Concept 18.2 Eukaryotic gene expression can be regulated at any stage. Like unicellular organisms, the tens of thousands of genes in the cells of multicellular eukaryotes are continuously turned on and off in response to signals from their internal and external environments. Gene expression must be controlled on a long-term basis during cellular differentiation, the divergence in form and function as cells in a multicellular organism specialize. Differential gene expression is the expression of different genes by cells with the same genome. A typical human cell probably expresses about 20% of its genes at any given time. Highly specialized cells, such as nerves or muscles, express only a tiny fraction of their genes. Although all the cells in an organism contain an identical genome, the subset of genes expressed in the cells of each type is unique. The differences between cell types are due to
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