still generate the pre-determined fate. An example is shown in Fig. 13.4 . Even mechanisms (studied in Ch. 11). In this chapter, we shall examine the genes that bring about the
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at 16-cell stage of a sea-urchin embryo, 4 small cells at the bottom (called micromeres) are already fated to develop into the digestive system. Transplanting 4 micromeres from another embryo to the top of a normal embryo results in the start of development of 2 digestive systems Fig. 13.4E
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Ch. 13_Developmental Genetics - Ch. 13 : Genetic Control of...

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