DiffGeneExpress211 - Differential Gene Expression in Growth...

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Differential Gene Expression in Growth and Development - 1 Differential Gene Expression Although all cells of a multicellular organism have the complete genome, not all genes are expressed at all times; genes needed for early growth and development may not function throughout the organism's lifetime. Other genes are not activated until later in life. As we have seen with our discussion of epigenetic control of gene expression, not all DNA is accessible in all cells at all times. Differentiation of cells for tissue formation in multicellular organisms requires differential gene expression, as do the morphological changes associated with growth and development. In multicellular organisms genes are expressed differently in different tissues. We have discussed how DNA methylation and histone modification control access to genes for transcription and they are common ways in which cells and tissues control gene activity. Before leaving the subject of gene regulation we will examine some effects of differential gene expression, particularly in early the growth and development of multicellular organisms. As seen briefly during our discussion of animal cloning, during development, cell lines become specialized for certain tissue formation. Genes in those lines are selectively activated or deactivated resulting in the differentiation of cells into specific cell and tissue types. For example, muscle cell specialization occurs in the mesoderm cell line. As cells determined to be muscle cells are formed, a transcription factor, called myoD (myoblast-determining gene) is coded, which activates the p21 gene transcription, whose gene product, p21, binds to the cyclin-dependent kinase in G 1 keeping the cell in G 1 , which then activates genes to differentiate those cells into the myoblasts and muscle cells. The myoD gene is not transcribed in other cells and tissues. Control of Differentiation in Muscle Cell
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Differential Gene Expression in Growth and Development - 2 Cell Line Determination in Early Development We have just seen one example of how selective gene transcription controls muscle cell specialization. Two processes in early development are important to specialization: cytoplasmic determination and induction. The cytoplasm of egg cells, and hence zygotes, is not uniform. Signal molecules and precursor molecules needed for development found within the cytoplasm of the egg cell may not be uniformly distributed. After a few cell divisions, some substances may not be found in all product cells. This segregation within the cytoplasm affects gene expression in many organisms. One example of cytoplasmic segregation is polarity. In many animals, the first division is critical. One of the two cells formed substantially develops into the embryo; the second does not. For most animals, polarity is established within a few divisions. Research on several species of animals has shown that if embryos are separated from
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DiffGeneExpress211 - Differential Gene Expression in Growth...

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