Using lac operon in E. coli and albumin gene in humans, compare and contrast the regulatory mechanisms of gene expression at the transcriptional level in prokaryotes and eukaryotes. In humans, somatic cells contain identical genes with the identical genome sequence (assuming no mutations or chromosome abnormalities are introduced). Yet, the cells in the brain are structurally and functionally quite different from the cells in the skeletal muscle. The major reason for this is that the cells in different organs express different sets of genes, which is called differential gene expression. Discuss how the differential gene expression is possible when all somatic cells carry identical genes and genome.