Concept 36: Different genes are active in different kinds of cells.
Cells differentiate because specific enzymes turn genes on and off in various cell types.
Most living things are composed of different kinds of cells specialized to perform different functions. A liver cell, for example, does not have the same biochemical duties as a nerve cell. Yet every cell of an organism has the same set of genetic instructions, so how can different types of cells have such different structures and biochemical functions? Since biochemical function is determined largely by specific enzymes (proteins), different sets of genes must be turned on and off in the various cell types. This is how cells differentiate.
This notion of cell-specific expression of genes is upheld by
hybridization experiments that can identify the unique
mRNAs in a cell type. More recently, DNA arrays and gene chips offer the opportunity to rapidly screen all gene activity of an organism. Co-expression of genes in response to external factors can thus be explored and tested.
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Igor Dawid and Thomas Sargent explain how they developed subtractive mRNA hybrization to find genes expressed by different cell types. Pat Brown and Steve Fodor show how genomes can be screened with DNA arrays and GeneChipsï¿½
Cognitive information is encoded in patterns of nervous activity and decoded by molecular listening devices at the synapse. Professor Seth Grant explains how different patterns of neural firing are critical to cognition.